Liquid Service

1. Have you purged all air/gas out of the impulse lines? Mounting the transmitter below the element keeps vapor out while in service, but does not ensure that the lines are self-purging.

2. Are impulse lines open and not plugged?

3. Element sized correctly, transmitter calibrated and connected correctly?

Bigg is on the right path.

Elevation should not affect the readings if
- tapping points are at same elevation
- installation of the transmitter is level, so that transmitter high and low ports are at the same elevation
- impulse lines are full of a medium consisting of a single phase liquid, which does not separate into multiple phases of different densities
- impulse lines are purged of air, vapor or gases
- liquid in impulse tubing is not boiling or subliming into its gas phase
- liquid doesn't freeze in the impulse tubing

I have experienced 44 other factors affect the DP readings, here's 4:
- equalizing valve in 3 valve manifold (or discrete valving) is leaking or not completely shut
- 3 valve manifold clogged
- poor installation of drilled ports in the piping
- transmitter setup is incorrect

As always, I am suspicious of those who claim the "reading is not accurate" when they fail to specify what reference they use to determine "accuracy", what the medium is, what the primary flow element is, nor provide examples of the magnitude of the observed errors over a range; all of which bring the evaluation itself into question and hinders any viable assessment of probable cause.