Calibration of Differential Pressure Transmitter

Calibration of Differential Pressure Transmitter To calibrate an instrument involves checking that the output of the given instrument corresponds to given inputs at several points throughout the calibration range of the instrument. For the analog DP transmitter, its output must be calibrated to obtain a zero percent (4mA) to 100 percent (20 mA) output proportional to the DP transmitter’s zero percent to 100 percent range of input pressures. In other words calibration of the transmitter is required to make the transmitter’s percent input equal to the transmitter’s percent output. This is accomplished by adjusting screws located and clearly marked as ZERO and SPAN on the analog transmitter’s outer casing. The ZERO and SPAN screws may also be referred to as the ZERO and RANGE adjustment screws for some manufacturers of DP transmitters. Whatever the model/manufacturer of your DP transmitter, it can be easily calibrated according to the manufacturers specific instruction on how to calibrate it. For every calibration you need to do, consult your manufacturer’s specific instruction for calibrating the specific DP transmitter. However there are general guidelines you need to follow before you calibrate any transmitter: Step 1: Ensure all the materials needed for the calibration are within reach (e.g meters, pressure source, pressure gauge, Digital multimeter, power supply module 24Vdc, etc) Step 2: Record and put down the following (can easily be sourced from transmitter nameplate): a) Transmitter make and model b) Transmitter calibration range c) Transmitter span d) Transmitter MWP(Maximum Working Pressure) Step 3: Connect all the equipment needed for the calibration exercise in the appropriate manner. To ensure you don’t make any mistake, you should draw a connection diagram for all equipment involved paying particular attention to polarity of transmitter and power source! Then connect them according to your connection diagram. A typical DP cell transmitter calibration diagram is shown below:

For your application, this could be modified slightly. For example if the pressure source is a hand pump, you can easily control the pressure applied to the DP cell. However, if you are doing field calibration that requires the

use of the actual process pressure, you will need a pressure regulator in conjunction with a pneumatic calibrator to help you control the pressure applied to the DP cell. Step 4: Most transmitter calibration done is a five point calibration. That is for 0%, 25%, 50%, 75% and 100% of input span or range (in this case pressure input into the DP transmitter). This should correspond exactly to 0%, 25%, 50%, 75% and 100% of the transmitter output span (4- 20mA). The graph below illustrates the correlation between input and output values.

Readings are taken for both increasing and decreasing input values and the corresponding transmitter output values are recorded Step 5: The low port of the DP transmitter cell is vented to the atmosphere (as shown in the connection diagram above) and the high port of the DP transmitter connected to a pressure source e.g a hand pump or any other suitable pressure source in a bench calibration or the actual process pressure through a pressure regulator and a pneumatic calibrator in a field calibration. So once your equipment is well setup, power it up and pressurize the high port of your DP transmitter. Record the current reading in (m A) which will be your first data point. Continue pressurizing the transmitter and recording your readings for the five points (0%, 25%, 50%, 75% and 100% of input pressure). All the reading obtained will be the as found readings. If you calibrate the DP transmitter before first testing and recording the as found data, the history of the device performance data will be lost. Step 6 Start to calibrate the DP transmitter. Note that during the calibration process, the transmitter’s zero percent, (LRV), is to be calibrated to the,

LRV, of the calibration range and the transmitter’s span is to be calibrated to the, URV, of the calibration range. For example, suppose a DP transmitter with output 4 – 20mA is to be used to measure pressure in the range 0 – 300 psig, then the transmitter zero percent (LRV) is 4mA and will be calibrated to 0psig and the transmitter’s 100%, URV, which is 20mA will be calibrated to 300psig. Locate the manufacturer’s instruction manual and with it locate the transmitters ZERO and SPAN adjustment screws sometimes called Zero and Range adjustment screws. Note that these screws each connect to a variable resistance (potentiometer) and can be turned indefinitely. That is the potentiometer is of a type that once fully adjusted clockwise or counter clockwise the screw may continue to turn without further varying the resistance for either direction. The potentiometer has a maximum of 20 turns between minimum and maximum resistance therefore turning the ZERO or SPAN screws clockwise or counter-clockwise for 20 turns will cause the potentiometer to be at either maximum or minimum. Please note that for the analog DP transmitter, the ZERO and SPAN adjustments are interactive. That is, adjusting one has an effect on the other. Specifically, changes made to the span adjustment almost always alter the DP transmitter’s ZERO point. This back and forth adjustment of the ZERO and SPAN is what makes the DP transmitter calibration sometimes tedious. Step 7: Turn the ZERO and SPAN screws both 20 turns clockwise. Next turn both screws 10 turns counter clockwise to approximately adjust the potentiometer to the mid resistance point (50%). Step 8: Apply the 0% (LRV) pressure to the transmitter’s High side, and the transmitter’s low side vented so that there is no differential pressure acting across the transmitter’s DP cell. Step 9: Adjust the ZERO screw on the transmitter while observing the current meter to cause the indication to be 4m A, which is the transmitters LRV output. This may not be exactly 4mA but depending on your plant’s acceptable margin of error, you should get a value very close to 4mA. Step 10: Next pressurize the high side of the DP transmitter to cause the pressure applied to the high side to increase to the 100 percent value (URV) of the calibration range. Step 11:

Adjust the SPAN screw while observing the meter’s current indication to cause the meter to indicate 20 mA, which is the 100% (URV) output value signal for the DP transmitter. Step 12: 100% input to the transmitter (pressure) exactly equals the transmitters 0 % through 100 % output (4 - 20 mA current). A correctly calibrated DP transmitter can be described as one where the % input equals the % output for all values between 0 and 100 percent. Once you are satisfied with the level of accuracy of the calibration result, you are done with the calibration of the DP transmitter otherwise you will have to continue fine tuning the calibration process until a reasonable accuracy is achieved. Effective Tips in Calibration of DP Pressure Transmitters Calibration of a DP pressure transmitter involves a process by which the output of the transmitter is adjusted to properly represent a known pressure input. Calibration is one of the most frequently performed maintenance operations on pressure transmitters. If well performed, the transmitter’s performance improves otherwise its performance could deteriorate with grave consequences. A pressure input is used to provide zero and span adjustments to the transmitter in the calibration process. The following tips are general guides that you should have at the back of your mind when calibrating a DP pressure transmitter: Tip 1: - Read and understand the calibration procedure in the manufacturers’ instruction manual. The calibration procedures in the manual should be followed carefully to ensure a proper calibration. Tip 2: - The use of proper calibration equipment is crucial. The pressure source and any readout device in use must be of greater accuracy than the instrument being calibrated. Some experts in calibration have posited that as a general rule, the pressure source and readout device should be at the minimum four times more accurate than the device being calibrated. High accuracy measurements cannot be obtained when the calibration is done with low-accuracy equipment. It should be a regular practice to check the accuracy of calibration equipment against a higher standard on a regular basis to maintain the accuracy of the calibration equipment. Tip 3: - When doing calibration, leaks are a potential source of error. Eliminate all leaks in the calibration system. Use TEFLON tape on all pressure connections.

Tip 4: - Trapped liquids in the pressure transmitter are also a potential source of error. Drain all liquids from the transmitter and impulse piping before starting calibrating. Tip 5: - Linearity adjustments are crucial in any calibration process involving transmitters. Linearity adjustments should only be made at one point. All other points should be used to check the adjustments only. Tip 6: - Most DP pressure transmitters come with an electronic damping pot for curbing erratic output. Therefore, damping should only be set after the pressure transmitter is placed in service. Tip 7: - Temperature is a critical parameter in transmitter calibration. Transmitter performance is affected by changes in ambient temperature. To minimize the effect of temperature change, calibration should be done at the expected ambient temperature. If temperature is expected to fluctuate, it will be good practice to calibrate between the extremes. Tip 8: - Transmitter performance is also affected by changes in static pressure. We can reduce these effects if we calibrate at the line pressure. If this is not practicable then the pressure transmitter should be put in service after calibration and re-zeroed after the transmitter has reached the operating pressure. **

Calibration Data Sheet – Instead of temperature units, use corresponding pressure units.

Common terms Used in DP Transmitter Calibration Lower Range Limit (LRL) This is the lowest value of the measured variable that a transmitter can be configured to measure. This is different from Lower Range Value (LRV). Lower Range Value (LRV) Lowest value of the measured variable that the analog output of a transmitter is currently configured to measure.

Transmitter Re-ranging Configuration function

that

changes

a

transmitter

4mA and 20mA settings

Upper Range Limit (URL) This is the highest value of the measured variable that a transmitter can be configured to measure. This is different from Upper Range Value (URV). Upper Range Value (URV) Highest value of the measured variable that the analog output of a transmitter is currently configured to measure Span Span is defined as the algebraic difference between the upper (URV) and lower range (LRV) values of the DP transmitter. Span = URV – LRV For example, if the DP transmitter is being used to measure a pressures in the range 0 – 300psig, then URV = 300, and LRV = 0 Therefore span = URV – LRV = 300 – 0 = 300 Calibration Range The calibration range of a DP transmitter is defined as “the region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper range values.” The limits are defined by the zero and span values of the DP transmitter. The zero value is the lower end of the range. For example a DP transmitter being used to measure pressures of 0 – 500 psig has a calibration range of 0 – 500 psig. Instrument Range

This refers to the capability of the DP transmitter. If a manufacturer has designed a DP transmitter for the range 0 – 700psig for example, then 0 – 700 psig is the instrument range of the transmitter. Under no circumstances should the DP transmitter be used in an application where the pressure is expected to be above 700psig as this will inevitably destroy the transmitter because its capability has been exceeded. The calibration range may be the same or differ from the instrument range. For example, a DP transmitter may have a nameplate instrument range of 0–700 psig and output of 4 - 20 mA. However, if an engineer has determined the instrument will be calibrated for 0to-300 psig = 4-to-20 mA. Then, the calibration range would be specified as 0-to-300 psig = 4to-20 mA. In this example, the zero input value is 0 psig and zero output value is 4 mA. The input span is 300 psig and the output span is 16 mA.

MWP MWP means the Maximum Working Pressure of the DP transmitter. MWP refers to the amount of gauge pressure common to each port (High and Low), not the differential pressure between ports, which the DP transmitter can safely handle without being damaged. Transmitter Damping Output function that increases the response time of a transmitter to smooth the output when there are rapid input variations Zero Trim A zero-based, one point adjustment used in different pressure applications to compensate for mounting position effects or zero shifts caused by static pressure.