Instrumentation

How to Calibrate and Adjust a Differential Pressure Switch Just like pressure switches, a differential pressure switch can be calibrated to a known set point. You can do a quick calibration of a differential pressure switch the same way you calibrate a pressure switch. However for more accurate calibrate, the procedure is slightly modified. I.Quick calibration procedure for a Differential Pressure Switch Equipment required includes: a. A variable pressure source b. A digital multimeter or continuity test lamp c. A test gauge Calibration Equipment setup

Calibration Steps Step 1: Connect the variable pressure source to a test gauge and the HI side pressure port of switch Step 2: Connect the test lamp or multimeter (set to the ohmmeter setting) across C-Common and NO-Normal Open switching element contacts as shown above Step 3: Raise pressure and note test gauge reading when circuit closes Step 4: Slowly drop pressure and note test gauge reading when circuit opens Step 5: Adjust set point screw to increase or decrease set point Step 6:

Repeat steps 3, 4 and 5 until contacts change at desired increasing or decreasing differential pressure set point II.Accurate Calibration Procedure To accurately calibrate a differential pressure switch, we need to simulate the required service conditions. Equipment required for this calibration process are: a. Differential pressure gauge (preferably a digital type) b. Variable pressure source c. Block, bleed and equalizer valves d. Continuity test lamp or digital multimeter First determine whether the set point occurs on increasing or decreasing differential pressure and calibrate using either of: a. Set point on Increasing Differential Pressure b. Set point on Decreasing Differential Pressure Calibration Steps for Set point on Increasing Differential Pressure Connect the setup as shown below and then proceed with the steps

Calibration steps Step 1: Connect the continuity test lamp or digital multimeter across the C-Common and NO-Normally Open switching element/contacts Step 2:

Close the bleed valves, open the equalizer valve and raise pressure equally on both HI and LO sides to the static pressure that the differential pressure switch will see under normal operating conditions Step 3: With static pressure stable, close the equalizer valve to isolate the HI side from the LO side Step 4: Keep HI side pressure steady, slightly open the LO side bleed valve to reduce the LO side pressure (increase diffrential pressure) until desired diffrential pressure set point appears on diffrential pressure gauge. Close bleed valve to stablize differential pressure. Check the status of the electrical contacts against the following possible scenarios and follow the instructions that match the status of the contact:

a. Set point is Okay if (see diagram above): Contacts make precisely at increasing differential pressure set point, repeat Steps 2-4 as desired to verify calibration. Calibration is complete. b. Contacts are Open – Set point too High: If contacts are open when increasing differential pressure is reached, adjust set point screw until contacts make. Repeat steps 2-4. c. Contacts Closed – Set point Too Low: If contacts are closed when increasing differential pressure is reached, adjust set point screw until contacts break. From this point, adjust set point again until contacts make. Repeat steps 2-4. Calibration Steps for Set point on Decreasing Differential Pressure Step 1: Connect the continuity test lamp or digital multimeter across the C-Common and NO-Normally Open switching element/contacts Step 2: Close the bleed valves, open the equalizer valve and raise pressure equally on both HI and LO sides to the normal operating static pressure Step 3: With normal HI side pressure stable, close the equalizer valve to isolate the HI side from the LO side Step 4: Slightly open the LO side bleed valve to reduce LO side pressure ( increase differential pressure) until the normal operating differential pressure appears on the differential pressure gauge. Close the bleed valve to stabilize differential pressure. Contacts should close ( make) by the time normal operating differential pressure is reached. If the contacts are still open at normal operating differential pressure, adjust the set point screw until the contacts make.

Step 5: Keeping the HI side pressure steady, slightly open the equalizer valve to increase LO side pressure (decrease differential pressure) until the desired differential pressure set point appears on the differential pressure gauge. Close the equalizer valve to stabilize differential pressure. Check the status of the electrical contacts against the following differential pressure scenarios and follow the instructions that match the status of the contacts:

a. Set point is Okay if (see diagram above): Contacts break precisely at decreasing differential pressure set point, repeat Steps 2-5 as desired to verify calibration. Calibration is complete. b. Contacts are Open – Set point too High: If contacts are open when deecreasing differential pressure is reached, adjust set point screw until contacts make. From this point, adjust set point screw again until contacts break. Repeat steps 2-5. c. Contacts Closed – Set point Too Low: If contacts are closed when decreasing differential pressure is reached, adjust set point screw until contacts break. Repeat steps 2-5.

Basics of The 4 - 20mA Current Loop The 4-20mA current loop is a very robust and popular sensor signalling standard. Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise. In a 4-20mA current loop, all the signalling current flows through all devices. All the devices in the loop drop voltage due to the signal current flowing through them. The signalling current is not affected by these voltage drops as long as the power supply voltage is greater than the sum of the voltage drops around the loop at the maximum signal current of 20mA

As shown in the diagram above, current supplied from the power supply flows through the loop wires with resistance, RW, to the trasmitter and the 4-20mA transmitter regulates the current flow within the loop. The current allowed by the transmitter is called the loop current and it is proportional to the parameter that is being measured. The loop current flows back to the controller through the wire, and then flow through resistor, R, to ground and returns to the power supply. The current flowing through R produces a voltage that is easily measured by the analog input of a controller. For a 250 ohm resistor, the voltage will be 1VDC at 4mA and 5VDC at 20mA. As the diagram above showns, there are four basic components in the 4-20mA current loop namely: a. b. c. d.

The power supply The 2-Wire transmitter A receiver resistor, R that converts the loop current into a voltage The loop wires that interconnects all devices or components in the loop.

1.The power supply Power supply package for 4-20mA, 2-wire transmitters must always be DC because the change in current flow is representative of the parameter that is being measured. For 4-20mA loops with 2-wire transmitters, common power supply voltages are 36VDC, 24VDC, 15VDC and 12VDC. Current loops using 3-wire transmitters can have either AC or DC power supplies. The most common AC power supply is the 24VAC control ttransformer. Be sure to check your transmitter’s intallation manual for the proper voltage requirements. 2.The 4-20mA, 2-wire transmitter The transmitter is the heart of the 4-20mA signal system. It converts a physical property such as temperature, flow or pressure into an electrical signal. This electrical signal is a current proportional to the temperature, flow or pressure being measured. In a 4-20mA loop, 4mA represents the low end of the measurement range and 20mA represents the high end. The voltage specification for most transmitters comes in a range. For example if the voltage of a 2-wire transmitter is specified as 15 to 24VDC, the lower voltage is the minimum voltage necessary to guarantee proper transmitter operation. The higher voltage is the maximum voltage the transmitter can withstand and operate to its stated specifications without damage or adverse consequences. 3.The Receiver Resistor, R In engineering, it is much easier to measure a voltage than it is to measure a current. Therefore, many current loop circuits use a Receiver Resistor (R in our case) to convert the current into a voltage. In the diagram above, R is a 250 ohm resistor. The current flowing through it produces a voltage that is easily measured by the analog input of a controller. For the250 ohm resistor, the voltage will be 1VDC at 4mA of loop current and 5VDC at 20mA of loop current. The mostcommon receiver resistor in a 4-20mA loop is 250 ohm; however, depending upon the particular application, resistance of 100 to 750 ohm may be used. 4.The Loop wires When current flows through a wire, it produces a voltage drop proportional to the length and thichness (gauge) of the wire. All the loop wires have resistance, usually expressed in Ohms per length. The voltage drop can be calculated using Ohm’s law: E=IxRw E: the voltage across the resistor in volts; I: the current flowing through the loop wires in amperes: Rw: the loop wire’s resistance in Ohms However, because the current flowing in a typical 4-20mA loop is small, voltage drops are ususally small although the loop wire runs should be considered then running instrumentation wiring to bring down voltage drops. These basic components exist in any 4-20mA loop that you will deal with. To successfully troubleshoot this loop you need to be familiar with all these components that have been discussed.

Instrumentation Basics: 4 - 20mA and 3 - 15psi Control Signals In the fied of instrumentation, analogue electronic signals and pneumatic signals are typically used for control purposes to actuate the final control element in a control loop which is usually a control valve. An “analogue” electronic signal is a voltage or current whose magnitude represents some physical measurement or control quatity. The most popular form of signal transmission used in mordern industrial instrumentation systems is the 4-20mA DC standard. This is an analog siganl standard, meaning that the electric current is used to proportionately represent measurements or control signals. Typically, a 4mA current value represents 0% of scale, a 20mA current value represents 100% of scale, and any current value in between 4 and 20mA represents a commensurate percentage in between 0% and 100%. In pneumatic systems, a standard signal range od 3 to 15 PSI (Pounds Per Square Inch) is used. Here, a varying air pressure signal represents some process measurement in an analogue (proportional) fashion. Typically, a 3 PSI pressure value represents 0% of scale, a 15 PSI pressure value represents 100% of scale, and any pressure value in between 3 and 15 PSI represents a commensurate percentage in between 0% and 100%. It is worthy of note to state here that pneumatic signals are commonly used in process industries for safety especially when there is a risk of fire or explosion. Relating 4-20mA signal to instrument variables: Calculating the equivalent milliamp value for any given percentage of signal range is quite easy. Given the linear relatonship between signal percentage and milliamps, the equation takes the form of the standard slope-intercept line equation C=mP+b Here, C is the equivalent current in milliamps, P is the desired percentage of signal, m is the span of the 4-20mA range (16mA), and b is the offset value, or 4mA: Current=(16mA)(Percentage/100%)+(4mA) This equation form is identical to the one used to calculate pneumatic instrument signal pressures (the 3 to 15 PSI standard): Pressure=(12PSI)(Percentage/100%)+(3PSI) The same mathematical relationship holds for any linear measurement range. Given a percentage of range P, the measured variable is equal to: Measured variable=(Span)(P/100%)+(LRV) Practical examples of calculations between milliamp current values and process variable values follow: An electronic temperature transmitter is ranged 40 to 140 degrees Fahrenheit and has a 4-20mA output signal. Calculate the current output by this transmitter if the measured temperature is 60 degrees Fahrenheit. Solution: First, we convert the temperature value of 60 degrees into a percentage of range based on the knowledge of the temperature range span (140 -40=100 degrees) and lower-range value (LRV=40 degrees). We may do so by manipulating the general formula: Measured variable=(Span)/(P/100%)+(LRV) Measured variable-(LRV)=(Span)(P/100%) Therefore, P=[(Measured variable-LRV)/(Span)]x100%=[(60-40)/(100)]x100%=20%

Next, we take this percentage value and translate it into a 4-20mA current value using formula: Current=(16mA)(P/100%)+(4mA)=(16mA)(20%/100%)+(4mA)=7.2mA Therefore, the transmitter should output a Process Value signal of 7.2mA at a temperature 60 F. An electronic loop controller outputs a signal of 8mA to a direct-responding control valve (where 4mA is shut and 20mA is wide open). How far open should be control valve be at this Manipulated Variable signal level? Solution: We must convert the milliamp signal value into a percentage of valve travle. This means determining the percentage value of the 8mA signal on the 4-20mA range. First we need to manipulated the percentagemilliamp formula to solve for percentage (P): Current=(16mA)(P/100%)+(4mA) P/100%=[(Current-4mA0/(16mA)] P=[(Current-4mA)/(16mA)]x100%=[(8mA-4mA)/(16mA)]x100%=25% Therefore, the control valve should be 25% open when the MV signal is at a value of 8mA Additional learning resources: Now that you understand basic control signal used in instrumentation. You could also check out this book “measurement and control basics”. It is a very good book for beginners to instrumentation and control. Established technicians and engineers could also get the book for their library. Closely related to the concept of the 4-20mA signal and 3-15PSI signal is the art of calibration. Many technicians and experts in instrumentation and control will agree with me that calibration is a critical aspect of plant management and process safety. To learn the art of calibration you might check out these books: -A Technician’s guide to calibration -Calibration Handbook. This book teaches you how to develope and management a calibration program in your plant

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Electronic Transmitters Wire Configuration Today’s electronic transmitters including prerssure transmitters are connected in different wire configurations. These connection methods are of great concern to the instrument engineer/technician. The “two wire, three wire and four wire classifications are often used to describe the method of connection of electronic transmitters: 1. Two wire transmitters: These are the simplest and most economical and should be used wherever load conditions will permit. They are often called loop powered instruments. In a two wire system, the only source of power to the

transmitter is from the signal loop. The 4mA zero-end current is sufficient do drive the internal circuitry of the transmitter and the current from 4 to 20mA represents the range of the measured process variable. The power supply and the instruments are usually mounted in the control room. The schematic diagram below shows the wire transmitter configuration:

2. Three wire transmitters: Some transmitters require more power than the signal loop (4-20mA. Etc) can supply their internal circuitry. A DC common wire is run from the instrument to the transmitter. This permits the transmitter to draw whatever power it needs from the power supply and produce the desired signal current at the transmitter output. The schematic diagram for a three wire transmitter is shown below:

3. Four wire transmitters: Four wire transmitters have their own internal power supply hence they are often referred to as selfpowered instruments. They require no connection to DC power supply. A 120 VAC sources is connected only to the receiving instrument. These are often used where an instrument is added to the load of the DC supplies. The disadvantage is the need for AC power at the instrument site. Below is shown the wire configuration of a four wire transmitter:

Note that in all the transmitter wire configurations shown above, a load resistor of 250ohm is used. Ususally process controllers used in instrumentation systems are not equipped to directly accept milliamp input signals, but rather voltage signals. For this reason a precision resistor is connected across the input terminals of the controllers to convert current signals from transmitters into standardized analog voltage signals that controllers can understand. A voltage signal range of 1 to 5 volts is standard, although some models of controllers use different voltage ranges and therefore require different precision resistor values. If the voltage range is 1-5 volts and the current range is 4-20mA, the precision resistor value must be 250 ohms. In the transmitter wire configurations discussed above, it is assumed that a voltage signal range of 1-5V and a standard current signal of 4-20mA are used.

Basics of Smart Transmitters Smart Transmitters are advancement over conventional analog transmitters. They contain microprocessors as an integral unit within the device. These devices have built-in diagnostic ability, greater accuracy (due to digital compensation of sensor nonlinearities), and the ability to communicate digitally with host devices for reporting of various process parameters. The most common class of smart transmitters incorporates the HART protocol. HART, an acronym for Highway Addressable Remote Transducer, is an industry standard that defines the communication protocol between smart field devices and a control system that employs traditional 4-20mA signal. Parts of a Smart Transmitter: To fully understand the main components of a smart transmitter, a simplified block diagram of the device is shown below:

The above block diagram is further simplified to give the one below:

As shown above in fig A, the smart transmitter consists of the following basic parts: a. Process Sensor b. An analog to digital converter (ADC) c. A microprocessor d. A digital to analog converter (DAC) These basic parts can be organized into three basic sections as shown in fig B: a. Input section b. Conversion Section c. Output section Input Section: The input section comprises the process sensor or transducer and the Analog to Digital Converter (ADC). The sensor measures the process variable of interest (be it pressure, temperature, flow etc) which is then converted into a proportional electrical signal. The measured electrical signal is then transformed to a digital count by the Analog to DigitalConverter (ADC). This digital count, representative of the process variable (PV), is then fed into the conversion section which contains the microprocessor. However, the microprocessor must rely upon some form of equation or algorithm to relate the raw count value of the electrical measurement to the actual process variable (PV) of interest such as temperature, pressure, or flow. The principal form of this algorithm is usually established by the manufacturers of the smart transmitters, but most HART transmitters include commands to perform field adjustments. This type of adjustment is often referred to as a sensor trim. The output of the input section is a digital representation of the process variable (PV). When you read the process variable using a hand held field communicator, this is the value that you see.

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Transmitters Used in Process Instrumentation

Basics of Smart Pressure Transmitter Calibration: