Page "Pressure measurement" Paragraph 35

Liquid column gauges consist of a vertical column of liquid in a tube that has ends which are exposed to different pressures.

The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube.

A very simple version is a U-shaped tube half-full of liquid, one side of which is connected to the region of interest while the reference pressure ( which might be the atmospheric pressure or a vacuum ) is applied to the other.

The difference in liquid level represents the applied pressure.

The pressure exerted by a column of fluid of height h and density ρ is given by the hydrostatic pressure equation, P = hgρ.

Therefore the pressure difference between the applied pressure P < sub > a </ sub > and the reference pressure P < sub > 0 </ sub > in a U-tube manometer can be found by solving.

In other words, the pressure on either end of the liquid ( shown in blue in the figure to the right ) must be balanced ( since the liquid is static ) and so.

If the fluid being measured is significantly dense, hydrostatic corrections may have to be made for the height between the moving surface of the manometer working fluid and the location where the pressure measurement is desired except when measuring differential pressure of a fluid ( for example across an orifice plate or venturi ), in which case the density ρ should be corrected by subtracting the density of the fluid being measured.