Need Clarification on Tuned DP Transmitters

I fully understand the seal temperature effect where a temperature change of the fill fluid in the capillary causes the fill fluid to expand or contract, and to change its density.

I'm still not sure why they call the change in fill fluid density 'head temperature effect'. There is fill fluid on the back side of the transmitter's diaphragm, inside the pressure body, as shown in the cross section of DP body sensor (minus the process heads).


This is not the remote seal diaphragm, rather the transmitter's diaphragms. The fill fluid transmits the pressure directly to the sensing element. Its volume is small in comparison to the volume of a remote seal with capillary fill. I suspect that head temperature effect might well refer to the fill fluid behind the transmitter's diaphragm.

If you note in the diagrams in Cahill's article, a tuned system uses only one capillary.

If you read further, Cahill agrees with you, that the seal and head temperature effects work against each other:

Tuned systems use the asymmetry of the system to balance seal temperature effects with head temperature effects. It takes advantage of the fact that the two temperature effects act against each other for a given change in ambient temperature.

Most people grossly underestimate the temperature effect on remote seals.

I created a 20mm offset in a liquid level measurement in about 5 minutes on a cold spring day by using the heat of my hands on a gauge pressure transmitter with a close coupled 50mm diaphragm seal on a vented tank with a static 2m of liquid. I guessing I raised the temperature less than 5 deg C. No capillary, just fill fluid in the pressure body and between the transmitter's diaphragm and the close-coupled diaphragm.

Some vendors, like Siemens publish a figures for temperature error. Others are totally silent.