Interview Questions for Fresher for Instrumentation

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Sensors used for measuring different parameters: A sensor; is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, most sensors are calibrated against known standards. A good sensor obeys the following rules:    

Is sensitive to the measured property Is insensitive to any other property likely to be encountered in its application Does not influence the measured property Resolution: The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made.

1. Thermal, heat, temperature:    

Bolometer, Calorimeter, Exhaust gas temperature gauge, Gardon gauge, Heat flux sensor Infrared thermometer , Micro bolometer, Microwave radiometer, Net radiometer Resistance temperature detector, Resistance thermometer, Silicon bandgap temperature sensor Temperature gauge, Thermistor, Thermocouple, Thermometer

2. Pressure 

Barograph, Barometer, Boost gauge, Bourdon gauge, Hot filament ionization gauge, Ionization gauge, McLeod gauge, Oscillating U-tube, Permanent Downhole Gauge, Pirani gauge, Pressure sensor ,Pressure gauge, Tactile sensor, Time pressure gauge

3. Flow, fluid velocity 

Air flow meter, Anemometer , Flow sensor, Gas meter, Mass flow sensor, Water meter

4. Force, density, level  

Bhangmeter, Hydrometer, Force gauge, Level sensor, Load cell, Magnetic level gauge Nuclear density gauge, Piezoelectric sensor, Strain gauge, Torque sensor ,Viscometer

5. Proximity, presence  

Alarm sensor, Motion detector, Occupancy sensor ,Passive infrared sensor ,Reed switch Stud finder, Triangulation sensor , Touch switch , Wired glove

6. Optical, light, imaging   

Charge-coupled device, Colorimeter , Contact image sensor , Electro-optical sensor Flame detector, Infra-red sensor , LED as light sensor, Nichols radiometer, Fiber optic sensors Photodetector, Photodiode, Photomultiplier tubes, Phototransistor, Photoelectric sensor

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Photoionization detector , Photomultiplier, Photoresistor, Photoswitch, Phototube, Proximity sensor Scintillometer, Shack-Hartmann, Wavefront sensor

7. Acoustic, sound, vibration 

Geophone , Hydrophone, Lace Sensor a guitar pickup , Microphone , Seismometer, Accelerometer

Transmitter: In industrial process control, a "transmitter" is any device which converts measurements from a sensor into a signal, conditions it, to be received, usually sent via wires, by some display or control device located a distance away. Typically in process control applications the "transmitter" will output an analog 4-20 mA current loop or digital protocol to represent a measured variable within a range. For example, a pressure transmitter might use 4 mA as a representation for 50 psig of pressure and 20 mA as 1000 psig of pressure and any value in between proportionately ranged between 50 and 1000 psig. (A 0-4 mA signal indicates a system error.) Older technology transmitters used pneumatic pressure typically ranged between 3 to 15 psig (20 to 100 kPa) to represent a process variable.

Why 4-20ma signal is preferred over 0-10v signal The most popular standards worldwide are (not in any particular order):- 0-5V, 0-10V, 1-5V, 2-10V, 15mA, 0-20mA, 4-20mA, 10-50mA. The first four are DC voltage signals whose uses are fairly obvious. 0-5V is especially popular with microprocessor applications as 5V is the popular supply voltage for these products. It does however suffer from a drawback that it is very difficult to tell when you are at zero. Take the example where a thermometer is calibrated so that 0°C = 0V and 100°C = 5V. Typically the circuit is powered by a single 5V supply so that the output cannot go negative. If we are reading 0V are we at 0°C or even lower. Could the output voltage have failed or perhaps a faulty sensor is giving us a false reading? If instead we use a 1-5V signal we can see if the temperature starts to go below zero as the reading drops below 1V and we can use a level close to 0V to warn us of loss of power. This type of offset is referred to as an ELEVATED ZERO and is the preferred type of signal in industry. The advantage of a voltage signal is simplicity. Almost everyone understands the concept. The signal is on two wires, one positive the other negative. Power must be provided to the electronics which drives the voltage signal and this may be provided as two extra wires, or a single extra wire using one of the signal wires (usually the negative) as a common. The disadvantages of voltage signals are primarily loss of accuracy caused by the input impedance of the measuring instrument and electrical interference from nearby power cables and radio transmitters. To overcome both of the above problems a current signal is more effective. Assume a situation where 020mA is used to represent 0-100°C. In our measuring instrument we have a 250ohm resistor through which the current passes and we measure the voltage across it. At 100°C we have 20mA passing through the resistor giving us a voltage of 5V. Ah-ha you say. This is just a 0-5V signal. Not so. This 5V signal remains the same even if the resistance of the wires carrying it changes, whereas with a voltage signal a change in wire resistance causes a change in signal at the measuring instrument unless it has an infinite impedance.

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This means that a current signal may be transmitted long distances through conductors of varying resistance with no loss of accuracy. Neat! The other advantage of the current signal is improved noise immunity. Spurious signals are superimposed on the signal we wish to measure by varying magnetic fields (maybe generated by AC power cables) which cut the conductors and induce currents in the same fashion as a generator. If our conductors are small and close together (theoretically in the same place) these fields generate equal and opposite currents in the two conductors which cancel. In practice a twisted pair of conductors of telephone cable sized wire works well. If we are using a 0-20mA signal we have to supply power to our electronics in a similar way as to power a voltage signal using one or two extra power supply lines. With a 4-20mA signal we always have at least 4mA flowing in the loop. This means that if we can power our electronics from the residual 4mA we can have our power supply AND signal on the same pair of conductors. This technique simplifies installation, especially in large plants, as only a twisted pair is needed to transmit a signal from a sensing device to a measuring or control instrument. An instrument which transmits a 4-20mA signal is usually referred to as a TRANSMITTER and if it derives its power from the 4mA residual current it is called a LOOP POWERED TRANSMITTER. With 0-20 mA you can not distinguish between minimum field value and connection break. With 2-20 mA, internal circuit can distinguish between connection break of minimum value. Normally when the value is minimum the transmitter will give you 4 mA while in case of connection breakage it will give 0 mA.

Difference between 2 wire, 3 wire & 4 wire transmitter: In 2 wire transmitter the power and signal are transmitted through same cable. In 3 wire transmitter the data signal and power are with respect to common ground. In 4 wire transmitter two wires for power supply and two for signals. Only current transmitters can be used as 2 wire transmitters.

smart transmitter: A “Smart” transmitter is a transmitter that uses a microprocessor as the heart of the electronics. in addition, a “Smart” transmitter will output some type of remote digital communications allowing you to read and set-up the device from a remote position. They can convert analog signals to digital signals (A/D), making communication swift and easy and can even send both analog and digital signals at the same time as denoted by D/A. A smart transmitter has a number of other capabilities as well. For instance, inputs can be varied, as denoted by A/D. If a temperature transmitter is a smart transmitter, it will accept millivolt signals from thermocouples and resistance signals from resistance temperature devices (RTDs), and thermistors. The controller takes the output signal from the transmitter and sends it back to the final control element. The communicator is shown on the right. The communicator is a hand-held interface device that allows digital "instructions" to be delivered to the smart transmitters. Testing, configuring , and supply or acquiring data are all accomplished through the communicator. The communicator has a display that lets the technician see the input or output information. The communicator can be connected directly to the smart transmitter, or in parallel any where on the loop.

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Smart transmitters also have the following features:     

Configuration Re-ranging Characteristics Signal conditioning Self-diagnosis

4.4.1 Configuration Smart transmitters can be configured to meet the demands of the process in which they are used. For example, the same transmitter can be set up to read almost any range or type of thermocouple, RTD, or thermistor. 4.4.2 Re-ranging The range that the smart transmitter functions under can be easily changed from a remote location, for example by the technician in a control room. The technician or the operator has access to any smart device in the loop and does not even have to be at the transmitter to perform the change. The communicator could be a PC, a programmable logic controller (PLC), or a hand-held device. The type of communicator depends on the manufacturer. There is a wide range of inputs that a smart transmitter will accept. For instance, with pressure units, the operator can determine ahead of time whether to use inches of water, inches of mercury, psi, bars, millibars, pascals, or kilopascals. 4.4.3 Characteristics Another characteristic of a smart transmitter is its ability to act as a stand-alone transmitter. In such a capacity, it sends the output signal to a distributed control system (DCS) or a PLC. 4.4.4 Signal conditioning Smart transmitters can also perform signal conditioning, scanning the average signal and eliminating any "noise" spikes. Signals can also be delayed (dampened) so that the response does not fluctuate. This is especially useful with a rapidly changing process. 4.4.5 Self-diagnosis Finally, a smart transmitter can diagnose itself and report on any problems in the process. For example, it can report on a circuit board which is not working properly.

field bus: fieldbus (or field bus) is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158.

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A complex automated industrial system — such as a manufacturing assembly line — usually needs an organized hierarchy of controller systems to function. In this hierarchy there is usually a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non-time-critical communications system (e.g. Ethernet). At the bottom of the control chain is the fieldbus which links the PLCs to the components which actually do the work such as sensors, actuators, electric motors, console lights, switches, valves and contactors. Fieldbus is an industrial network system for real-time distributed control. It is a way to connect instruments in a manufacturing plant. Fieldbus works on a network structure which typically allows daisy-chain, star, ring, branch, and tree network topologies. Previously computers were connected using RS-232 (serial connections) by which only two devices could communicate. This would be the equivalent of the currently used 4-20 mA communication scheme which requires that each device has its own communication point at the controller level, while the fieldbus is the equivalent of the current LAN-type connections, which require only one communication point at the controller level and allow multiple (100's) of analog and digital points to be connected at the same time. This reduces both the length of the cable required and the number of cables required. Furthermore, since devices that communicate through fieldbus require a microprocessor, multiple points are typically provided by the same device. Some fieldbus devices now support control schemes such as PID control on the device side instead of forcing the controller to do the processing. Standards There are a wide variety of concurring fieldbus standards. Some of the most widely used ones include:  

AS-Interface CAN

Industrial Ethernet based: 

Interbus , LonWorks , Modbus , PROFIBUS, BITBUS, Compu Bus , Safety BUS p

Cost advantage The amount of cabling required is much lower in Field bus than in 4-20mA installations. This is because many devices share the same set of cables in a multi-dropped fashion rather than requiring a dedicated set of cables per device as in the case of 4-20mA devices. Moreover, several parameters can be communicated per device in a Fieldbus network whereas only one parameter can be transmitted on a 4-20mA connection. Disadvantages There are disadvantages to using fieldbus compared to the 4-20 mA analog signal standard (or to 4-20 mA with HART):    

Fieldbus systems are more complex, so users need to be more extensively trained or more highly qualified The price of fieldbus components is higher Fieldbus test devices are more complex compared to a (high-spec) multimeter that can be used to read and simulate analog 4-20 mA signals Slightly longer reaction times with fieldbus, depending on the system

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Device manufacturers have to offer different versions of their devices (e.g. sensors, actuators) due to the number of different (incompatible) fieldbus standards. This can add to the cost of the devices and to the difficulty of device selection and availability. One or more fieldbus standards may predominate in future and others may become obsolete. This increases the investment risk when implementing fieldbus.

Modbus: Modbus is a serial communications protocol published by Modicon in 1979 for use with its programmable logic controllers (PLCs). It has become a de facto standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices. The main reasons for the extensive use of Modbus over other communications protocols are: 1. It is openly published and royalty-free 2. Relatively easy industrial network to deploy 3. It moves raw bits or words without placing many restrictions on vendors Modbus allows for communication between many devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Limitations 

Since Modbus was designed in the late 1970s to communicate to programmable logic controllers, the number of data types is limited to those understood by PLCs at the time. Large binary objects are not supported.

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No standard way exists for a node to find the description of a data object, for example, to determine if a register value represents a temperature between 30 and 175 degrees.

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Since Modbus is a master/slave protocol, there is no way for a field device to "report by exception" (except over Ethernet TCP/IP, called open-mbus)- the master node must routinely poll each field device, and look for changes in the data. This consumes bandwidth and network time in applications where bandwidth may be expensive, such as over a low-bit-rate radio link.

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Modbus is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a master station (once again Ethernet TCP/IP proving the exception).

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Modbus transmissions must be contiguous which limits the types of remote communications devices to those that can buffer data to avoid gaps in the transmission.

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Modbus protocol provides no security against unauthorized commands or interception of data.

PROFIBUS (Process Field Bus) is a standard for field bus communication in automation technology and was first promoted (1989) by BMBF (German department of education and research). It should not be confused with the PROFINET standard for Industrial Ethernet.

Protocol:

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A protocol is a set of guidelines or rules. A communications protocol is a formal description of digital message formats and the rules for exchanging those messages in or between computing systems and in telecommunications. Protocols may include signaling, authentication and error detection and correction capabilities. A protocol describes the syntax, semantics, and synchronization of communication and may be implemented in hardware or software, or both. The information exchanged between devices on a network or other communications medium is governed by rules that can be set out in a technical specification called a communication protocol standard. The nature of the communication, the actual data exchanged and any state-dependent behaviors are defined by the specification. This approach is often taken for protocols in use by telecommunications. In digital systems, the rules can be expressed by algorithms and datastructures, raising the opportunity of hardware independency. Expressing the algorithms in a portable programming language, makes the protocol operating system independent. In other words, a protocol is a machine independent description of communications between systems. To implement a protocol, the protocol software modules are to be interfaced with a framework assumed to be implemented on the machine's operating system. This framework implements the networking functionality of the operating system. Obviously, the framework needs to be as simple as it can be, to allow for an easier incorporation into the operating systems. At the time the Internet was formed, layering had proven to be a succesful design approach for both compiler and operating system design and given the similarities between programming languages and communication protocols, it was intuitively felt that layering should be applied to the protocols as well. This gave rise to the concept of layered protocols which nowadays forms the basis of protocol design. Systems do not use a single protocol to handle a transmission. Instead they use a set of cooperating protocols, sometimes called a protocol family or protocol suite. To cooperate the protocols have to communicate with each other, so there is an unnamed 'protocol' to do this. A technique used by this 'protocol' is called encapsulation, which makes it possible to pass messages from layer to layer. The protocols can be arranged on functionality in groups, for instance there is a group of transport protocols. The functionalities are mapped on the layers, each layer solving a distinct class of problems relating to, for instance: application-, transport-, internet- and network interface-functions. To transmit a message, a protocol has to be selected from each layer, so some sort of multiplexing/demultiplexing takes place. The selection of the next protocol, also part of the aforementioned 'protocol' is accomplished by extending the message with a protocolselector for each layer.

RS 232, RS485: In telecommunications, RS-232 (Recommended Standard 232) is a standard for serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuitterminating Equipment). It is commonly used in computer serial ports. protocol is one or a few sets of hardware and software rules agreed to by all communication parties for exchanging data correctly and efficiently.

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Synchronous and Asynchronous Communications Synchronous Communication requires the sender and receiver to share the same clock. The sender provides a timing signal to the receiver so that the receiver knows when to "read" the data. Synchronous Communication generally has higher data rates and greater error-checking capability. A printer is a form of Synchronous Communication. Asynchronous Communication has no timing signal or clock. Instead, it inserts Start/Stop bits into each byte of data to "synchronize" the communication. As it uses less wires for communication (no clock signals), Asynchronous Communication is simpler and more cost-effective. RS-232/RS-485/RS-422/TTL are the forms of Asynchronous Communications. Drilling Down: Bits and Bytes Internal computer communications consists of digital electronics, represented by only two conditions: ON or OFF. We represent these with two numbers: 0 and 1, which in the binary system is termed a Bit. A Byte consists of 8 bits, which represents decimal number 0 to 255, or Hexadecimal number 0 to FF. As described above, a byte is the basic unit of Asynchronous communications. Baud rate, Data bits, Parity, and Stop bit

RS-232 Logic Waveform (8N1) The baud rate is the communication speed that measures the number of bit transfers per second. For example, 19200 baud is 19200 bits per second. Data bits are a measurement of the actual data bits in a communication packet. For example, the above graphic shows eight (8) data bits in a communication packet. A communication packet refers to a single byte transfer, including Start/Stop bits, Data bits and Parity. If you are transferring a standard ASCII code (0 to 127), 7 data bits are enough. If it is an extended ASCII code (128 to 255), then 8 data bits are required. Parity is a simple way to error-check. There are Even, Odd, Mark and Space indicators. You can also use no parity. For Even and Odd parity, the serial port sets the parity bit (the last bit after the data bit) to a value to ensure that the data packet has an Even or Odd number of logic-high bits. For example, if the data is 10010010, for Even parity, the serial port sets the parity bit as 1 to keep the number of logic-high bits Even. For Odd parity, the parity bit is 0 so that the number of logic-high bits is Odd. Mark parity simply sets the parity bit to logic-high and Space sets the parity bit to logic-low, so that the receiving party can determine if the data is corrupted. Stop bits are used to signal the end of a communication packet. This also helps to synchronize different clocks on the serial devices. Handshaking (Flow Control): Handshaking is also called "Flow Control". The main purpose of Handshaking is to prevent receiver overloading. By using Handshaking signals, receivers will be able to tell the sending device to pause data

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transmission if the receiver is overloaded. There are three types of handshaking: Software handshaking, Hardware handshaking and Both. Software handshaking uses two control characters: XON and XOFF. The receiver sends these control characters to pause transmitter during communication. XON is decimal 17 and XOFF is decimal 19 in the ASCII chart. The drawback of Software handshaking is that these two control characters can not be used in data. This is quite important when you are transmitting Binary data as you might need to use these two codes in your data. Hardware handshaking makes use of actual hardware lines, such as RTS/CTS, DTR/DSR, and DCD/RI (for modem). In DTE/DCE communication, RTS (Request to Send) is an output on the DTE and input on the DCE. CTS (Clear to Send) is the answering signal coming from the DCE. Before sending a data, the DTE asks permission by setting its RTS output to high. No data will be sent until the DCE grants permission by using the CTS line. The DTE uses the DTR (Data Terminal Ready) signal to indicate it is ready to accept information, whereas the DCE uses the DSR signal for the same purpose. DTR/DSR are normally ON or OFF for the whole connection session (e.g. Off-hook), while RTS/CTS are ON or OFF for each data transmission. DCD (Data Carrier Ready) is used by the modem when a connection has been established with remote equipment, while RI (Ring Indicator) is used by the modem to indicate a ring signal from telephone line Data formats (Binary, Hex, Dec, Oct, and ASCII) Serial devices use Binary for communication, which consists of just two unique numbers: 0 and 1. Binary is the Base-2 numbering system. One byte of data consists of 8 binary digits, from 0000 0000 to 1111 1111. Hexadecimal is the base-16 system, which consists of 16 numbers: 0 to 9 and the letters A to F (decimal number 15). The Hexadecimal numbering system is useful because it can represent every byte as two consecutive hexadecimal digits, and it is easier for humans to read Hexadecimal numbers than Binary numbers. Most of the manufacturers use Hexadecimal in their protocol documentation. It is simple to convert a value from Hexadecimal to Binary. Just translate each Hexadecimal digit into its 4-bit binary equivalent. E.g. Hexadecimal number F3 equals Binary number 1111 0011. Octal refers to the base-8 numbering system, which uses just eight unique symbols (0 to 7). Programmers often use Octal format because it is relatively easy for people to read and can easily be translated into binary format: each Octal digit represents 3 binary digits. E.g. Octal number 73 equals to Binary number 111 011. Decimal refers to numbers in base 10, which is the numbering system we use most in everyday life. It's not as easy as Hexadecimal and Octal to converter Decimal to Binary number, but it is easier for us to understand Decimal. ASCII (American Standard Code for Information Interchange) is a character encoding based on the English alphabet. ASCII codes (both readable and unreadable) are widely used in communications, such as Modem communications. Letters A to Z and numbers 0 to 9 are readable ASCII codes. Some ASCII codes are unreadable, such as the control codes: XON and XOFF, which are used in Software flow control.

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Checksum: Many serial protocols use checksum (additional bytes added at the end of the data string) to check the data integrity, as errors might occur during data transmission. There are many types of checksum, from the simplest uses of it in Modula or BCC to sophisticated CRC calculation. Using Modula as an example, we learn that before data transmission, the sender adds all command bytes together then mod it by 255 (decimal) to get an additional byte. This is to be added at the end of the command string. When the receiver receives the command string, it will first check the added byte to see whether data remain unchanged or not. If that is the case, it will accept the data, and if not, it will ask the sender to resend the data.

Rs 485: EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. The standard is published by the ANSI Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the EIA-485 standard can be used effectively over long distances and in electrically noisy environments.

Actuator: An actuator is a mechanical device for moving or controlling a mechanism or system. It takes energy, usually transported by air, electric current, or liquid, and converts that into some kind of motion. Examples and applications Mechanical actuators operate by coversion of rotary motion into linear motion, or vice versa. Conversion is commonly made via a few simple types of mechanism including: Screw: Screw jack, ball screw and roller screw actuators all operate on the principle of the simple machine known as the screw. By rotating the actuator's nut, the screw shaft moves in a line. By moving the screw shaft, the nut rotates. Etc. Wheel and axle: Hoist, winch, rack and pinion, chain drive, belt drive, rigid chain and rigid belt actuators operate on the principle of the wheel and axle. By rotating a wheel/axle (e.g. drum, gear, pulley or shaft) a linear member (e.g. cable, rack, chain or belt) moves. By moving the linear member, the wheel/axle rotates. In engineering, actuators are frequently used as mechanisms to introduce motion, or to clamp an object so as to prevent motion. In electronic engineering, actuators are a subdivision of transducers. They are devices which transform an input signal (mainly an electrical signal) into motion. Specific examples include: electrical motors, pneumatic actuators, hydraulic actuators, linear actuators, comb drive, piezoelectric actuators and amplified piezoelectric actuators, thermal bimorphs, micromirror devices and electroactive polymers. Motors are mostly used when circular motions are needed, but can also be used for linear applications by transforming circular to linear motion with a bolt and screw transducer. On the other hand, some actuators are intrinsically linear, such as piezoelectric actuators.

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Thermodynamic efficiency The efficiency of an actuator is a standard tool used to calculate or estimate the usefulness of any actuating mechanism. It is a dimensionless quantity which is generally lower than 1 expressing the energy conversion factor. For better explanation see Thermodynamic efficiency. Most of the wasted energy (due to friction, magnetic losses, eddy currents etc.) is thermally dissipated.

Actuators for valves can be classified up several characteristics: By the type of movement    

Multi-turn. Quarter-turn. Linear. Lever.

By the energy source   



Manual Electric: they can be drived by direct and alternate current. Pneumatic: they use pressured air or gas to create motion. They are widely used in the industry due to their low cost. In case of failure they are easy to diagnose or repair in field, rather than electric actuators. Oleo-Hydraulic

Functionality    

On / Off valve service Positioning to % open Modulating to control changes on flow conditions Emergency Shut Down (ESD)

Other characteristics   

Explosion-proof Safety integrity level (SIL) Enclosure: IP and NEMA.

NEMA stands for the National Electrical Manufacturers Association. IP stands for International Protection

How RTD Works: A resistance temperature detector (RTD) is a temperature measuring device made of materials such as platinum whose resistance change is known at different temperatures. The R v/s T curve is used to determine the temperature. The platinum RTD's are the most stable, linear and accurate among all the

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RTD's The Common material used to make RTD's are   

Platinum Nickel Copper

However Tungsten and Balco are also used to make RTD's but they are very rare. RTD's are commonly categorized by their nominal resistance at 0 degree celcius. Typical nominal resistance values for platinum RTD's include 100ohm and 1000ohm. The most common RTD's are PT100 and PT1000. The Advantages of using RTD's for temperature measurement are   



Accurate Provide good stability and repeatability RTD's are also relatively immune to electrical noise and therefore well suited for temperature measurement in industrial environments, especially around motors, generators and other high voltage equipments. The resistance changes linearly with temperature so the measurement is quite simple.

The Disadvantages of RTD's are   

Lower temperature measuring range High cost The RTD's are not well suited to use in harsh environments compared to thermocouple.

Resistance Temperature Detectors (RTDs) are temperature sensors that contain a resistor that changes resistance value as its temperature changes.They have been used for many years to measure temperature in laboratory and industrial processes,and have developed a reputation for accuracy, repeatability, and stability. Why use an RTD instead of a thermocouple or thermistor sensor? Each type of temperature sensor has a particular set of conditions for which it is best suited. RTDs offer several advantages: • A wide temperature range (-50 to 500°C for thin-film and -200 to 850°C for wire-wound) • Good accuracy (better than thermocouples) • Good interchangeability • Long-term stability With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals such asplatinum, they are very stable and are not affected by corrosion or oxidation.Other materials such as nickel, copper, and nickel-ironalloy have also been used for RTDs. However, thesematerials are not commonly used since they havelower temperature capabilities and are not as stable orrepeatable as platinum.

Wiring Configuration: The wiring configuration is the last of those parameters typically specified by the instrument manufacturer, although the system designer does have some control based on the application. An RTD is inherently a 2wire device, but lead wire resistance can drastically reduce the accuracy of the measurement by adding additional, uncompensated resistance into your system. Most applications therefore add a third wire to help the circuit compensate for lead wire resistance, and thus provide a truer indication of the measured temperature.

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Four-wire RTD's provide slightly better compensation, but are generally found only in laboratory equipment and other areas where high accuracy is required. When used in conjunction with a 3-wire instrument, a 4-wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3-wire RTD; if the fourth wire is connected, new errors will be introduced. Connecting a 3-wire RTD to a 4-wire instrument can cause serious errors or simply not work at all, depending on the instrument circuitry. A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the un compensated resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according to the instrument manufacturer's recommendations.

The advantages of 2, 3 and 4 wire Pt100 measurement. INTRODUCTION Two, Three and Four wire measurement techniques have been developed for measuring accurately the resistance of resistive temperature detectors (RTD). This application note looks at the new techniques being adopted in „Smart‟ instruments where, using the intelligence of a microprocessor, the traditional drawbacks of 3 wire systems no longer apply. THREE WIRE MEASUREMENT The traditional method of accurately measuring a resistance, is to incorporate the resistance into a Wheatstone bridge circuit (see figure below). A voltage excites the bridge and the voltage across the bridge is proportional to the resistance of the RTD. A problem occurs when we introduce lead resistances (See figure below). It is apparent that any resistance in the lead looks as though there is additional resistance in the element to be measured.

To minimise these errors the three wire compensated bridge was introduced (see figure below).

This has the effect of removing the error introduced by the lead resistance as long as lead resistances RL1 and RL3 are matched. However the effect of the lead resistance can be to cause less current to flow in the detector leg and hence introduce a small but possibly significant span error. This can be eliminated by exciting the bridge from a constant current source rather than a constant voltage and so whatever the lead resistance, the same current always flows through the detector. With this method there are no lead resistance errors introduced as long as the lead resistances are equally matched. In practice they are very closely matched as long as the wire used is part of the same multi-core cable.

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The exception to this is when the sensor is used in a Hazardous area and connected to the bridge circuit via a Zener Barrier. Here any mis-match in the resistance of the two legs of the Zener barrier can appear as a sensor error. Although still small, this error can be as much as 0.15 ohm or approx. 0.3°C.(MTL 155 Barrier). For analogue transmitters Status Instruments Ltd. have traditionally used a variation of this technique using an in house active bridge circuit. The exception being the new „Smart‟ series of instruments which use a different technique which will be explained later.

Another way of measuring Pt100 elements is to use a 4 wire current and voltage method (see figure below).

Here the detector is excited by a constant current and the voltage across the detector measured by an amplifier with a high impedance input. If the current source is perfect and the input impedance of the voltage measuring circuit is infinite, then there is no error whatsoever introduced by the lead resistances even if they are mismatched.

THE ‘SMART’ WAY FORWARD This paragraph describes the method used to measure Pt100 on the new DM3000 series instruments. The current trend for so called SMART instruments is to have a universal input capable of supporting a wide range of inputs. It is inconvenient (and unnecessary) to dedicate input pins and electronics to support a constant current supply and a bridge arrangement. The input circuit measures voltages to a high degree of accuracy and the microprocessor performs the calculation in the figure below.

Rc is used solely to limit the current flowing and Rs is a stable reference resistor. Having computed the resistance, the microprocessor applies the corrections required and translates the resistance to an accurate temperature reading. In addition, the microprocessor can determine which if any, of the RTD inputs has become disconnected and detect other errors such as RTD short circuit. This is an improvement over both conventional three and four wire circuits because you can now have a predictable failure mode which does not depend upon which of the three wires has become disconnected. This technique removes lead resistance effects as long as they are equal. Again, we have the problem when using Zener barriers, in that if the legs of the barrier are not accurately matched, then a small error could be introduced.

Temperature range supported by RTD:

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PT100 Platinum Resistance Thermometers Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature range (from -200 to +850 °C). Standard Sensors are are available from many manufacturers with various accuracy specifications and numerous packaging options to suit most applications. Unlike thermocouples, it is not necessary to use special cables to connect to the sensor. The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C. There are also PT1000 sensors that have a resistance of 1000 ohms at 0 °C. The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4 °C. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90). This linearisation is done automatically, in software, when using Pico signal conditioners. The linearisation equation is: Rt = R0 * (1 + A* t + B*t2 + C*(t-100)* t3) Where: Rt is the resistance at temperature t, R0 is the resistance at 0 °C, and A= 3.9083 E-3 B = -5.775 E-7 C = -4.183 E -12 (below 0 °C), or C = 0 (above 0 °C) For a PT100 sensor, a 1 °C temperature change will cause a 0.384 ohm change in resistance, so even a small error in measurement of the resistance (for example, the resistance of the wires leading to the sensor) can cause a large error in the measurement of the temperature. For precision work, sensors have four wirestwo to carry the sense current, and two to measure the voltage across the sensor element. It is also possible to obtain three-wire sensors, although these operate on the (not necessarily valid) assumption that the resistance of each of the three wires is the same. The current through the sensor will cause some heating: for example, a sense current of 1 mA through a 100 ohm resistor will generate 100 µW of heat. If the sensor element is unable to dissipate this heat, it will report an artificially high temperature. This effect can be reduced by either using a large sensor element, or by making sure that it is in good thermal contact with its environment. Using a 1 mA sense current will give a signal of only 100 mV. Because the change in resistance for a degree celsius is very small, even a small error in the measurement of the voltage across the sensor will produce a large error in the temperature measurement. For example, a 100 µV voltage measurement error will give a 0.4 °C error in the temperature reading. Similarly, a 1 µA error in the sense current will give 0.4 °C temperature error.

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Because of the low signal levels, it is important to keep any cables away from electric cables, motors, switchgear and other devices that may emit electrical noise. Using screened cable, with the screen grounded at one end, may help to reduce interference. When using long cables, it is necessary to check that the measuring equipment is capable of handling the resistance of the cables. Most equipment can cope with up to 100 ohms per core. The type of probe and cable should be chosen carefully to suit the application. The main issues are the temperature range and exposure to fluids (corrosive or conductive) or metals. Clearly, normal solder junctions on cables should not be used at temperatures above about 170 °C. Sensor manufacturers offer a wide range of sensors that comply with BS1904 class B (DIN 43760): these sensors offer an accuracy of ±0.3 °C at 0 °C. For increased accuracy, BS1904 class A (±0.15 °C) or tenthDIN sensors (±0.03 °C). Companies like Isotech can provide standards with 0.001 °C accuracy. Please note that these accuracy specifications relate to the SENSOR ONLY: it is necessary to add on any error in the measuring system as well. Related standards are IEC751 and JISC1604-1989. IEC751 also defines the colour coding for PRT sensor cables: the one or two wires attached to one end of the sensor are red, and the one or two wires at the other end are white.

Differencies between PT1000 and PT100: A PT1000 is however a temperature-dependent resistance. It has 1000 ohms with zero °C, with 1°C 1003.85 ohms with 10°C 1038.5 ohms and so on. The temperature coefficient amounts to depending upon platinum material +3.85 Ohm/°C with the PT1000 and +0.385 Ohm/°C with the PT100. A PT1000 has therefore a larger slope and makes possible thereby a higher resolution. The sensor has small currents, a self-heating of sensors small is minimized. With a PT100 it is easy to develop thermometers for a large temperature range.

What is a Thermocouple: Thermocouple is the devise used extensively for measurement of the temperature of the body. Temperature is the fundamental property just like the mass and time and is frequently measured quantity. Thermocouple comprises of at least two metals joint together to form two junctions. One is connected to the body whose temperature is to be measured; this junction is called as hot or measuring junction. The other junction is connected to the body of known temperature; this is called as cold or reference junction. Thus the thermocouple enables measuring the unknown temperature of the body with reference to the known temperature of the other body. Principle of Working of Thermocouple The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. All these have been described in brief below. 1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals. 2) Peltier effect: As per the Peltier effect when two dissimilar metals are joined together to form two junctions, the emf is generated within the circuit due to different temperatures of the two junctions of the circuit.

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3) Thomson effect: As per Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit. In most of the cases the emf suggested by Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect play prominent role in the working principle of the thermocouple.

How Thermocouple Works? The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises to two dissimilar metals A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T1 and T2 respectively. Remember that the thermocouple cannot be formed if there are no two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions. If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperature, the emf‟s will not become zero and there will be net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable devise. For measurement of the temperature of the body, one junction of the thermocouple is connected to the body whose temperature is to be measured. This junction is called as hot junction or the measuring junction. The other junction is connected to the body whose temperature is known. This junction is called as cold or reference junction. Within the circuit of the thermocouple the devise for measuring the current or emf flowing the circuit is connected. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In the figure 2 above the two junctions of the thermocouple and the devise used for measurement of emf (potentiometer) are shown. Now, the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly. Devices Used for Measuring emf within the Thermocouple Circuit The amount of emf developed within the thermocouple circuit is very small, which is usually in millivolts, hence some highly sensitive instruments should be used for measuring the emf generated in the

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thermocouple circuit. The two devices used commonly for measuring emf within the thermocouple circuit are ordinary galvanometer and voltage balancing potentiometer. The manually or automatically balancing potentiometer is used more commonly. The figure 2 above shows potentiometer connected in the thermocouple circuit. The junction p is connected to the body whose temperature is to be measured. The junction q is the reference junction, whose temperature can be measured by the thermometer. In some cases the reference junctions can also be maintained at the ice temperature by connecting it to the ice bath (see figure 3). This devise can be calibrated in terms of the input temperature so that its scale can give the value directly in terms of temperature.

Materials used for Thermocouples: For the formation of the thermocouple at least two metals should be joined together to form two junctions. The thermocouple cannot be formed if the two junctions are not formed. Both the junctions are maintained at different temperatures. One is at the known temperature and the other at the unknown temperature. There are several combinations of the metals that can be used to form the thermocouple. However, there certain metals and their combinations that work better than the other combinations and they have become standard for certain ranges of temperature and for specific applications. Some of the elements used commonly for thermocouple are: 1) Copper as independent element 2) Iron as independent element 3) Platinum as independent element 4) Rhodium as an independent element 5) Iridium as an independent element 6) Constantan: combination of 60% copper and 40% nickel 7) Chromel: combination of 10% chromium, 90% nickel 8) Alumel: combination of 2% aluminum, 90% nickel and remainder silicon and manganese Properties of Some Combinations of Materials Used for Thermocouples Here some of the properties of commonly used combinations of the elements for thermocouples. 1) Copper - constantan: used for -300 to 650 F. Inexpensive, high emf output 2) Chromel - constantan: used for 0 to 1000 F. Highest emf output, good stability 3) Iron - constantan: used for 0 to 1500 F. Inexpensive, high emf output, iron oxidizes after 1500F 4) Chromel - alumel: used for 600 to 2000 F. Resistant to the temperature within the specified range

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5) Platinum - 10% rhodium: used for 1300 to 2850 F. Expensive and gives low output, resistant to oxidation, stable, used only for high temp. How to Choose the Thermocouple Elements: Here are some of the factors that should be considered while selecting the material combinations for thermocouples. 1) The combination of the materials should produce sufficiently high electromotive force (emf) when they are connected. This ensures that the emf can be measured easily and the value of the temperature can be obtained correctly. 2) The combination of materials should have high resolution or fast response time: This means for every degree change in the temperature of the junctions there is sufficient emf generated within the circuit so that it can be sensed and measured easily. 3) Higher stability: The properties of the combination should remains stable for the specified temperature range. 4) Resistant to oxidation: The materials used for the thermocouple should be resistant to the oxidation, this is especially very important for the materials that are used at high temperatures since they tend to oxidize at those temperatures. For all the material combinations the size of the wire is very important. If higher temperatures are to be measured, the diameter of the wire should be more, but in that case time response to the temperature change increases. Hence some compromise will have to be made between the response of the thermocouple and its life.

Selecting a Device for Cold-Junction Temperature Measurement: As explained above, to implement cold-junction compensation, the temperature of the cold junction must be determined. This calculation can be accomplished with any type of temperature-measurement device. Among the more popular devices are temperature-sensing ICs, thermistors, and RTDs. Each family of devices offers advantages and disadvantages over the others, so the requirements of a specific application will determine which type to use. For applications requiring extreme accuracy, a calibrated platinum RTD offers the best performance across the widest temperature range. This performance comes, however, with high cost. Thermistors and silicon temperature-sensing ICs are cost-effective alternatives to RTDs for applications that do not require such high accuracy. Thermistors operate across a wider temperature range than silicon ICs. Nonetheless, temperature-sensing ICs are often preferred over thermistors because of their linearity; correcting a thermistor's nonlinearity can require too much work from the system's microcontroller. The temperature-sensing ICs provide excellent linearity but operate across a narrower temperature range. In summary, a cold-junction temperature-measurement device must be selected to match the requirements of the system. As with any temperature-measurement application, accuracy, temperature range, cost, and linearity are all significant considerations in the selection process. Each requirement must be weighed carefully in order to select the optimal combination of cost and performance.

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Conclusion: When working with thermocouples, you must establish a reference point because thermocouples are differential temperature-measurement devices. A thermocouple provides a voltage that represents the temperature difference between the hot and cold junctions. If you know both the temperature of the cold junction and the temperature of the hot junction relative to the cold-junction temperature, you can determine the actual hot-junction temperature. The main selection criteria for the appropriate cold-junction compensation device are accuracy, cost, linearity, and temperature range. Some platinum RTDs offer the best accuracy, but at high cost. Thermistors are inexpensive and operate over a wide temperature range, but their lack of linearity can be problematic. Silicon temperature-sensing ICs operate over a narrow temperature range, but offer reasonable accuracy, linearity, and low cost, thus making them a suitable choice for many thermocouple cold-junction compensation applications.

How capacitive & Inductive sensors function:

Inductive Proximity Sensors

Inductive proximity sensors detect the presence of metallic objects. Their operating principle is based on a coil and high frequency oscillator that creates a field in the close surroundings of the sensing surface. The presence of metal in the operating area causes a change in the oscillation amplitude. This change is identified by a threshold circuit, which changes the output of the sensor. The operating distance of the sensor depends on the coil‟s size as well as the target‟s shape, size and material.

Capacitive Proximity Sensors

Capacitive sensors are used for non-contact detection of metallic objects & nonmetallic objects (liquid, plastic, wooden materials and so on). Capacitive proximity sensors use the variation of capacitance between the sensor and the object being detected. When the object is at a preset distance from the sensitive side of the sensor, an electronic circuit inside the sensor begins to oscillate. The rise or fall of such oscillation is identified by a threshold circuit that drives an amplifier for the operation of an external load. A screw placed on the backside of the sensor allows regulation of the operating distance. This sensitivity regulation is useful in applications, such as detection of full containers and non-detection of empty containers.

Photoelectric Proximity Sensors

Photoelectric sensors use light sensitive elements to detect objects and are made up of an emitter (light source) and a receiver. We offer three types of photoelectric sensors are available.

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Photoelectric sensor types: Direct Reflection (Diffused) - emitter and receiver are housed together and use the light reflected directly off the object for detection.

Reflection with Reflector (Retro-reflective) - emitter and receiver are housed together and require a reflector. An object is detected when it interrupts the light beam between the sensor and reflector.

Thru Beam - emitter and receiver are housed separately and detect an object when it interrupts the light beam between the emitter and receiver.

Magnetic Proximity Sensors Magnetic sensors are actuated by the presence of a permanent magnet. Their operating principle is based on the use of reed contacts, which consist of two low reluctance ferro-magnetic reeds enclosed in glass bulbs containing inert gas. The reciprocal attraction of both reeds in the presence of a magnetic field, due to

magnetic induction, establishes an electrical contact.