How this calculator works
A single column or open U-tube compares pressure with an air reference whose density is neglected. A differential U-tube with equal-elevation taps uses the difference between the manometer liquid density and the carried-fluid density. Use the general three-segment path for unequal elevations or multiple fluids: trace from reference B to unknown A and enter positive vertical drops and negative rises. Surface tension and gas-column pressure changes are excluded.
Formula
Open column: pA - pB = rho_m g h. Equal-elevation differential U-tube: pA - pB = (rho_m - rho_f) g h. General path: pA = pB + sum(rho_i g delta z_down,i). Gauge pressure = absolute pressure - local atmospheric pressure.
Worked example
A 0.1 m mercury column at density 13600 kg/m3 and standard gravity represents 13337.044 Pa. With water above the mercury in an equal-elevation differential tube, the same height represents 12356.379 Pa.
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Frequently asked questions
Does an inclined tube use its sloping length?
No. Use the vertical height difference between interfaces, obtained from the tube geometry if necessary.
How do I set an open reference?
Set reference pressure equal to local atmospheric pressure. The gauge result then equals the calculated column pressure difference.
Can I represent only two path segments?
Yes. Set the third vertical drop to zero; its density then has no effect on pressure.