Manometers: Pressure Measuring Devices

Manometers are simple devices that employ liquid columns for measuring pressure difference between two points.

In Figure(L 8.1), some of the commonly used manometers are shown.

In all the cases, a tube is attached to a point where the pressure difference is to be measured and its other end left open to the atmosphere. If the pressure at the point P is higher than the local atmospheric pressure the liquid will rise in the tube. Since the column of the liquid in the tube is at rest, the liquid pressure P must be balanced by the hydrostatic pressure due to the column of liquid and the superimposed atmospheric pressure, Patm .

This simplest form of manometer is called a Piezometer . It may be inadequate if the pressure difference is either very small or large.

U - Tube Manometer

In (Fig : L -8.2), a manometer with two vertical limbs forms a U-shaped measuring tube. A liquid of different density r1 is used as a manometric fluid. We may recall the Pascal's law which states that the pressure on a horizontal plane in a continuous fluid at rest is the same. Applying this equality of pressure at points B and C on the plane gives

Inclined Manometer

A manometer with an inclined tube arrangement helps to amplify the pressure reading, especially in low pressure range. A typical arrangement of the same is shown in Fig. L-8.3.

At the same pressure difference, Equations (1) and (2) indicate that inclined tube manometer amplifies the length of measurement by , which is the primary advantage of such type of manometer.