A manometer is a device used to measure the pressure of a gas supply. The simplest type is a U-tube manometer, which uses a column of liquid (usually water or mercury) to compare the gas pressure with atmospheric pressure.
How a U-tube manometer works
A U-shaped tube is partially filled with liquid. One end is connected to the gas supply. The other end is open to the atmosphere.
If the gas pressure equals atmospheric pressure, the liquid levels in both arms are the same.
If the gas pressure is greater than atmospheric pressure, the liquid in the arm connected to the gas is pushed down, and the liquid in the open arm rises. The difference in height (h) between the two liquid levels is related to the difference in pressure.
If the gas pressure is less than atmospheric pressure, the liquid levels shift the other way.
Calculating gas pressure
gas pressure = atmospheric pressure + (density of liquid x g x h)
where h is the height difference between the two liquid levels, measured in metres.
If the gas-side level is lower, the gas pressure is above atmospheric. If it is higher, the gas pressure is below atmospheric.
Mercury vs water manometers
Mercury is denser than water (13,600 kg/m cubed vs 1000 kg/m cubed), so a mercury manometer gives a much smaller height difference for the same pressure. Mercury manometers are used for measuring higher pressures. Water manometers are more sensitive for small pressure differences.
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