Mercury pressure gauges measure gas pressure using a vertical tube filled with liquid mercury. Closed-tube gauges are preferred due to mercury’s toxicity. They are used to measure atmospheric pressure and flow rates in air conditioning systems. Maintenance is required as mercury slowly vaporizes, leading to the use of alternative liquids.
A mercury pressure gauge is a vertical glass or plastic tube filled with liquid mercury used to measure the pressure of gases. There are open and closed versions, the difference being that a closed-tube manometer has a vacuum above the mercury in the closed end, while the open-tube manometer is open to air. Open pressure gauges measure the pressure difference between the sample gas and the ambient air. Closed-tube devices measure the absolute pressure of the sample gas based on the height of the mercury in the tube.
The most typical application of mercury is in enclosed pressure gauges due to the toxic nature of mercury. Although mercury is a liquid at room temperature, it releases vapor which can be dangerous to humans over a period of time. It is important to contain the mercury in a closed system to prevent any adverse reactions and a closed tube device allows the mercury to be used with minimal risk.
Measurement of atmospheric and barometric pressure is common for laboratory research. Escalators mounted along the gauge column allow the user to adjust for local pressure differences. This is accomplished by setting a zero scale at the open end of the column which is aligned with the mercury level. A standard atmosphere used as a research reference is 29.92 inches (760 millimeters) of mercury, which is why meteorological observations typically show atmospheric pressure in these units.
Open pressure gauges are often used to measure flows in air conditioning systems or to build ducts in a ventilation system. Inserting a probe into the air stream will affect the height of the mercury in the gauge column. This height can be calibrated and used to measure the pressure drop, which can be used to calculate flow rates. Using valves to shut off the mercury gauge when not in use can minimize mercury exposure in open gauges.
A mercury pressure gauge requires occasional maintenance because liquid mercury slowly vaporizes and oxidizes when in contact with air. Vaporization and the need to handle and replace mercury has led to the replacement of mercury with oils, alcohols or water. These materials also vaporize at different rates, and some gauge liquids may react with the gases being measured. Before using a pressure gauge, users may wish to check the compatibility of the fluid with the gases they will be measuring.
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