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Wet Leg Level Measurement with Both Legs Connected
Reason for wet leg level measurement with both legs connected

What is the reason in Wet leg level measurement, both leg is connected?

As you can see I the picture https://ibb.co/mrACik (Lp Heater).

thank you.

It should not cause any problems.

It is possible, that with the purged reference leg, that the tube connection allows a "slight" purge to protect the lower connection from pluggage or build up...

A DP level transmitter always has a 'wet' connection at the low elevation tank connection.

A wet leg is used for the high elevation connection when the vapors at the top of the tank tend to condense to liquid in the impulse line. When the condensate/liquid collects in a dry leg, it applies a hydrostatic head pressure that is an offset pressure on that leg, creating an error in the measurement.

A wet leg attempts to ensure that there is constant hydrostatic head pressure in the wet leg so a correct measurement can be obtained.

Your explanation is what I'm usually see on the internet, but in this case both leg connected.

Wet leg measurement is used to build the hydrostatic head pressure to transmitter. DP should be made zero by sensor trim, otherwise it will create additional DP. Generally Wet leg builds the head pressure and error due to condensate can be made zero.

The reference fluid should be the same as the fluid in the tank, otherwise it will eventually get contaminated. If it's something like steam condensate, you should have a condensate pot to ensure the reference height is always constant.

I see no point in having the transmitter mounted below the vessel. It only contributes to errors.

measurements like this are critically dependent on your process.

Given that the current installation is functional, what is the process fluid, what are the temperatures, and what liquid leg fill is being used.

Transmitter location/elevation is often set by accessibility for maintenance.