A mercury manometer is a vertical glass or plastic tube filled with liquid mercury that is used to measure pressure of gases. There are open and closed tube manometers. The difference is that a closed-tube manometer has a vacuum above the mercury in the closed end of the tube, while the open-tube is open to the air. Open-end manometers measure the pressure difference between the gas being sampled and the room air. Closed-tube manometers measure the absolute pressure of the sampled gas by the height of the mercury in the tube.

The most typical application for mercury is in closed-end manometers because of the toxic nature of mercury. Although mercury is a liquid at room temperature, it does release vapor that can be hazardous to humans over a period of time. It is important to contain the mercury in a closed system to prevent adverse reaction, and a closed-tube mercury manometer allows mercury to be used with minimal risk.

Measuring atmospheric and barometric pressure, or the pressure around us, is common for laboratory research. Moveable scales mounted along the mercury manometer column allow the user to adjust for local pressure differences. This is accomplished by setting a zero scale on the open end of the column that is lined up with the mercury level. A standard atmosphere used as a research reference is 29.92 inches of mercury (760 millimeters), which is why weather observations typically show atmospheric pressure in inches or millimeters of mercury.

Open-end manometers are often used for measuring flows in air conditioning systems, or building ducts in a ventilation system. Inserting a probe into the air stream will affect the height of mercury in the manometer column. This height can be calibrated and used to measure pressure drop, which can be used to calculate flow rates. Using valves to close the mercury manometer when it is not being used can minimize exposure to mercury in open-end manometers.

Starting in the late 20th century, a concern over mercury toxicity led to manometers using other liquids. A mercury manometer requires occasional maintenance, because liquid mercury vaporizes and oxidizes slowly in contact with air. Vaporization and the need to handle and replace the mercury has led to replacement of mercury with oils, alcohols or water. These materials also vaporize at different rates, and some manometer liquids could react with the gases being measured. Before using a manometer, users may wish to check for compatibility of the manometer fluid with the gases they will be measuring.