The u tube monometer consists of glass tube in u shape and partially filled with a liquid. Water and mercury are used because of their known specific weights for various temperatures. Also they do not stick to the tube. One end of the tube is connected to low pressure tap and other end is connected to high pressure tap. When there is pressure difference between two ends of the tube, the liquid level goes down from high pressure end and goes up from low pressure end. The difference in liquid indicates the difference in pressure. If P1 and P2 are pressure and h is difference in level or head,
Differential pressure (P) = P1 – P2
P = (ρ1 – ρ2)gh
ρ1 = density of liquid.
Ρ2 = density of fluid whose pressure is to be measured.
h = (h1 – h2) difference in fluid level
g = acceleration due to gravity.