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A mercury manometer is a vertical glass or plastic tube filled with liquid mercury that is used to measure the pressure of gases. There are open and closed versions, with the difference being that a closed-tube manometer has a vacuum above the mercury in the closed end, 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 devices 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 any adverse reaction, and a closed-tube device allows mercury to be used with minimal risk.
Measuring atmospheric and barometric pressure is common for laboratory research. Movable scales mounted along the 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 (760 millimeters) of mercury, which is why weather observations typically show atmospheric pressure in these units.
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.
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 fluid with the gases they will be measuring.