How to Calibrate a Pressure Transmitter

1. How to Calibrate a Pressure Transmitter?

2. Pressure Transmitters are used to sense pressure and convert it into electrical signal. The conversion of Pressure in to electrical signal is achieved by change in resistance of the Wheatstone circuit of the strain gauge bonded on the diaphragm. • The pressure applied causes deflection of the diaphragm which induces strain in the circuit. The resistance caused is proportional to the applied pressure and further converted in to Voltage or Current configurations. • The term Transducer or Transmitters are often confused. Transducer is a term generally used where the analog output of the sensor is voltage, whereas the word Transmitter is used where the output of the sensor is current.

3. Voltage configurations are conditioned outputs and are preferred to use on equipment and test stands due to their stability and Current configurations are preferred to use where the signal has to be transmitted to long distances. • Pressure Transmitters are reliable and durable instruments and have wide usage across equipment manufacturing, utility applications and Process industries. • Pressure transmitters require periodic maintenance and calibration to ensure optimal performance.

4. There is no specific rule for calibration of Pressure Transmitters; however, it depends on the regulations the company has to follow and purpose of the calibration such as safety norm, application demands, process conditions or as a part of standard maintenance. • The general industry practice is to calibrate the pressure transmitters every one to three years depending on aforementioned conditions. • The pressure transmitters performance specifications are rated by the manufacturers under standard test conditions; however, they undergo wear during continuous usage.

5. Due to this there will be effect on the long term stability of the instrument. A slight deflection in the accuracy can also be observed. Hence during calibration we have to take in to account the maximum permissible errors. • A proper and accurate test equipment setup is essential to simulate the testing of the pressure transmitters. The test equipment should be traceable to National or International standards. • As a general recommendation, the test equipment should be 3-4 times more accurate than the pressure transmitter being calibrated.

6. Procedure For Calibration • Install the 2 wire 4-20mA analog output Pressure Transmitter on the test setup • Connect the power source lead wire to Positive terminal and Output lead wire to the negative terminal • Now pressurize the transmitter to 100% of the range and hold the pressure for few seconds and release the pressure. • The Digital Mutlimeter should show the reading of 4mA at Zero pressure and 20mA at Full scale value. Check lower pressure value and higher pressure value.

7. Perform zero and span adjustments if any deviation is observed by turning the potentiometers provided on the pressure transmitters. MASS® Pressure Transmitters are calibrated by Terminal point method wherein the accuracy is including zero and span, repeatability, hysteresis errors. Hence zero and span adjustment screws are not available on our Transmitters. • Note the readings at three points up and three points down at 0%, 50%, 100% of the pressure range. The 4-20mA output shall be 4 mA, 12mA, 20 mA at three points. Upon reaching to a test point, allow the device under test and measuring instrument to stabilize before moving to the next point. The pressure transmitter can be calibrated at more than three points depending on the application requirement.

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