Inverted Tube Manometer

For the measurement of small pressure differences in liquids, an inverted U-tube manometer is used.

Fig 4.8 An Inverted Tube Manometer

Here and the line PQ is taken at the level of the higher meniscus to equate the pressures at P and Q from the principle of hydrostatics. It may be written that

where represents the piezometric pressure, (z being the vertical height of the point concerned from any reference datum). In case of a horizontal pipe (z₁= z₂) the difference in piezometric pressure becomes equal to the difference in the static pressure. If is sufficiently small, a large value of x may be obtained for a small value of . Air is used as the manometric fluid. Therefore, is negligible compared with and hence,

(4.5)

Air may be pumped through a valve V at the top of the manometer until the liquid menisci are at a suitable level.