Manometer Calculator (U Tube and Inclined)
Convert a U tube or inclined manometer reading into differential pressure, accounting for the density of the gauge liquid and the fluid above it.
- h = 100 mm
- ΔP = (ρm − ρf) × g × h = (998.2 − 1.2) × 9.807 × 0.1 = 977.7 Pa
Manometer Equations
Inclined: h = L × sin θ
ρm is the density of the gauge liquid, ρf the density of the fluid above it (air, gas or a lighter liquid), g = 9.80665 m/s², h the vertical height difference and L the reading along an inclined tube at angle θ to horizontal.
Worked example
A water U tube reading 100 mm with air above it gives ΔP = (998.2 − 1.2) × 9.807 × 0.1 = 977.7 Pa (9.78 mbar). An inclined tube at 10° reading 100 mm gives only 169.8 Pa, but with 5.8 times the resolution.
Frequently Asked Questions
Why subtract the density of the fluid above?
The process fluid fills the other part of both legs. For gas it is negligible; for liquid over mercury it matters.
Why use mercury?
It is 13.6 times denser than water, so a short tube can read high pressures. It is now often replaced for safety reasons.
How accurate is a manometer?
Very accurate if read carefully and the liquid density and temperature are known. It is a primary standard for low pressure.
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.
Read Full Article