As stated above, the pressure of a gas is the force it exerts per unit area. This force can push a column of liquid up a tube: the higher the pressure, the larger the force for a given area, and consequently the more liquid that is pushed up. This behavior is exploited in the measuring of pressure. There are two different kinds of setups or devices: the barometer and the manometer.
The simplest form of barometer consists of a container of liquid open to the atmosphere, with an inverted tube placed in it. Care is taken so that there is no air trapped within the inverted tube. The pressure exerted by the air in the environment (i.e., the atmospheric pressure) pushes the liquid up the tube. The pressure is reported as the height to which the liquid rises. A value of 760 mm Hg, for example, which is a typical reading for atmospheric pressure, means that liquid mercury (Hg) rises to a height of 760 mm.
If, instead of measuring just the atmospheric pressure, we wish to measure the pressure of a gas generated in a reaction, a slightly different setup is needed, although it is one that uses the same principle. In an open-tube manometer, the liquid (usually mercury) is placed in a U-tube with one end exposed to the atmosphere and the other end connected to the closed vessel holding the gas whose pressure we want to measure:
It is the difference in heights of the two ends of the mercury that enables us to calculate the pressure of the gas in the vessel. If the end exposed to the atmosphere is x mm lower than the other end (as in the diagram above), then the pressure of the gas is (760 – x) mm Hg: The pressure is lower than atmospheric pressure, hence the mercury is pushed down further on the open-end side. Conversely, if the exposed end is x mm higher than the other end, the pressure of the gas is higher than atmospheric pressure, more exactly (760 + x) mm Hg.
A closed-tube manometer setup, considered a hybrid between the barometer and the open-end manometer, is also possible. The end that used to be exposed is now sealed off as well, with any trapped air again evacuated. This time, the difference in heights is read off directly as the pressure of the gas, without having to reference the atmospheric pressure. In the diagram below, then, the pressure of the gas is simply x mm Hg.
This setup is generally used only to measure pressures that are lower than atmospheric pressure.