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. Consult my previous post: How to Calibrate Your DP Transmitter for a detailed guide on how to calibrate a DP pressure transmitter.
Owing to the fact that a plant could go berserk, if one or two critical pressure transmitters are wrongly calibrated, it is important the calibration process and procedure be done properly. The following tips are general guides that you should have at the back of your mind when calibrating a DP pressure transmitter:
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.
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.
When doing calibration, leaks are a potential source of error. Eliminate all leaks in the calibration system. Use TEFLON tape on all pressure connections.
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.
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.
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.
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.
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.