How to calibrate Differential pressure level transmitter for capillary type [with offset]?

 

Procedure  Differential pressure level transmitter for capillary type [with offset]:

*Hook up HART Communicator and verify some parameters by refer to data sheet. Typical parameters are, tag number, PV, LRV and URV.
*Isolate the instrument from the process.
WARNING – If the process is hazardous, please unsure proper flushing is done to remove the entire hazard.
Open vent valve at drip ring and open plug at the top of the drip ring (if available) to release the process pressure
*Clean the liquid inside the drip ring and put back the plug for high side only (if available).
*Expose the low side to atmosphere
*Hook up a multimeter in series with the signal to the DCS to measure current signal.
*In this condition both capillary flange will be atm pressure at difference high
*Multimeter should show 4mA and PV at HART communicator should shows same with LRV
*If not, do zero adjustment at transmitter using HART Communicator
Connect pressure calibrator to high side flange (drip ring)
Apply pressure depend on calculation for span= (A x Product S.G)
Multimeter should show 20mA and PV at HART communicator should shows same with URV
*If not, do span adjustment at transmitter using HART Communicator
Verify the linearity by increasing and decreasing the pressure (0%,25%,50%,75%,100%,75%,50%,25% and 0%of range)
*After completion of the job ask panel operator to put loops back in normal mode or normalize the MOS

Example calculation:

Product S.G=0.635
Capillary S.G=1.07
A= 235mm (measurement length)
B= 400mm
Dp = pressure at high side – pressure at low side
LRV    = 0 – (B x capillary S.G)
= 0 – (400 x 1.07)
= – 428mmH2O
URV     = (A x Product S.G) – (B x capillary S.G)
= (235 x 0.635) – (400 x 1.07)
= 149.225 – 428
= – 278.775 mmH2O
The span is (A x Product S.G) = 149.225 mmH20
Note: This example is for onsite calibration without bringing down the flange.

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