Pressure Transmitter Elevation (Wet Leg) Correction
When a pressure transmitter is mounted below or above its process tap and the impulse line is full of liquid, the liquid column adds or subtracts a fixed pressure. This calculator finds that offset and the corrected calibration range.
- Offset = ±ρ × g × h = 998.2 × 9.807 × 3 m = 29,367 Pa (adds)
- Calibrated range = process range + offset: 0.29367 bar to 10.294 bar
Elevation Correction Formula
Transmitter below tap: calibrated range = process range + ΔPhead
Transmitter above tap: calibrated range = process range − ΔPhead
ρ is the density of the liquid in the impulse line (process liquid, condensate or seal fluid), g = 9.80665 m/s² and h the vertical distance between the tap and the transmitter diaphragm. Horizontal pipe length does not matter. Gas filled lines need no correction.
Worked example
A transmitter 3 m below a water line tap sees an extra 0.2937 bar, so a process range of 0 to 10 bar is calibrated as 0.2937 to 10.2937 bar (or the zero is trimmed in the transmitter).
Frequently Asked Questions
What is zero suppression?
Shifting the calibrated range up because the transmitter sees a positive head when the process is at zero. It is needed when the transmitter is below the tap.
What is zero elevation?
Shifting the range down because the transmitter is above the tap and the liquid column pulls the reading lower.
Can I trim the offset instead of reranging?
Yes. Most smart transmitters allow a zero trim with the process at a known pressure, which removes the head offset.
Learn more on the blog
Pressure transmitter installation tips
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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