An open-tube manometer is a device used to measure pressures slightly below or above atmospheric pressure. The pressure of a substance, such as a gas, is given by the difference in levels of the manometric fluid.
Mercury is a commonly used manometric fluid. A difference in the heights of the mercury levels indicates the pressure difference in relation to atmospheric pressure. For example, if the pressure of the gas being measured is greater than atmospheric pressure, the mercury level of the arm exposed to the atmosphere is higher than that of the arm attached to the confined gas. The opposite occurs when the atmospheric pressure is greater than that of the enclosed gas: the mercury level of the arm attached to the enclosed gas is higher than that of the arm exposed to the atmosphere. When the pressure of the confined gas is equal to atmospheric pressure, the mercury levels in the two arms are equal.
A closed-tube manometer, as opposed to an open-tube manometer, only measures pressures that are below atmospheric pressure. The structure of a closed-tube manometer is very similar to that of the open-tube manometer, the only difference being that the arm that is open in an open-tube manometer is sealed and contains a vacuum in a closed-tube manometer.