Differential Pressure Transmitter

Can we use differential pressure transmitter to measure closed vessel or closed line pressure? All the dpt transmitter are absolute pressure transmitter. so if we insert one leg of that transmitter in close line that is not exposed to the atmosphere and other leg left open in the atmosphere, then can it measure exact gauge pressure. someone said that it will not measure exact pressure as dpt can only be used to measure pressure exposed to the atmosphere as per my above arrangement explained? If not why.

Thanks in advance.

All pressure measurement is inherently differential, meaning pressure is measured with respect to a reference pressure.

For gage pressure measurements, the reference is the atmosphere.

For absolute pressure measurements, the reference is a chamber evacuated to hard vacuum assumed to be absolute zero or as close to it as can be managed.

For Differential pressure measurements, the reference is a measured reference pressure.

A gage pressure transmitter has a port that vented to atmosphere for its reference.

A DP pressure transmitter can report gage pressure when the low side is left open to atmosphere.

>Can we use differential pressure transmitter to measure closed vessel or closed line pressure?

Yes. If you want a gage pressure measurement referenced to atmosphere, leave the low side port open to atmosphere (actually in a lot of places a sintered filter is put into the port to prevent bugs and critters from making a nest and gumming things up).

>All the dpt transmitter are absolute pressure transmitter.
No they are not absolute xmtrs.

>so if we insert one leg of that transmitter in close line that is not exposed to the atmosphere and other leg left open in the atmosphere.

An absolute pressure transmitter has only ONE port. The reference side is a vacuum chamber without external access or an external port. So your two port transmitters are differential pressure transmitters, NOT absolute pressure transmitters.

>and other leg left open in the atmosphere, then can it measure exact gauge pressure?
Yes. It is a DP transmitter and the low side open to atmosphere is the exactly the same measurement as a gage pressure transmitter.

If you are measuring level in a closed vessel a DP transmitter (2) two ports should be used with the High (H) port connected to the lower vessel connection and the Low (L) port connected to an upper vessel connection, above the maximum level. The calibration will consider the transmitter location in reference to the connections, specific gravity of liquid, and whether there are condensable vapors above the liquid. the zero may need to be adjusted to negate these.

To add to what David said

Absolute pressure transmitters are not so common. they are usually only purchased for use on processes that are effected by atmospheric pressure such as an evaporator or condenser.

That used to be the case, but I haven't had to do it for ages.
Just connect LP to the bottom HP to top and suppress the zero so it reads zero with the wet leg full.