Use of U-tube manometer
The U-tube manometer is used to measure pressure differences between two gases; typically one of these is air at atmospheric pressure.
pressure difference formula used
The U-tube manometer operates on the principle that P = ρgh.
The pressure difference Δp = liquid density ρ × g × height difference between the two arms h.
Use and Units
This instrument is so frequently used that the value for the pressure difference is sometimes quoted, not in Pascal, but in length units of the liquid in the manometer.
When water is used and the difference in height between liquid levels is 45 mm, then the pressure is quoted as “45 mm of water pressure”.
Another liquid frequently used in the U-tube manometer is mercury (although safety precautions must be taken to prevent the poisonous mercury vapour from reaching the lungs of a user), hence the unit is mm Hg.