A. Overview Differential pressure sensors are common in the process industry and cover a variety of applications. To understand what a differential pressure sensor is, it becomes important to put it in contrast to other pressure measurement types. The most common types of pressure measurement are absolute, gauge and differential.
B. Calibrating a HART Differential Pressure Transmitter To calibrate an instrument involves checking that the output of the given instrument corresponds to given at several points throughout the calibration range of the instrument. For analog DP Transmitter, its output must be calibrated to obtain a zero percent (4 mA) to 100 percent (20 mA ) output propotional to the DP transmitter’s zero percent to 100 percent range of input pressures. In other words calibration of the transmitter is requeired to make the transmitter’s percent input equal to the transmitter’s percent output. This is accomplished by adjysting 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 adjusment screws for some manufacturers of DP transmitters.
To Perform The Test For every calibration, its need to refer DP transmitter’s specification manual instruction for calibrating the specific DP transmitter and also concern to DP transmitter position installation on field as following below.
Step Items Description Ensure all the materials needed for calibration are within ready e.g Digital Multimeter, Digital Pressure Gauge, Power Supply 24VDC, hand pump pnuematic/hydraulic, hand tools, etc Record and put down the following from transmitter nameplate like as transmitter model, transmitter calibration range, transmitter span, transmitter Maximum Working Pressure. Close inlet valve from line process to DP transmitter. Connect all the equipment needed for the calibration like as picture above. To ensure you do not make any mistake. If the pressure source is a hand pump, you can easily control the pressure applied to the DP cell. If you are doing field calibration that requires the use of the actual process pressure, you will need a pressure regulator in conjuction with a pnuematic
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calibrator to help you control the pressure applied to the DP cell. For field calibration, an adjustable air supply and a pressure measuring device are required. For example, a dead weight tester or an adjustable clean air supply and pressure gauge can be used. The pressure source can be connected to the transmitter process connection with pipe fittings or it can be connected to the vent screw assembly using a calibration screw. If a calibration screw is being used, remove the vent screw and replace it with the calibration screw. Connect the pressure source to the calibration screw using 6 x 1 mm or 0.250 inch tubing. If a calibration screw is not being used, remove the entire vent screw assembly or drain plug (as applicable) from the high pressure side of the transmitter. Connect calibration tubing using a suitable thread sealant. Most transmitter calibration done is a five point calibration. That is for 0%, 25%, 50%, 75% and 100% of input span or range. Readings are taken for both increasing and decreasing input
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values and the corresponding transmitter output value are recorded. Close the bypass valve opened if ready bypass valve on field. Complete the calibration.
