Monday, June 1, 2020

Calliberation of DP transmitter

The DP cell can be calibrated in place by using a pressure calibrator and Digital multimeter
which can measure less than 30 mA precisely.

Procedure:
Break the current signal loop and connect the multi meter by observing the correct polarity at ammeter range, suitable for measuring 4-20 mA precisely. Disconnect the high pressure side and low pressure side connections after isolating the DP Cell. Now the current on the meter display should read 4 mA. Connect the pressure calibrator to high pressure side and low pressure side to remain open to atmosphere. Slowly press up the calibrator to the DP Cell’s maximum differential pressure range by taking care not to overshoot the pressure. At this pressure, the meter should read the current as 20 mA.

Offset Adjustments:
 When both the sides are vented at atmospheric pressure, the meter should
read 4 mA. If there is any offset, adjust the zero adjusting potentiometer with a small screw driver. After adjusting zero, press up the calibrator to maximum range and if the meter is reading more or less than 20 mA, then turn the span adjusting screw slowly to read this pressure as 20 mA. Whenever span is adjusted the zero also will be shifted little. So zero checking and correction again has to perform. Similarly the span checking and fine adjustment has to perform again. This zero,span checking and adjustments to do again until an acceptable accuracy achieved. Once the zero and span has been set, the linearity of the transducer has to be checked in minimum 4 steps, say at 25%,50%,75% and 100%. The linearity can be checked with more steps for better accuracy. The same way, hysterysis of the transducer also has to be checked in steps.

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