The flat, stable output signal during a HART output force means that the problem is not electrical, it’s a pressure measurement issue.
For a DP to read consistently low means either
- high side (bottom) is reading too low, or
- Low side (upper flange) is reading too high
What can cause erroneous pressure readings?
Air bubbles in the seal/capillary fill fluid will produce low readings. My experience is that factory filled capillaries are properly made, but I have seen shop fills that have left an air bubble in a capillary.
If the construction is not ‘all-welded’ (capillary welded at both ends to the seal and the DP process head) mechanics have been known to unscrew the capillary from the seal and then screw it back together. That always introduces an air bubble. DP’s with capillary seals should never have ‘vents’ in the process heads but I’ve seen it happen. Opening a vent will put an air bubble into the system and cause a low reading.
A leaking flange gasket will cause a low reading but a leak should be obvious with a blanket pressure of 4 bar.
Seal/capillary temperature differences between upper and lower seals will produce a measurement error. A higher temperature causes the fill fluid to exert a higher pressure. A lower temperature causes the fill fluid to exert a lower pressure.
If Heat trace were turned on only on the low side, that only could elevate the low side fill fluid temperature, drive the low side measurement up with the result that the DP (DP = high side minus low side) is low.
A kink (sharp bend) in the capillary can cause noise as the pressure jumps as fill fluid moves across the obstruction. But more often the kink passes no pressure. If the kink is in the high side, the level appears to be ‘stuck’ at one level. The level changes when there is a kink in the low side with an error based on the difference between the trapped pressure and actual pressure. All that said and your datasheet spells out armored sleeving on the capillary which helps prevent kinking.
There’s always the potential for a bad sensor in the transmitter. It isn’t common, but does happen.