DP Level Transmitter Questions

Usually, the pressure transmitter tap on the lower elevation side is not 'filled' with fluid, rather the process fills that line through gravity: liquid flows downhill. That's how the head pressure of column of liquid is transmitted to the pressure transmitter.

The upper elevation tap is called the reference leg.

The key performance criterion for a reference leg is that the reference leg provide (or represent) a constant, known, hydrostatic head; it's the reference point.

When filled with a liquid to provide a constant reference leg pressure, it's because

- the tank is pressurized and the measurement has to be a differential measurement, not a gauge pressure measurement referencing the atmosphere

- the liquid in the tank is likely to condense and collect in the impulse tube which will directly affect the accuracy of the reading. With condensate in the impulse line, the condensate will apply not just a blanket pressure to that leg, but blanket pressure plus the condensate's head pressure.

So to avoid varying amounts of condensate head pressure and its associated error in the reference leg, the reference leg is filled to start with, and is left filled and the fill fluid constitutes a known hydrostatic head reference point.