Differential Pressure Calibration Problem 1

[typhonic]

We use an orifice plate for natural gas flow measurement. We calibrate the Differential Pressure Transmitter (DPT) by placing a reference pressure on the high side and venting the low side. So for example, we may have 100 inches of water column gauge pressure on the high side and atmospheric pressure on the low side.

Once, by chance, we found a DPT which calibrated OK, but under normal flowing pressure of 300 PSIG, the transmitter was out of calibration. This DPT was sent back to the factory.

My question is: how many of you have test or calibration procedures which would identify a defective transmitter such as this one? I have been told that this is a rare occurance and I do not know of anyone who is prepared to detect it.

...John

 

[hacksaw]

this is a well known problem. the amount of shift is part of the manufacturers specification, but you have to look for it.

yes you can check for it in the bench calibration and at one time was a routine check.

that said, with the emergence of smart transmitters, the usuall assumption is that such tests have been performed by the manufacturer.

you can do the bench test yourself, but you have to have access to the right equipment and personnel.

 

[typhonic]

Thanks. At your suggestion, I checked the specifications and found information about the drift at different pressures.

During our next calibration, we plan to connect a spare transmitter next to the working transmitter and make a spot check at the current operating conditions.

This may be a waste of time, but I'll be more comfortable in the future after I have seen a pattern of DP transmitters not drifting under operating pressure.

...John