A manometer consists of a U-tube filled with a liquid (mercury, water or oil) with a certain density.
The manometer is used to measure the difference in pressure between the two sides of the U-tube.
When the manometer is not connected to the gas supply, i.e. when both arms are open to the atmosphere, the liquid levels in both arms are equal.
To measure the pressure of a gas, the other arm is connected to the gas pipe and the gas pressure acts on the surface of the liquid in the respective arm.
if the gas pressure is greater than the atmospheric pressure, the liquid in the respective arm (say arm B) will be pushed downwards. Under equilibrium conditions, (same pressure from both arms), the level of the liquid will be at the same level.
2. Bourdon Gauge
A Bordon gauge consists of a coil of flattened copper tube with an oval cross section connected to a lever system.
When the gas supply is connected, the pressure in the gas acts to straighten the copper coil.
The movement of the copper coil is transferred to the lever system which actuates a pointer to move across a scale which has been calibrated to give readings of pressure.
The unit of measurement used in the Bourdon gauge is Pascal. Bourdon gauges are normally connected to gas cylinders to give an indication of the quantity of gas in the cylinders.
Bourdon gauges are more robust than manometers and more suitable for measuring higher pressures. But they have to be calibrated before they can be used.