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Zero Shift on DP Level Transmitters

Zero Shift on DP Level Transmitters

Zero Shift on DP Level Transmitters

Hey all,

I recently setup two level transmitters on a small vessel. They are a DP style transmitters with the head flange mounted at the bottom and a wet leg (Glycol filled) running to the top tapping point.

I zeroed the cell with both legs open to atmosphere and an empty wet leg. Then gave both a fresh batch of Glycol and set that value up as the LRV.

When I introduced process pressure to both tappings one transmitter stayed at 0% but the other shifted up to 6%.

I've read around a few topics and see that zeroing the transmitter at Process pressures as apposed to Atmospheric is not uncommon. However I'd like to understand why one transmitter was drastically affected by the process pressure and the other was not? (I confirmed that the vessel is completely empty and both transmitters are mounted exactly the same however on different sides of the vessel)

I have ran into this problem on other similar setups too, It has only now become a problem as the following has happened:

Setup DP Level transmitter to be read 0% at process pressure despite reading 6% at Atmospheric. Then the vessel in question was de-pressured which resulted in the transmitter reading negative 6% and initiating it's Trip function.

I recently did a test on this exact transmitter and confirmed that the Cell can accurately sense 0.0kpa DP at Atmospheric and at Process Pressure. However when the Wet leg in introduced the Equal process pressure shifts the Zero by 6% (160mmH20 accross a 5meter wet leg) in this case.

Any help or thoughts are appreciated,


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