A manometer consists of a U-tube containing mercury (or some other liquid). It is used to find the difference in pressures between the gas enclosed in a vessel and atmospheric pressure.
In order to measure the pressure of the gas enclosed in a vessel one arm A of the manometer is connected to the vessel and the other is kept open to the atmosphere. The gas exerts pressure on B while the atmosphere exerts pressure on C. If the pressure of the gas is higher than that of the atmosphere, the liquid rises in arm C until the pressure, at two points B, C on the same horizontal level connected by a single fluid becomes equal to the gas pressure. Clearly,
Pressure of gas = Pressure of + Pressure due atmosphere to column CD p = p0 + hdg
Where h is the level difference in manometer arms and d is the density of the liquid, p0 is atmospheric pressure and p is gas pressure.
Fluid pressures are expressed with respect to some reference. When the reference level is complete vacuum, the measured pressure is known as absolute pressure. However, when the fluid pressure is expressed taking the local atmospheric pressure as the reference, the pressure so measured is called gauge pressure. A mercury barometer measures absolute pressure, while a manometer gives only the difference between the pressure of the gas and atmospheric pressure as such it directly measures the gauge pressure which is given by hρg.
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