EXPANSION PROCESSES OF A PERFECT GAS
Instruction Manual TH5 ISSUE 11 November 2010
Table of Contents Copyright and Trademarks .................................................................................... 1 General Overview ..................................................................................................... 2 Equipment Diagrams ................................................................................................. 3 Important Safety Information ..................................................................................... 8 Introduction ............................................................................................................ 8 The COSHH Regulations ....................................................................................... 8 Electrical Safety ..................................................................................................... 9 General Safety Rules............................................................................................. 9 Description .............................................................................................................. 14 Overview ............................................................................................................. 14 Installation ............................................................................................................... 17 Advisory............................................................................................................... 17 Installation Process.............................................................................................. 17 Electrical Supply .................................................................................................. 17 Optional Teaching Software & Data Logging Accessory ...................................... 19 Commissioning .................................................................................................... 19 Electrical Wiring Diagram..................................................................................... 21 Operation ................................................................................................................ 22 Operating the Software ........................................................................................ 22 Operating the Equipment ..................................................................................... 32 Equipment Specifications ........................................................................................ 36 I/O Port Pin Connections ..................................................................................... 36 Environmental Conditions .................................................................................... 37 Routine Maintenance .............................................................................................. 38 Responsibility ...................................................................................................... 38 General................................................................................................................ 38 Laboratory Teaching Exercises ............................................................................... 39 Index to Exercises ............................................................................................... 39 Adiabatic, Isothermal, Reversible and Irreversible Processes .............................. 39 ii
Table of Contents Nomenclature ...................................................................................................... 40 Data Sheet 1: Relative and absolute pressure ..................................................... 41 Data Sheet 2: Technical data ............................................................................... 42 Data Sheet 3: Relationship between Resistance and Temperature for Thermistors used on TH5 (Nominal values)............................................................................. 43 Exercise A - Determination of Heat Capacity Ratio ................................................. 44 Exercise B - Determination of Ratio of Volumes using an Isothermal Process......... 48 Project work ............................................................................................................ 51 Contact Details for Further Information .................................................................... 52
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Disclaimer This document and all the information contained within it is proprietary to Armfield Limited. This document must not be used for any purpose other than that for which it is supplied and its contents must not be reproduced, modified, adapted, published, translated or disclosed to any third party, in whole or in part, without the prior written permission of Armfield Limited. Should you have any queries or comments, please contact the Armfield Customer Support helpdesk (Monday to Friday: 0800 – 1800 GMT). Contact details are as follows: United Kingdom
International
(0) 1425 478781 (calls charged at local rate)
+44 (0) 1425 478781 (international rates apply)
Email:
[email protected] Fax: +44 (0) 1425 470916
Copyright and Trademarks Copyright © 2009 Armfield Limited. All rights reserved. Any technical documentation made available by Armfield Limited is the copyright work of Armfield Limited and wholly owned by Armfield Limited. Brands and product names mentioned in this manual may be trademarks or registered trademarks of their respective companies and are hereby acknowledged.
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General Overview The TH range is designed to introduce the fundamental principles of thermodynamics to the student. The range of equipment starts at basic concepts such as temperature and pressure measurement and leads on to introducing the relationships between these fundamentals, the first and second law of thermodynamics, the principles of reversibility, entropy, enthalpy etc. The equipment allows the student to gain a true understanding of these principles. The small scale of the equipment allows the relevant teaching exercises to be carried out in a relatively short period of time. This instruction manual describes the operation of the TH5 'Expansion Processes of a Perfect Gas' apparatus that has been designed by Armfield to introduce students to a range of basic thermodynamic processes using air as the working fluid. The hardware consists of two interconnected rigid vessels, one equipped for operation under pressure and the second under vacuum. An electric air pump and appropriate valves and tappings allow the use of each vessel independently or both vessels together to allow different thermodynamic processes to be evaluated. Pressure sensors connected to each vessel and temperature sensors located inside each vessel allow the changes in properties of the air inside the vessels to be monitored. All measurements are available as voltage signals for direct connection to a PC via an optional interface device (a PC interface and Windows based Educational Software is available to support the TH5 Expansion Processes of a Perfect Gas apparatus).
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Equipment Diagrams
Figure 1: Top View of TH5
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Armfield Instruction Manual
Figure 2: Front View of TH5
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Equipment Diagrams
Figure 3: Rear View of TH5
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Armfield Instruction Manual
Figure 4: Front View of Electrical Console
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Equipment Diagrams
Figure 5: Rear View of Electrical Console
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Important Safety Information Introduction All practical work areas and laboratories should be covered by local safety regulations which must be followed at all times. It is the responsibility of the owner to ensure that all users are made aware of relevant local regulations, and that the apparatus is operated in accordance with those regulations. If requested then Armfield can supply a typical set of standard laboratory safety rules, but these are guidelines only and should be modified as required. Supervision of users should be provided whenever appropriate. Your TH5 'Expansion Processes of a Perfect Gas' Apparatus has been designed to be safe in use when installed, operated and maintained in accordance with the instructions in this manual. As with any piece of sophisticated equipment, dangers exist if the equipment is misused, mishandled or badly maintained. Before proceeding to install, commission or operate the equipment described in this instruction manual we wish to alert you to potential hazards so that they may be avoided. Although designed for safe operation, any laboratory equipment may involve processes or procedures which are potentially hazardous. The major potential hazards associated with this particular equipment are listed below.
INJURY THROUGH MISUSE
INJURY FROM ELECTRIC SHOCK
INJURY FROM INCORRECT HANDLING
DAMAGE TO CLOTHING
Accidents can be avoided provided that equipment is regularly maintained and staff and students are made aware of potential hazards. A list of general safety rules is included in this manual, to assist staff and students in this regard. The list is not intended to be fully comprehensive but for guidance only. Please refer to the section regarding the Control of Substances Hazardous to Health Regulations.
The COSHH Regulations The Control of Substances Hazardous to Health Regulations (1988) The COSHH regulations impose a duty on employers to protect employees and others from substances used at work which may be hazardous to health. The regulations require you to make an assessment of all operations which are liable to expose any person to hazardous solids, liquids, dusts, vapours, gases or microorganisms. You are also required to introduce suitable procedures for handling these substances and keep appropriate records. Since the equipment supplied by Armfield Limited may involve the use of substances which can be hazardous (for example, cleaning fluids used for maintenance or chemicals used for particular demonstrations) it is essential that the laboratory
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Important Safety Information supervisor or some other person in authority is responsible for implementing the COSHH regulations. Part of the above regulations are to ensure that the relevant Health and Safety Data Sheets are available for all hazardous substances used in the laboratory. Any person using a hazardous substance must be informed of the following: Physical data about the substance Any hazard from fire or explosion Any hazard to health Appropriate First Aid treatment Any hazard from reaction with other substances How to clean/dispose of spillage Appropriate protective measures Appropriate storage and handling Although these regulations may not be applicable in your country, it is strongly recommended that a similar approach is adopted for the protection of the students operating the equipment. Local regulations must also be considered.
Electrical Safety The equipment described in this Instruction Manual operates from a mains voltage electrical supply. It must be connected to a supply of the same frequency and voltage as marked on the equipment or the mains lead. If in doubt, consult a qualified electrician or contact Armfield. The equipment must not be operated with any of the panels removed. To give increased operator protection, the unit incorporates a Residual Current Device (RCD), alternatively called an Earth Leakage Circuit Breaker, as an integral part of this equipment. If through misuse or accident the equipment becomes electrically dangerous, the RCD will switch off the electrical supply and reduce the severity of any electric shock received by an operator to a level which, under normal circumstances, will not cause injury to that person. At least once each month, check that the RCD is operating correctly by pressing the TEST button. The circuit breaker MUST trip when the button is pressed. Failure to trip means that the operator is not protected and the equipment must be checked and repaired by a competent electrician before it is used.
General Safety Rules 1. Follow Relevant Instructions a. Before attempting to install, commission or operate equipment, all relevant suppliers’/manufacturers’ instructions and local regulations should be understood and implemented. b. It is irresponsible and dangerous to misuse equipment or ignore instructions, regulations or warnings. 9
Armfield Instruction Manual c. Do not exceed specified maximum operating conditions (e.g. temperature, pressure, speed etc.). 2. Installation a. Use lifting tackle where possible to install heavy equipment. Where manual lifting is necessary beware of strained backs and crushed toes. Get help from an assistant if necessary. Wear safety shoes where appropriate. b. Extreme care should be exercised to avoid damage to the equipment during handling and unpacking. When using slings to lift equipment, ensure that the slings are attached to structural framework and do not foul adjacent pipework, glassware etc. When using fork lift trucks, position the forks beneath structural framework ensuring that the forks do not foul adjacent pipework, glassware etc. Damage may go unseen during commissioning creating a potential hazard to subsequent operators. c. Where special foundations are required follow the instructions provided and do not improvise. Locate heavy equipment at low level. d. Equipment involving inflammable or corrosive liquids should be sited in a containment area or bund with a capacity 50% greater than the maximum equipment contents. e. Ensure that all services are compatible with the equipment and that independent isolators are always provided and labelled. Use reliable connections in all instances, do not improvise. f.
Ensure that all equipment is reliably earthed and connected to an electrical supply at the correct voltage. The electrical supply must incorporate a Residual Current Device (RCD) (alternatively called an Earth Leakage Circuit Breaker - ELCB) to protect the operator from severe electric shock in the event of misuse or accident.
g. Potential hazards should always be the first consideration when deciding on a suitable location for equipment. Leave sufficient space between equipment and between walls and equipment. 3. Commissioning a. Ensure that equipment is commissioned and checked by a competent member of staff before permitting students to operate it. 4. Operation a. Ensure that students are fully aware of the potential hazards when operating equipment. b. Students should be supervised by a competent member of staff at all times when in the laboratory. No one should operate equipment alone. Do not leave equipment running unattended. c. Do not allow students to derive their own experimental procedures unless they are competent to do so. d. Serious injury can result from touching apparently stationary equipment when using a stroboscope to `freeze´ rotary motion. 10
Important Safety Information 5. Maintenance a. Badly maintained equipment is a potential hazard. Ensure that a competent member of staff is responsible for organising maintenance and repairs on a planned basis. b. Do not permit faulty equipment to be operated. Ensure that repairs are carried out competently and checked before students are permitted to operate the equipment. 6. Using Electricity a. At least once each month, check that ELCB's (RCCB's) are operating correctly by pressing the TEST button. The circuit breaker must trip when the button is pressed (failure to trip means that the operator is not protected and a repair must be effected by a competent electrician before the equipment or electrical supply is used). b. Electricity is the commonest cause of accidents in the laboratory. Ensure that all members of staff and students respect it. c. Ensure that the electrical supply has been disconnected from the equipment before attempting repairs or adjustments. d. Water and electricity are not compatible and can cause serious injury if they come into contact. Never operate portable electric appliances adjacent to equipment involving water unless some form of constraint or barrier is incorporated to prevent accidental contact. e. Always disconnect equipment from the electrical supply when not in use. 7. Avoiding fires or explosion a. Ensure that the laboratory is provided with adequate fire extinguishers appropriate to the potential hazards. b. Where inflammable liquids are used, smoking must be forbidden. Notices should be displayed to enforce this. c. Beware since fine powders or dust can spontaneously ignite under certain conditions. Empty vessels having contained inflammable liquids can contain vapour and explode if ignited. d. Bulk quantities of inflammable liquids should be stored outside the laboratory in accordance with local regulations. e. Storage tanks on equipment should not be overfilled. All spillages should be immediately cleaned up, carefully disposing of any contaminated cloths etc. Beware of slippery floors. f.
When liquids giving off inflammable vapours are handled in the laboratory, the area should be ventilated by an ex-proof extraction system. Vents on the equipment should be connected to the extraction system.
g. Students should not be allowed to prepare mixtures for analysis or other purpose without competent supervision.
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Armfield Instruction Manual 8. Handling poisons, corrosive or toxic materials a. Certain liquids essential to the operation of equipment, for example mercury, are poisonous or can give off poisonous vapours. Wear appropriate protective clothing when handling such substances. Clean up any spillage immediately and ventilate areas thoroughly using extraction equipment. Beware of slippery floors. b. Do not allow food to be brought into or consumed in the laboratory. Never use chemical beakers as drinking vessels. c. Where poisonous vapours are involved, smoking must be forbidden. Notices should be displayed to enforce this. d. Poisons and very toxic materials must be kept in a locked cupboard or store and checked regularly. Use of such substances should be supervised. e. When diluting concentrated acids and alkalis, the acid or alkali should be added slowly to water while stirring. The reverse should never be attempted. 9. Avoiding cuts and burns a. Take care when handling sharp edged components. Do not exert undue force on glass or fragile items. b. Hot surfaces cannot, in most cases, be totally shielded and can produce severe burns even when not `visibly hot´. Use common sense and think which parts of the equipment are likely to be hot. 10. Eye protection a. Goggles must be worn whenever there is a risk to the eyes. Risk may arise from powders, liquid splashes, vapours or splinters. Beware of debris from fast moving air streams. Alkaline solutions are particularly dangerous to the eyes. b. Never look directly at a strong source of light such as a laser or Xenon arc lamp. Ensure that equipment using such a source is positioned so that passers-by cannot accidentally view the source or reflected ray. c. Facilities for eye irrigation should always be available. 11. Ear protection a. Ear protectors must be worn when operating noisy equipment. 12. Clothing a. Suitable clothing should be worn in the laboratory. Loose garments can cause serious injury if caught in rotating machinery. Ties, rings on fingers etc. should be removed in these situations. b. Additional protective clothing should be available for all members of staff and students as appropriate. 13. Guards and safety devices
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Important Safety Information a. Guards and safety devices are installed on equipment to protect the operator. The equipment must not be operated with such devices removed. b. Safety valves, cut-outs or other safety devices will have been set to protect the equipment. Interference with these devices may create a potential hazard. c. It is not possible to guard the operator against all contingencies. Use common sense at all times when in the laboratory. d. Before starting a rotating machine, make sure staff are aware how to stop it in an emergency. e. Ensure that speed control devices are always set at zero before starting equipment. 14. First aid a. If an accident does occur in the laboratory it is essential that first aid equipment is available and that the supervisor knows how to use it. b. A notice giving details of a proficient first-aider should be prominently displayed. c. A `short list´ of the antidotes for the chemicals used in a particular laboratory should be prominently displayed.
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Description Where necessary, refer to the drawings in the Equipment Diagrams section.
Overview The pressure and vacuum vessel assembly: The equipment comprises two floor-standing interconnected rigid vessels on a common baseplate (4), the larger vessel (3) equipped for operation under pressure and the smaller vessel (6) equipped for operation under vacuum. A freestanding electrically operated air pump (9), together with valves and tappings on the top plate (2) allow the appropriate vessel to be pressurised or evacuated as required to suit the teaching exercise. The vessels can be used independently or together to allow different thermodynamic processes to be evaluated. A pressure sensor (P & V) connected to each vessel through the top plate (2) and a temperature sensor (T1 & T2) inside each vessel allow the properties of the air contained within the vessels to be monitored continuously. Both vessels are constructed from clear rigid plastic which affords light insulation between the air inside the vessel and the surroundings to reduce heating/cooling but allows each vessel and its contents to return to ambient temperature reasonably quickly. Each end of the clear acrylic tube is located in a groove in the top plate (2) and bottom plate (4). The joint is sealed by an ‘O’ ring (5) in the groove that is compressed by a series of tie rods (8) that surround each vessel. The capacity of the pressurised vessel (3) is approximately 23 litres. The capacity of the evacuated vessel (6) is approximately 9 litres. Each vessel incorporates the following features: A connection from the top of the vessel to the inlet/outlet of the air pump (9) to allow the vessel to be evacuated/pressurised as required for the appropriate demonstration. An isolating valve in the connection to each vessel (valve V4 on the pressure vessel and valve V7 on the vacuum vessel) allows the vessel to be isolated from the air pump. A connection to a piezo-resistive pressure sensor (P & V described below) to measure the pressure and vacuum inside each vessel (range of both sensors +/-34.48 kN/m2) A connection to a large bore pipe and valve to allow rapid depressurisation/pressurisation of the vessel to/from the atmosphere. Valve V1 allows air to exit the pressure vessel to atmosphere when the vessel has been pressurised. Valve V3 allows air to enter the vacuum vessel from the atmosphere when the vessel has been evacuated. Interconnection between the two vessels via a large bore pipe and large bore valve V2 to allow fast changes to occur. Opening valve V2 allows air to flow from the pressurised vessel to the evacuated vessel when a pressure difference exists between the two vessels. Interconnection between the two vessels via a small bore pipe and needle valve V5 to allow gradual changes to occur. Needle valve V5 can be adjusted to change the rate that air flows between the vessels. Since valve V5 cannot be fully closed, isolating valve V6 allows this connection to be closed and also allows the setting of V5 to be preserved between demonstrations.
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Description A tapping for a fast-response thermistor type temperature probe (T1 & T2 described below) to monitor the temperature of the air inside each vessel. A pressure relief valve, (1) on the pressure vessel and (7) on the vacuum vessel, to prevent over-pressurisation of either vessel. Pressure sensors (P & V) Both sensors are piezo-resistive and produce a voltage output that changes linearly with changing pressure. Sensor P measures the pressure inside the larger pressure vessel (3). Sensor V measures the vacuum inside the smaller vacuum vessel (6). This type of solid state pressure sensor allows the small changes in pressure to be monitored without the oscillations that would occur if using a traditional U tube manometer. Note: Since V is an indication of vacuum inside the smaller vessel the sign convention means that positive values are indicated when below atmospheric pressure and negative values are indicated when above atmospheric pressure. Temperature probes (T1 & T2) Each temperature probe consists of a miniature semiconductor thermistor bead (10), incorporating extremely fine connecting leads, that is installed between two support wires at the tip of the temperature probe assembly. The thermistor is a thermally sensitive variable resistor that exhibits a highly non-linear and negative characteristic (resistance falls with increasing temperature). The extremely small size of the thermistor bead and connecting leads means that the thermal capacity of the sensor is small and therefore the first–order time constant is extremely small (the speed of response is fast when the air temperature changes). The response of the thermistor can never be as fast as a pressure sensor because of the finite size of the bead and connecting leads but it is sufficiently fast to indicate the temperature changes that accompany the changes in pressure. The Electrical Console: All power supplies, signal conditioning circuitry etc are contained in a simple electrical console (11) with appropriate current protection devices (23, 24 & 25) and an RCD (26) for operator protection. The console is designed to stand on a suitable bench above the pressure and vacuum vessel assembly and incorporates the necessary electrical connections for the air pump and sensors. All circuits inside the console are operated by a mains on/off switch (12) on the front of the console. The various circuits inside the console are protected against excessive current by miniature circuit breakers, as follows: O/P (23) This breaker protects the electrical output marked OUTPUT (27) at the rear of the console. The socket is used to power accessories such as a chart recorder (not supplied) or the older IFD3 interface used for data logging. A mains connection is not necessary when using the newer IFD5 interface. CTRL (24) This breaker protects the power supplies and circuits inside the console.
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Armfield Instruction Manual PUMP (25) This breaker protects the electrical supply to the air pump used to pressurise or evacuate the vessels. The air pump is started and stopped using the switch marked ‘Air Pump’ (13) on the front of the console. Appropriate sockets at the rear of the console allow the air pump and sensors to be connected to the console as follows: AIR PUMP (18) PRESSURE SENSOR TANK 1 (19)
Mains supply lead to the air pump Pressure sensor P in pressure vessel
THERM TANK 1 (20) vessel
Temperature sensor T1 in pressure
VACUUM SENSOR TANK 2 (21)
Pressure sensor V in vacuum vessel
THERM TANK 2 (22) vessel
Temperature sensor T2 in vacuum
The pressure P, vacuum V and temperatures T1 & T2 measured inside the two vessels are displayed on a common digital meter (16) with a rotary selector switch (14). Measurements are indicated directly in the engineering units appropriate to the demonstration. I.e. pressure P and vacuum V are indicated in kNm-2. Temperatures T1 and T2 are indicated in Ohms (the resistance of the corresponding thermistor sensor) from which the actual temperature may be determined using the table on page (xv) of the teaching manual. When using the optional teaching software (TH5304 with IFD6 interface) the resistance readings are automatically converted to equivalent readings of temperature in oC. All signals are simultaneously connected to a 50 way I/O Port connector (15) for connection to a PC using an optional interface device (TH-IFD) with educational software package (TH5-304). Alternatively, appropriate signals can be connected to a user supplied chart recorder if required. See I/O Port Pin Connections for information. Note: As the teaching exercises require the transient pressure and temperature responses to be observed and recorded, one of these recording options is necessary to provide a clear demonstration of the thermodynamic processes involved. A barometer (not supplied by Armfield) will be required to measure the local atmospheric pressure for accurate results when using the TH5.
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Installation Advisory Before operating the equipment, it must be unpacked, assembled and installed as described in the steps that follow. Safe use of the equipment depends on following the correct installation procedure. Where necessary, refer to the drawings in the Equipment Diagrams section.
Installation Process Carefully remove the TH5 ‘Expansion Processes of a Perfect Gas' apparatus from its carton. Place the electrical console on a suitable work bench adjacent to a mains electrical socket then place the baseplate, incorporating the two rigid vessels and air pump, on the floor in front of the bench. The air pump should be positioned on the bench alongside the console in a position convenient for connecting the flexible tubing to the tops of the two vessels. Do not connect the electrical supply at this stage. Note: It is suggested that the pressure and vacuum vessel assembly is placed on the floor when in use as it is necessary to operate the ball valves on top of the vessels with a rapid snap-action. This action is difficult to perform with the valves at high level. When not in use it is suggested that the assembly be moved onto a bench top or other location to prevent damage to the equipment through accidental knocks etc. The equipment is supplied with the thermistor temperature sensors T1 and T2 fitted inside the vessels to prevent damage in transit or handling. The thermistor bead at the tip of each sensor is extremely delicate and the sensor must be handled with care if removed from the vessel for any reason. Connections between the electrical console, the air pump and the pressure/vacuum vessel assembly are described in the Commissioning section.
Electrical Supply Before connecting the TH5 to the mains electrical supply ensure that the apparatus has been assembled as described in the Installation Process section of this instruction manual. Before connecting the appropriate electrical supply check the following: Ensure that the mains on/off switch (12) on the front of the console is in the OFF position. Ensure that the RCD/RCCB (26) and three miniature circuit breakers marked PUMP (25), CTRL (24) and O/P (23) are in the OFF (down) position. Ensure that the ball valves V1, V2 and V3 on top of the vessels are fully open. Ensure that the isolating valves V4 and V7 from the air pump to the pressure and vacuum vessels are fully open. ELECTRICAL SUPPLY FOR VERSION TH5-A: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 230V, 50Hz. Check that the voltage
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Armfield Instruction Manual and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Two alternative mains leads are supplied with this version of the equipment with an appropriate plug fitted to suit European or UK style electrical sockets, so it will not be necessary for an electrician to terminate any electrical connections when installing the equipment. For information the supply cable and equipment wiring use the following convention: GREEN and YELLOW
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EARTH (GROUND)
BROWN or BLACK
-
LIVE (HOT)
BLUE or WHITE
-
NEUTRAL
Supply fuse rating
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2 AMPS
ELECTRICAL SUPPLY FOR VERSION TH5-B: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 120V, 60Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. The mains lead supplied with this equipment is fitted with a NEMA 15-5P plug so it will not be necessary for an electrician to terminate any electrical connections when installing the equipment. For information the supply cable and equipment wiring use the following convention: GREEN and YELLOW
-
EARTH (GROUND)
BROWN or BLACK
-
LIVE (HOT)
BLUE or WHITE
-
NEUTRAL
Supply fuse rating
-
3 AMPS
ELECTRICAL SUPPLY FOR VERSION TH5-G: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 220V, 60Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. The mains lead supplied with this equipment is fitted with a NEMA 15-6P plug so it will not be necessary for an electrician to terminate any electrical connections when installing the equipment. For information the supply cable and equipment wiring use the following convention:
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GREEN and YELLOW
-
EARTH (GROUND)
BROWN or BLACK
-
LIVE (HOT)
BLUE or WHITE
-
NEUTRAL
Supply fuse rating
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2 AMPS
Installation
Optional Teaching Software & Data Logging Accessory An I/O Data Port connector (15) on the right hand side of the console allows the voltage signals from each of the measurements to be connected to a suitable PC using an Armfield interface device (IFD3 or IFD5). The IFD3 obtains its power from a mains outlet socket (27) marked OUTPUT at the rear of the console. The IFD5 obtains its power via the USB cable from the PC and requires no additional power supply. This interface device together with the appropriate Windows based software is available as an optional accessory to accompany the TH5. The operation of the interface is described in the instruction leaflet supplied with the interface device. The operation of the Windows based software is described in the help text included as part of the software. The TH5 Expansion Processes of a Perfect Gas apparatus is ready for use and should be commissioned as described in the Commissioning section. Refer to the Operation section for details on how to operate the equipment.
Commissioning The following procedure is used for checking that the TH5 ‘Expansion Processes of a Perfect Gas’ apparatus is operating correctly. Ensure that the equipment has been set up in accordance with the previous sections of this manual. 1. Ensure that the mains on/off switch (12) on the console is in the OFF position and the air pump switch (13) is also set to off. 2. Ensure that the ball valves V1, V2 and V3 on top of the vessels are fully open (Atmospheric pressure inside both vessels). 3. Ensure that the isolating valves V4 and V7 from the air pump to the pressure and vacuum vessels are fully open. 4. Connect the inlet on the air pump to the tapping on top of the small vessel and the outlet on the air pump to the tapping on top of the large vessel using the flexible tubing supplied as shown in the diagram on page 7. Although the air pump is shown diagrammatically located on top of the air vessels it is most convenient to locate the air pump alongside the electrical console on a suitable bench. 5. Connect the lead from the air pump to the socket marked AIR PUMP (18) at the rear of the electrical console. Connect the lead from each of the sensors to the appropriate socket at the rear of the electrical console as follows:Pressure sensor on pressure vessel (large diameter vessel) to socket marked PRESSURE SENSOR TANK 1 (19). Temperature sensor inside pressure vessel to socket marked THERM TANK 1 (20).
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Armfield Instruction Manual Pressure sensor on vacuum vessel (smaller diameter vessel) to socket marked VACUUM SENSOR TANK 2 (21). Temperature sensor inside vacuum vessel to socket marked THERM TANK 2 (22). 6. Ensure that the mains electrical supply is connected and switched on. Check the operation of the RCD/RCCB (26) by pressing the TEST button. The RCD must trip when the button is pressed. If the RCD does not trip or it trips before pressing the test button then it must be checked by a competent electrician before the equipment is used. 7. Ensure that the RCD/RCCB (26) and the three miniature circuit breakers marked PUMP (25) CTRL (24) and O/P (23) on the rear of the console are in the ON (up) position. Set the mains on/off switch (12) on the front of the console to the ON position. Observe that the digital panel meter (16) is illuminated. 8. Set the rotary selector switch (14) to each position in turn and check that the readings are sensible as follows: With the selector switch set to P or V observe that the pressure and vacuum readings are zero (atmospheric pressure). With the selector switch set to T1 or T2 observe that the resistance of the thermistor is indicated in Ohms, typically 2000 at 25 oC (resistance will increase with falling temperature). 9. Close ball valve V1 from the large pressure vessel to atmosphere. Close ball valve V2 between the large pressure and small vacuum vessels. Close isolating valve V6 between the large pressure and small vacuum vessels (located on top of small vacuum vessel) Ensure that isolating valve V4 is open to allow the air pump to pressurise the large pressure vessel. 10. Set the selector switch (14) to position P to observe the pressure inside the large vessel then switch on the air pump (9) using the switch (13) on the console. Observe that the pressure P gradually rises. When the pressure reaches approximately 30 kN/m2 close isolating valve V4 and switch off the air pump. Set the selector switch to position T1 and observe that the temperature of the air has risen slightly (indicated by a small fall in the resistance of the thermistor T1). 11. Rapidly open then close ball valve V1 to allow a small amount of air to escape from the large vessel. Observe that the pressure falls instantly then gradually recovers to a value below the original pressure. Check that pressure P settles after a few minutes and does not continue to fall (a continuing fall in pressure indicates a leak).
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Installation The transient change in pressure (and corresponding change in temperature T1) can be viewed clearly using the optional teaching software (TH5-304). 12. Close ball valve V3 from the small vessel to atmosphere. Ensure that isolating valve V7 is open to allow the air pump to evacuate the small vessel. 13. Set the selector switch to position V to observe the vacuum inside the small vessel then switch on the air pump (9) using the switch (13) on the console. Observe that the vacuum V gradually rises. When the vacuum reaches approximately 30 kN/m2 close isolating valve V7 and switch off the air pump. Set the selector switch to T2 and observe that the temperature of the air has fallen slightly (indicated by a small rise in the resistance of the thermistor T2). 14. Rapidly open then close ball valve V3 to allow a small amount of air to enter the small vessel. Observe that the vacuum falls instantly then gradually recovers to a value below the original vacuum. Check that vacuum V settles after a few minutes and does not continue to fall (a continuing fall in vacuum indicates a leak). The transient change in vacuum (and corresponding change in temperature T2) can be viewed clearly using the optional teaching software (TH5-304). 15. Open the ball valves V1, V2 and V3 to return the vessels to atmospheric pressure. Switch off the equipment using the mains switch (12) on the console. The basic operation of the TH5 'Expansion Processes of a Perfect Gas’ apparatus has been confirmed. Refer to the Operation section for further information.
Electrical Wiring Diagram Click on the relevant link to invoke the Wiring Diagram: Wiring Diagram CDM28103 Printed Versions of this Instruction Manual Please note, all wiring diagrams are appended at the rear of this manual
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Operation Where necessary, refer to the drawings in the Equipment Diagrams section.
Operating the Software Note: The diagrams in this section are included as typical examples and may not relate specifically to the individual product described in this instruction manual. The Armfield Software is a powerful Educational and Data Logging tool with a wide range of features. Some of the major features are highlighted below, to assist users, but full details on the software and how to use it are provided in the presentations and Help text incorporated in the Software. Help on Using the Software or Using the Equipment is available by clicking the appropriate topic in the Help drop-down menu from the upper toolbar when operating the software as shown:
Before operating the software ensure that the equipment has been connected to the IFD5 Interface (where IFD5 is separate from the equipment) and the IFD5 has been connected to a suitable PC using a USB lead. For further information on these actions refer to the Operation manual. Load the software. If multiple experiments are available then a menu will be displayed listing the options. Wait for the presentation screen to open fully as shown:
Before proceeding to operate the software ensure that IFD: OK is displayed at the bottom of the screen. If IFD:ERROR is displayed check the USB connection between the IFD5 and the PC and confirm that the red and green LED’s are both illuminated. If the problem persists then check that the driver is installed correctly (refer to the Operation manual).
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Operation
Presentation Screen - Basics and Navigation As stated above, the software starts with the Presentation Screen displayed. The user is met by a simple presentation which gives them an overview of the capabilities of the equipment and software and explains in simple terms how to navigate around the software and summarizes the major facilities complete with direct links to detailed context sensitive ‘help’ texts. To view the presentations click Next or click the required topic in the left hand pane as appropriate. Click More while displaying any of the topics to display a Help index related to that topic. To return to the Presentation screen at any time click the View Presentation icon from the main tool bar or click Presentation from the dropdown menu as shown:
For more detailed information about the presentations refer to the Help available via the upper toolbar when operating the software.
Toolbar A toolbar is displayed at the top of the screen at all times, so users can jump immediately to the facility they require, as shown:
The upper menu expands as a dropdown menu when the cursor is placed over a name. The lower row of icons (standard for all Armfield Software) allows a particular function to be selected. To aid recognition, pop-up text names appear when the cursor is placed over the icon.
Mimic Diagram The Mimic Diagram is the most commonly used screen and gives a pictorial representation of the equipment, with continuously updated display boxes for all the various sensor readings, calculated variables etc. directly in engineering units. To view the Mimic Diagram click the View Diagram icon from the main tool bar or click Diagram from the View drop-down menu as shown:
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Armfield Instruction Manual
A Mimic diagram is displayed, similar to the diagram as shown:
The details in the diagram will vary depending on the equipment chosen if multiple experiments are available. In addition to measured variables such as Temperature, Pressure and Flowrate (from a direct reading flowmeter), calculated data such as Motor Torque, Motor Speed and Discharge / Volume flowrate (from pressure drop across an orifice plate) are continuously displayed in data boxes with a white background. These are automatically updated and cannot be changed by the user. Manual data input boxes with a coloured background allow constants such as Orifice Cd and Atmospheric Pressure to be changed by over-typing the default value, if required. The data boxes associated with some pressure sensors include a Zero button alongside. This button is used to compensate for any drift in the zero value, which is an inherent characteristic of pressure sensors. Pressing the Zero button just before starting a set of readings resets the zero measurement and allows accurate pressure measurements to be taken referenced to atmospheric pressure. This action must be 24
Operation carried out before the motor is switched on otherwise the pressure readings will be offset. The mimic diagram associated with some products includes the facility to select different experiments or different accessories, usually on the left hand side of the screen, as shown:
Clicking on the appropriate accessory or exercise will change the associated mimic diagram, table, graphs etc to suit the exercise being performed.
Control Facilities in the Mimic Diagram A Power On button allows the motor to be switched off or on as required. The button always defaults to off at startup. Clicking this button switches the power on (1) and off (0) alternately. A box marked Motor Setting allows the speed of the motor to be varied from 0 to 100% either stepwise, by typing in values, or using the up / down arrows as appropriate. It is usual to operate the equipment with the motor initially set to 100%, then reduce the setting as required to investigate the effect of reduced speed on performance of the equipment. When the software and hardware are functioning correctly together, the green LED marked Watchdog Enabled will alternate On and Off. If the Watchdog stops alternating then this indicates a loss of communication between the hardware and software that must be investigated. Details on the operation of any automatic PID Control loops in the software are included later in this section.
Data Logging Facilities in the Mimic Diagram There are two types of sampling available in the software, namely Automatic or Manual. In Automatic logging, samples are taken regularly at a preset but variable interval. In Manual logging, a single set of samples is taken only when requested by
25
Armfield Instruction Manual the operator (useful when conditions have to be changed and the equipment allowed to stabilize at a new condition before taking a set of readings). The type of logging will default to manual or automatic logging as appropriate to the type of product being operated. Manual logging is selected when obtaining performance data from a machine where conditions need to stabilize after changing appropriate settings. To record a set of set of data values from each of the measurement sensors click the main toolbar. One set of data will be recorded each time the
icon from the icon is clicked.
Automatic logging is selected when transients need to be recorded so that they can be plotted against time. Click the the
icon from the toolbar to start recording, click
icon from the toolbar to stop recording.
The type of logging can be configured by clicking Configure in the Sample dropdown menu from the upper toolbar as shown:
In addition to the choice of Manual or Automatic sampling, the parameters for Automatic sampling can also be set. Namely, the time interval between samples can be set to the required number of minutes or seconds. Continuous sampling can be selected, with no time limit or sampling for a fixed duration can be set to the required number of hours, minutes or seconds as shown:
Tabular Display To view the Table screen click the View Table icon click Table from the View dropdown menu as shown:
26
from the main tool bar or
Operation
The data is displayed in a tabular format, similar to the screen as shown:
As the data is sampled, it is stored in spreadsheet format, updated each time the data is sampled. The table also contains columns for the calculated values. New sheets can be added to the spreadsheet for different data runs by clicking the icon from the main toolbar. Sheets can be renamed by double clicking on the sheet name at the bottom left corner of the screen (initially Run 1, Run 2 etc) then entering the required name. For more detailed information about Data Logging and changing the settings within the software refer to the Help available via the upper toolbar when operating the software.
Graphical Display When several samples have been recorded, they can be viewed in graphical format.
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Armfield Instruction Manual
To view the data in Graphical format click the View graph icon tool bar or click Graph from the View drop-down menu as shown:
from the main
The results are displayed in a graphical format as shown:
(The actual graph displayed will depend on the product selected and the exercise that is being conducted, the data that has been logged and the parameter(s) that has been selected). Powerful and flexible graph plotting tools are available in the software, allowing the user full choice over what is displayed, including dual y axes, points or lines, displaying data from different runs, etc. Formatting and scaling is done automatically by default, but can be changed manually if required. To change the data displayed on the Graph click Graph Data from the Format dropdown menu as shown:
28
Operation
The available parameters (Series of data) are displayed in the left hand pane as shown:
Two axes are available for plotting, allowing series with different scaling to be presented on the same x axis. To select a series for plotting, click the appropriate series in the left pane so that it is highlighted then click the appropriate right-facing arrow to move the series into one of the windows in the right hand pane. Multiple series with the same scaling can be plotted simultaneously by moving them all into the same window in the right pane. To remove a series from the graph, click the appropriate series in the right pane so that it is highlighted then click the appropriate left-facing arrow to move the series into the left pane. The X-Axis Content is chosen by default to suit the exercise. The content can be changed if appropriate by opening the drop down menu at the top of the window. The format of the graphs, scaling of the axes etc. can be changed if required by clicking Graph in the Format drop-down menu as shown:
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Armfield Instruction Manual
For more detailed information about changing these settings refer to the Help available via the upper toolbar when operating the software.
PID Control Where appropriate, the software associated with some products will include a single or multiple PID control loops whereby a function on the product can be manually or automatically controlled using the PC by measuring an appropriate variable and varying a function such as a heater power or pump speed. The PID loop can be accessed by clicking the box labelled PID or Control depending on the particular software:
A PID screen is then displayed as shown:
30
Operation
The Mode of operation always defaults to Manual control and 0% output when the software is loaded to ensure safe operation of the equipment. If appropriate, the operator can retain manual operation and simply vary the value from 0 to 100% in the Manual Output box, then clicking Apply. Alternatively, the PID loop can be changed to Automatic operation by clicking the Automatic button. If any of the PID settings need to be changed from the default values then these should be adjusted individually before clicking the Apply button.
The controller can be restored to manual operation at any time by clicking the Manual button. The value in the Manual Output box can be changed as required before clicking the Apply button. Settings associated with Automatic Operation such as the Setpoint, Proportional Band, Integral Time, Derivative Time and Cycle Time (if appropriate) can be changed by the operator as required before clicking the Apply button. Clicking Calculations displays the calculations associated with the PID loop to aid understanding and optimization of the loop when changing settings as shown:
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Armfield Instruction Manual
Clicking Settings returns the screen to the PID settings. Clicking OK closes the PID screen but leaves the loop running in the background. In some instances the Process Variable, Control variable and Control Action can be varied to suit different exercises, however, in most instances these boxes are locked to suit a particular exercise. Where the variables can be changed the options available can be selected via a drop-down menu.
Advanced Features The software incorporates advanced features such as the facility to recalibrate the sensor inputs from within the software without resorting to electrical adjustments of the hardware. For more detailed information about these advanced functions within the software refer to the Help available via the upper toolbar when operating the software.
Operating the Equipment The following notes are provided to assist an operator who requires further details on how to use the equipment. The individual laboratory teaching exercises provide the necessary settings/sequences for operating the equipment as required to complete a particular exercise Refer to the Equipment Diagrams. Although the air pump is show located on top of the air vessels it is most convenient to locate the air pump alongside the electrical console on a suitable bench. Note: Pressure P in the large vessel and Vacuum V in the small vessel are indicated by positive values on the digital meter (16). A negative reading for P means that the large vessel is below atmospheric (I.e. vacuum). A negative reading for V means that the small vessel is above atmospheric (I.e. pressure).
32
Operation
Using the air pump to pressurise the large vessel Before using the air pump check that the outlet of the pump is connected to the tapping on top of the large vessel as show in Figure 1 using flexible tubing. Set the selector switch (14) to position P to observe the pressure inside the large vessel (3) on the digital meter (16). Close ball valve V1 from the large vessel to atmosphere. Close ball valve V2 that connects the large vessel to the small vessel (via a large bore pipe). Close isolating valve V6 that connects the large vessel to the small vessel (via needle valve V5). Open isolating valve V4 to allow the air pump to pressurise the large vessel. Open isolating valve V7 to allow the air pump to draw air from the small vessel. Switch on the air pump (9) using the switch (13) on the console. Allow the pressure to rise until the required reading is indicated on the digital meter. Note: Most demonstrations are performed with a maximum starting pressure of 30 kN/m2. The pressure can be taken above this value if required but readings in excess of 35 kN/m2 cannot be monitored using a computer or chart recorder that has been connected to the I/O Port (15). Pressures in excess of this will operate the pressure relief valve (1) that protects the large vessel from damage. When the required pressure is achieved close the isolating valve V4 and switch off the air pump. The pressure reading will fall slightly after closing the isolating valve. This is normal and due to temperature changes (This effect is explained in the relevant teaching exercise).
Using the air pump to evacuate the small vessel Before using the air pump check that the inlet of the pump is connected to the tapping on top of the small vessel as show in Figure 1 using flexible tubing. Set the selector switch (14) to position V to observe the Vacuum inside the small vessel (6) on the digital meter (16). Close ball valve V3 from the small vessel to atmosphere. Close ball valve V2 that connects the large vessel to the small vessel (via a large bore pipe). Close isolating valve V6 that connects the large vessel to the small vessel (via needle valve V5). Open isolating valve V7 to allow the air pump to evacuate the small vessel. Open isolating valve V4 to allow the air pump to deliver air to the large vessel. Switch on the air pump (9) using the switch (13) on the console. Allow the vacuum to rise until the required reading is indicated on the digital meter.
33
Armfield Instruction Manual Note: Most demonstrations are performed with a maximum starting vacuum of 30 kN/m2. The vacuum can be taken above this value if required but readings in excess of 35 kN/m2 cannot be monitored using a computer or chart recorder that has been connected to the I/O Port (15). When the required vacuum is achieved close the isolating valve V7 and switch off the air pump. The vacuum reading will fall slightly after closing the isolating valve. This is normal and due to temperature changes (This effect is explained in the relevant teaching exercise). Note: If the small vessel accidentally becomes pressurised in excess of 35 kN/m2 then the pressure relief valve (7) will protect the small vessel from damage.
Creating a step change in the large vessel Having created a pressure inside the vessel wait until the pressure reading stabilises (temperature of the air inside the vessel achieves room temperature). The required step change is a small but rapid change in the pressure inside the vessel. This is achieved by opening then closing ball valve V1 rapidly with a snap action. It may take a little practice but when performed correctly the air will be heard to rush out of the vessel for a very short duration. The effect of the step change is described in the relevant teaching exercise.
Creating a step change in the small vessel Having created a vacuum inside the vessel wait until the vacuum reading stabilises (temperature of the air inside the vessel achieves room temperature). The required step change is a small but rapid change in the vacuum inside the vessel. This is achieved by opening then closing ball valve V3 rapidly with a snap action. It may take a little practice but when performed correctly the air will be heard to rush into the vessel for a very short duration. The effect of the step change is described in the relevant teaching exercise.
Creating a step change between the small and large vessels Having created a vacuum inside the small vessel and/or a pressure inside the large vessel wait until the vacuum and pressure readings stabilise (temperature of the air inside the vessels achieves room temperature). The required step change is a small but rapid change in the vacuum/pressure inside the vessels. This is achieved by opening then closing ball valve V2 rapidly with a snap action. It may take a little practice but when performed correctly the air will be heard to rush from the large vessel to the small vessel for a very short duration. The effect of the step change is described in the relevant teaching exercise.
34
Operation
Creating a gradual change between the small and large vessels Having created a vacuum inside the small vessel and/or a pressure inside the large vessel wait until the vacuum and pressure readings stabilise (temperature of the air inside the vessels achieves room temperature). The required change is a very gradual change in the vacuum/pressure inside the vessels, in effect a slow leak that does not affect the temperature of the air inside the vessels. This is achieved by closing needle valve V5, opening isolating valve V6 then opening needle valve V5 very slightly until the pressure/vacuum readings start to change. If the movement of air is audible then the needle valve V5 has been opened too far and must be closed slightly. Similarly if readings of T1 or T2 are observed to change then needle valve V5 has been opened too far and must be closed. As the pressure difference between the two vessels reduces valve V5 can be opened further to reduce the time of the exercise provided that it is not opened sufficiently to affect T1 and T2 The effect of the gradual change is described in the relevant teaching exercise.
Converting resistance values to temperature Readings of T(R)1 and T(R)2 from the console are resistance values for the thermistor inside each vessel. These resistance readings can be converted to corresponding temperature values T1 and T2 using the chart provided in Data Sheet 3. The thermistors are supplied as a matched pair which means that readings from the sensors will be closely matched. However, the thermistor has a wide tolerance band which means that some temperature offset will be inevitable in the readings. The measured values of resistance and the corresponding calculated values of temperature are not used in any calculations; they are simply used to demonstrate the changes in temperature in the system so an offset is not important. However, if the user wishes to correct the offset for the specific thermistors supplied with the TH5 then the resistance / temperature equation can be corrected to give actual temperature readings, corrected for manufacturing tolerance as follows: Determine the resistance measurement with the thermistor at 25°C (E.g. 1800 Ohms). For this thermistor with nominal characteristics (R = 2000 Ohms at 25°C) the equation that best describes the temperature/resistance relationship is: Temperature (°C) = -0.021 R3 + 2.976 R2 – 177.1 R + 4894 If the actual resistance reading is 1800 Ohms at 25°C (200 Ohms lower that the nominal) then the equation becomes: Actual Temperature (°C) = -0.021 R3 + 2.976 R2 – 177.1 R + 4894 - 200) Actual Temperature (°C) = --0.021 R3 + 2.976 R2 – 177.1 R + 4694
35
Equipment Specifications I/O Port Pin Connections To allow access to the measurement signals in applications other than when using an Armfield IFD3, the connections to the 50 way connector (15) are listed below for information: Pin No
Channel No
Signal Function
Analog Outputs (0-5 V dc exported from socket) 1
Ch 0 Signal
2
Ch 0 Return
3
Ch 1 Signal
4
Ch 1 Return
5
Ch 2 Signal
6
Ch 2 Return
7
Ch 3 Signal
8
Ch 3 Return
9
Ch 4 Signal
10
Ch 4 Return
11
Ch 5 Signal
12
Ch 5 Return
13
Ch 6 Signal
14
Ch 6 Return
15
Ch 7 signal
16
Ch 7 return
17-21
Not used
Not used on TH5
Not used on TH5
P Pressure (0V = 0 kN/m2, 5V = 34.48 kN/m2)
T(R)1 Thermistor (0V = 0, 5V = 5102 )
T(R)2 Thermistor (0V = 0, 5V = 5102 )
V Vacuum (0V = 0 kN/m2, 5V = 34.48 kN/m2)
Not used on TH5
Not used on TH5
Analog Inputs (0-5V dc input from socket)
36
Equipment Specifications
22-25
Not Used
Digital Outputs (0-5V dc) 26-37
Not Used
Digital Inputs (0-5V dc) 38-50
Not used
Environmental Conditions This equipment has been designed for operation in the following environmental conditions. Operation outside of these conditions may result reduced performance, damage to the equipment or hazard to the operator. a. Indoor use; b. Altitude up to 2000 m; c. Temperature 5 °C to 40 °C; d. Maximum relative humidity 80 % for temperatures up to 31 °C, decreasing linearly to 50 % relative humidity at 40 °C; e. Mains supply voltage fluctuations up to ±10 % of the nominal voltage; f.
Transient over-voltages typically present on the MAINS supply; NOTE: The normal level of transient over-voltages is impulse withstand (overvoltage) category II of IEC 60364-4-443;
g. Pollution degree 2. Normally only nonconductive pollution occurs. Temporary conductivity caused by condensation is to be expected. Typical of an office or laboratory environment
37
Routine Maintenance Responsibility To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment.
General In addition to regular maintenance the following notes should be observed: 1. The TH5 should be disconnected from the electrical supply when not in use. 2. When not in use it is suggested that the assembly incorporating the pressure and vacuum vessels be moved onto a bench top or other location to prevent damage to the equipment through accidental knocks etc. 3. The large ball valves V1, V2 and V3 on top of the vessels should be left open when the equipment is not in use to prevent pressurisation of the vessels with changes in temperature/changes in atmospheric pressure. 4. Do not use solvents when cleaning the clear acrylic vessels. The vessels can be disassembled if cleaning of the inside becomes necessary. The clear acrylic tube is sealed into groves in the top and bottom plates using ‘O’ rings that are compressed by the tie rods. The tubes can therefore be removed for cleaning by unscrewing the tie rods. Ensure that the lock nuts are tightened on the tie rods when reassembling the vessels. NOTE: The temperature sensor located inside each of the vessels is extremely delicate and must be handled with care if removed from the vessel when cleaning the vessel. For accurate results the thermistor beads located at the tip of each temperature sensor are a matched pair. If one bead is damaged it is therefore necessary to replace both beads. 5. When not in use for an extended period, cover the equipment with a dust cloth. 6. For safe operation of the equipment the pressure relief valves and air pump must not be modified or replaced by alternatives. 7. Zero adjustment of pressure sensors: The conditioning circuits associated with the two pressure sensors installed in the Pressure and Vacuum vessels incorporate zero adjustment potentiometers to eliminate any zero offset. If P or V need adjustment to read zero, open lid of console then adjust VR4 with the rotary switch set to V or VR13 with the rotary switch set to P to display true zero readings on the display..
38
Laboratory Teaching Exercises Index to Exercises Exercise A - Determination of Heat Capacity Ratio Exercise B - Determination of Ratio of Volumes using an Isothermal Process Project work When using the appropriate Armfield Teaching Software (optional accessory) the Teaching Exercises may differ slightly and it is suggested that reference is made to the help text incorporated in the software rather than this manual.
Adiabatic, Isothermal, Reversible and Irreversible Processes Exercises A and B describe, respectively, an adiabatic isentropic reversible process and an isothermal irreversible process. The following may be helpful when devising additional demonstrations of thermodynamic principles in the context of the TH5 apparatus, such as those described in the Project Work exercise.
System The system is defined by the experiment and is chosen to suit the demonstration. For most experiments, the system will be defined as being either one vessel or both vessels plus the pipework to the valves, and therefore the system is of a fixed volume. For most purposes the atmosphere surrounding the system (i.e. the laboratory) can be assumed to be of infinite volume and constant temperature and pressure.
Adiabatic Processes In an adiabatic process, no thermal energy is gained or lost by the system. There is no chemical process that causes the system to generate heat internally, and there is no heat transfer from or to the surrounding atmosphere. The total energy of the system can still change, providing this is not due to a transfer of thermal energy. For example, there may be a gain or loss in mechanical energy due to changes in pressure or volume. Changes in pressure produce adiabatic heating or cooling, i.e. a change in the temperature of the system that is not due to heat transfer with the surroundings or an internal chemical process. Any process may be assumed to be adiabatic for the purposes of demonstration if the process is rapid enough that there is no time for significant heat transfer to occur.
Isothermal Processes In an isothermal process, as implied by the name (iso: ‘same’ and thermal: ‘relating to heat’), the temperature remains constant. An isothermal process may also be adiabatic (i.e. the process occurs at a constant temperature with no heat transfer between the system and its surroundings), but more usually an isothermal process is one that occurs very slowly, so that there is sufficient time for heat transfer between the system and its surroundings and the system remains in thermal equilibrium with its surroundings. Any process may be assumed to be isothermal for the purposes of demonstration if it occurs over a sufficiently long time period to allow all parts of the system to remain at ambient temperature.
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Armfield Instruction Manual
Isentropic Processes In an isentropic process, the entropy of the system remains constant (as indicated by the name, which comes from ‘iso’ and ‘entropy’). From the second law of thermodynamics: where Q is the thermal energy gained by the system due to heating T is the temperature of the system, and dS is the change in entropy. For practical purposes, the process may be considered isentropic if the following conditions are satisfied:
There is no thermal transfer to or from the system (Q = 0)
There is no significant change in mass (and thus no corresponding entropy flux to or from the system)
Temperature changes are small with respect to the initial system temperature
The demonstration is so arranged as to minimise any other changes in entropy
Reversible Processes A process is considered thermodynamically reversible if the net change in the combined entropy of the system and its surroundings is zero. It is not possible to physically demonstrate a process occurring in one direction, then to reverse that process; technically, it is impossible to demonstrate a true reversible process, as this requires 100% efficiency which cannot be achieved. However, for practical purposes a reversible process can be demonstrated by keeping the change in entropy of the system very close to zero. This can be approximated if there is no change in thermal energy of the system and minimal entropy flux into or out of the system. The standard method for demonstrating a reversible process involves a very small, very rapid expulsion of air from a pressurised volume of gas. Because the mass change is small there is minimal entropy flux, and because the process is rapid there is minimal heat transfer.
Nomenclature The following nomenclature has been used for the theory and calculations presented in this manual: Name Measured pressure in large vessel
40
Symbol
P
Unit
N/m
2
Type
Definition
Recorded
Instantaneous pressure (gauge) inside large vessel. Sign convention: +ve when above atmospheric pressure
Laboratory Teaching Exercises
Measured vacuum in small V vessel
N/m²
Recorded
Instantaneous vacuum (gauge) inside vessel. Sign convention: +ve when below atmospheric pressure
Measured resistance in large vessel
T(R)1
Ohms
Recorded
Instantaneous resistance of thermistor sensor inside large vessel
Measured resistance in small vessel
T(R)2
Ohms
Recorded
Instantaneous resistance of thermistor sensor inside large vessel
Volume of large Vol1 vessel
m3
Given
Nominal volume = 0.0224 m3
Volume of small Vol2 vessel
m3
Given
Nominal volume = 0.0091 m3
o
Calculated
Derived from resistance T(R)1 using data sheet 1
o
Calculated
Derived from resistance T(R)2 using data sheet 1
N/m²
Recorded
Ambient (atmospheric) pressure of the surroundings
Temperature in T1 large vessel Temperature in T2 small vessel Barometric pressure Absolute pressure in large vessel
Patm
P1abs
C
C
N/m²
Calculated
Applied pressure relative to the pressure of total vacuum. = P + Patm Applied pressure relative to the pressure of total vacuum.
Absolute pressure in small vessel
P2abs
Subscript
s
Denotes start condition
Subscript
i
Denotes intermediate condition
Subscript
f
Denotes final condition
N/m²
Calculated
= Patm - V
Data Sheet 1: Relative and absolute pressure The measurement of any physical property relies upon comparison with some fixed reference point. Pressure is one such property, and pressure measurement must begin by defining a suitable fixed point. An obvious reference is that of the ambient pressure of the surroundings. Pressure scales have been based around a zero point
41
Armfield Instruction Manual of the pressure of the atmosphere at sea level. Pressures lower than atmospheric are assigned negative values; pressures higher than atmospheric have positive values. Gauges for measuring pressure give readings relative to this zero point by comparing the pressure of interest to the pressure of the surrounding air. Pressure measured with such a gauge is given relative to a fixed value, and is sometimes termed gauge pressure. Gauges measure pressure difference between the pressure to be measured and the barometric (ambient) pressure. This may then need adjusting, to take into account any difference between barometric pressure and the pressure at sea level. Many calculations using equations derived from fundamental physical laws require absolute pressure values. Absolute pressure is the pressure relative to a total absence of pressure (ie. a total vacuum). On an absolute pressure scale, all pressures have a positive value. The following chart illustrates the difference between gauge pressure, barometric pressure, and absolute pressure.
Data Sheet 2: Technical data The following information may be of use when using this apparatus: Nominal height of large and small vessels:
0.590 m
Nominal cross-sectional area of large vessel:
0.038 m2
Nominal cross-sectional area of small vessel:
0.0154 m²
Approximate volume of large vessel:
0.0224 m3
Approximate volume of small vessel:
0.0091 m3
42
Laboratory Teaching Exercises
Data Sheet 3: Relationship between Resistance and Temperature for Thermistors used on TH5 (Nominal values) Temperature (oC)
Resistance (Ohms) Temperature (oC)
Resistance (Ohms)
8
3777
31
1570
9
3650
32
1509
10
3525
33
1450
11
3403
34
1394
12
3285
35
1341
13
3169
36
1291
14
3055
37
1244
15
2945
38
1199
16
2838
39
1158
17
2733
40
1119
18
2632
41
1083
19
2533
42
1050
20
2437
43
1020
21
2344
44
993
22
2253
45
969
23
2166
50
890
24
2082
25
2000
26
1921
27
1845
28
1772
29
1702
30
1635
43
Exercise A - Determination of Heat Capacity Ratio
Pressure-Volume Diagram
Objective This experiment is a modern version of the original experiment attributed to the names Clement and Desormes (or alternatively to Shoemaker). The heat capacity ratio = Cp/Cv can be determined for air near standard temperature and pressure. The demonstration gives students experience with properties of an ideal gas, adiabatic processes and the first law. It also illustrates how P-V-T data are used to measure other thermodynamic properties.
Method The experiment involves a two-step process. In the first step a pressurised vessel is depressurised briefly by opening then closing a large bore valve very quickly. The gas inside the vessel expands from Ps to Pi - a process that can be assumed to be adiabatic and reversible (P/T(-1/) is constant). NOTE: It has been argued that this is in fact an irreversible expansion doing work against atmospheric pressure. The resulting equation assuming an irreversible process yields virtually identical results to that where reversible conditions are assumed, providing the pressure differential between the vessel and atmosphere is small with respect to atmospheric pressure. Thus the assumption of a reversible process is reasonable under the experimental conditions described. The volume of gas inside the vessel is then allowed to return to thermal equilibrium, attaining a final pressure Pf. The second step is therefore a constant volume process (P/T is constant).
Theory For a perfect gas, Cp = Cv + R Where Cp = molar heat capacity at constant pressure, and Cv = molar heat capacity at constant volume.
44
Exercise A For a real gas a relationship may be defined between the heat capacities, which is dependent on the equation of state, although it is more complex than that for a perfect gas. The heat capacity ratio may then be determined experimentally using a two step process: 1. An adiabatic reversible expansion from the initial pressure Ps to an intermediate pressure Pi
2. A return of the temperature to its original value Ts at constant volume Vol1i
For a reversible adiabatic expansion dq = 0 From the First Law of Thermodynamics, dU = dq + dw Therefore during the expansion process dU = dW
or
dU = -pdV
At constant volume the heat capacity relates the change in temperature to the change in internal energy dU = Cv dT Substituting in to equation x, Cv dT = -pdV Substituting in the ideal gas law and then integrating gives
Now, for an ideal gas
therefore
Rearranging and substituting in from equation x,
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Armfield Instruction Manual
During the return of the temperature to the starting value,
Thus
Rearranging gives the relationship in its required form:
Equipment Set Up Before starting the exercise ensure that both rigid vessels are at atmospheric pressure by opening ball valves V1 and V3 on top of the vessels (open to atmosphere). Close all other valves before commencing the exercise. A data logger (teaching software) or chart recorder will be required to observe the transient nature of the pressure and temperature inside the vessel and to obtain an accurate value for the instantaneous intermediate pressure. The logger/recorder should be configured and ready for use to record P and T(R)1 with respect to time when required during the exercise.
Procedure Measure and record Patm using a barometer (not supplied by Armfield). Close ball valves V1 and V3 and open valve V4. Start the data logger/chart recorder as appropriate. Pressurise the large vessel by switching on the air pump. When P reaches approximately 30 kN/m2 (indicated on console) switch off the air pump and close valve V4. Wait until pressure P in the large vessel has stabilised (P will fall slightly as the vessel contents cools to room temperature). Record the starting pressure Ps Open then close valve V1 very rapidly with a snap action to allow a small amount of air to escape from the vessel. Record Pi (accurate instantaneous value can obtained from the data logger or chart recorder).
46
Exercise A Allow the vessel contents to return to ambient temperature then record the final pressure Pf. The exercise can be repeated at different initial pressures in the vessel (Pf becoming Ps for the subsequent run) as the pressure falls towards atmospheric pressure following each step change. If required the exercise can also be repeated using the small vessel that has been initially evacuated and the step changes causing a rising pressure inside the vessel (operate valve V7 to pressurise the small vessel).
Results Record your results under the following headings: Atmospheric pressure (absolute)
Patm ______ N/m2
Starting pressure (measured)
P1s _______ N/m2
Starting pressure (absolute)
P1abss _____ N/m² (= Ps + Patm)
Intermediate pressure (measured)
Pi ________ N/m²
Intermediate pressure (absolute)
P1absi _____ N/m² (= Pi + Patm)
Final pressure (measured)
Pf ________ N/m²
Final pressure (absolute)
P1absf _____ N/m² (= Pf + Patm)
For each step response calculate the heat capacity ratio (Cp/Cv)for air as follows:
Observe the transient changes in the air pressure and temperature following each step change (note that increasing resistance of the thermistor means decreasing temperature).
Conclusions Why can the initial expansion process be considered adiabatic? How well does the result obtained compare to the expected result? Give possible reasons for any difference. Comment on any differences in the transient responses of the pressure and temperature sensors.
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Exercise B - Determination of Ratio of Volumes using an Isothermal Process Objective The ratio of volumes for two vessels can be determined by using an isothermal expansion process.
Method One vessel is initially pressurised and allowed to stabilise at ambient temperature. Then air is allowed to leak very slowly from the pressurised vessel into another vessel of different size via a needle valve. This process is isothermal. Observation of the pressure before and after the process enables the ratio of the volumes of the vessels to be calculated.
Theory The theory for this experiment makes the assumption that air behaves as a perfect gas. The final equilibrium pressure Pabsf can be determined from the ideal gas equation of state:
….. (Eq. 1) Where m is the sum of the initial mass present in the two vessels, m1 + m2 Vol is the total volume of the two vessels, Vol1 + Vol2, and T is the final equilibrium temperature. Substituting in for m and V gives
….. (Eq. 2) Both vessels are at room temperature before the valve is opened. As the process is isothermal, the initial temperature will be the same as the final temperature, (T = T1s = T2s = T1f = T2f). Taking the ideal gas equation of state once again gives:
….. (Eq. 3) for the volume of the first vessel, and
….. (Eq. 4) for the volume of the second vessel. Substituting in to equation 2 then gives
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Exercise B
….. (Eq. 5) Cancelling R and T, and rearranging gives
….. (Eq. 6) Dividing top and bottom by Vol2, this becomes
….. (Eq. 7) This can be rearranged to give the equation for the volume ratio of the vessels,
….. (Eq. 8)
Equipment Set Up Before starting the exercise ensure that both rigid vessels are at atmospheric pressure by opening ball valves V1 and V3 on top of the vessels (open to atmosphere). Close all other valves before commencing the exercise. A data logger (teaching software) or chart recorder will be required to observe the transient nature of the pressure/vacuum and temperatures inside the vessels. The logger/recorder should be configured and ready for use to record P, V, T(R)1 and T(R)2 with respect to time when required during the exercise.
Procedure Measure and record Patm using a barometer (not supplied by Armfield). Close ball valves V1 and V3 and valve V5. Open valve V4. Start the data logger/chart recorder as appropriate. Pressurise the large vessel by switching on the air pump. When P reaches approximately 30 kN/m2 (indicated on console) switch off the air pump and close valve V4. Wait until pressure P in the large vessel has stabilised (P will fall slightly as the vessel contents cools to room temperature). Record the starting pressure Ps Ensure that needle valve V5 is fully closed then open isolating valve V6. Open needle valve V5 very slightly to allow air to leak from the large vessel to the small vessel. Adjust V5 to so that P falls slowly with no change in T1 or T2 (if the flow of air is too fast then T1 and T2 will change and the exercise must be repeated). 49
Armfield Instruction Manual As the pressure P falls in the large vessel and the pressure rises in the small vessel (-ve readings for V) valve V5 can be opened slightly to reduce the duration of the exercise. Allow the contents of both vessels to stabilise in pressure and temperature then record the final pressure Pf (Vf = - Pf) The exercise can be repeated at different initial pressures in the large vessel.
Results Record your results under the following headings: Constant temperature for both vessels
T _________°C
Atmospheric pressure (absolute)
Patm ______ N/m2
Initial pressure for first vessel (measured)
Ps ________ N/m²
Initial pressure for first vessel (absolute)
P1abss _____ N/m² (= Patm + Ps)
Initial vacuum for second vessel (measured) Vs ________ N/m² Initial pressure for second vessel (absolute)
P2abss _____ N/m² (= Patm – Vs )
Final pressure of vessels (measured)
Pf _________ N/m² (= -Vf)
Final pressure of vessels (absolute)
P1absf ______ N/m² (= Patm + Pf)
Calculate the volume ratio for the two vessels.
Conclusions How well does the result obtained compare to the expected result? Give possible reasons for any difference. Comment on the effect if the rate of change of pressure was sufficient to affect the temperature of the air inside the vessels.
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Project work The equipment can be used for project work by using the two vessels and interconnecting valves in different combinations to evaluate different thermodynamic processes:
Effect of vessel volume The exercises described in the product manual can be repeated using the alternative vessel pressurised or evacuated as appropriate.
Determination of Specific Heat Ratio The specific heat ratio can be determined for air by using an adiabatic expansion process. A vessel is evacuated and allowed to stabilise at ambient temperature. Air from the atmosphere rushes into the vessel when a large bore valve is opened. Analysis of this process (assumed to be adiabatic as it is rapid, but irreversible due to the large influx of mass and corresponding change in entropy) can be made using the first law of thermodynamics. The increase in internal energy in the vessel is equated to the entering enthalpy flux, and the analysis of the subsequent constant volume process, restoring the temperature to the ambient value, yields a value for .
Determination of Ratio of Volumes using an Adiabatic Process The ratio of the volumes for the two vessels can be determined using an adiabatic process. One vessel is initially pressurised and the second smaller vessel is evacuated. After allowing both vessels to stabilise at ambient temperature a large bore valve connecting the two vessels is suddenly opened and air is allowed to rush from one vessel to the other. This process may be taken to be adiabatic and therefore at constant internal energy. The two vessels eventually stabilise at the same pressure and temperature. Observations of the pressure before and after this process enable the ratio of the volumes of the vessels to be calculated.
SAFETY NOTE Although the equipment can be used for project work it must not be modified in such a way that it could be dangerous to an operator. The air pump supplied by Armfield is designed to pressurise/evacuate the vessels without producing excessive pressure or vacuum. This pump must not be replaced by a different pump, alternative air supply etc. Both vessels incorporate a pressure relief valve to prevent over-pressurisation. These relief valves must not be modified or removed.
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Contact Details for Further Information Main Office:
Armfield Limited Bridge House West Street Ringwood Hampshire England BH24 1DY Tel: +44 (0)1425 478781 Fax: +44 (0)1425 470916 Email:
[email protected] [email protected] Web: http://www.armfield.co.uk
US Office:
Armfield Inc. 436 West Commodore Blvd (#2) Jackson, NJ 08527 Tel: (732) 928 3332 Fax: (732) 928 3542 Email:
[email protected]
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