Quiz Corner: Location of differential pressure flow transmitter for liquid service

Where and how should a differential pressure flow transmitter be located for liquid service?

A. Above the flow element with the impulse tubes sloping upward to the transmitter    

B. Above the flow element with the impulse tubes sloping upward and then downward to the transmitter 

C. Below the flow element with the impulse tubes sloping upward and then downward to the transmitter   

D. Below the flow element with the impulse tubes sloping downward to the transmitter

Accurately transmitting the differential pressure generated by the flow element to the transmitter involves keeping the impulse tubing full of liquid. Locating the transmitter above the flow element enables gas to be trapped and potentially compressed, whereby it can adversely affect accurate transmission of the differential pressure signal to the transmitter — eliminating Answer A and Answer B. 

Upward sloping impulse tubing also eliminates Answer C. Answer D is correct. 

Additional complicating factors

In some installations, it is not possible to install impulse tubes that constantly slope downward to the transmitter, so other means may be used to effect accurate transmission of the differential pressure signal. These include diaphragm seals or pots where gas can accumulate without affecting the measurement.