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Mercury barometer. A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high. . Currently, it is defined as exactly 133.322 387 415 pascals, or approximately [a] 1 torr = 1 / 760 atmosphere = 101 325 / 760
Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth.The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, [1] 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. [2]
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters of mercury (mmHg). A torr was originally defined as 1 mmHg. The pressure is quoted as the level of the mercury's height in the vertical column.
The experiment uses a simple barometer to measure the pressure of air, filling it with mercury up until 75% of the tube. Any air bubbles in the tube must be removed by inverting several times. After that, a clean mercury is filled once again until the tube is completely full. The barometer is then placed inverted on the dish full of mercury.
The parent of all mercury pressure gauges is the mercury barometer invented by Evangelista Torricelli in 1643. [15] An early engineering application of the mercury pressure gauge was to measure pressure in steam boilers during the age of steam. The first use on steam engines was by James Watt while developing the Watt steam engine between 1763 ...
So, for example, pressure head might be written "742.2 mm Hg" or "4.2 in H 2 O at 59 °F" for measurements taken with mercury or water as the manometric fluid respectively. The word "gauge" or "vacuum" may be added to such a measurement to distinguish between a pressure above or below the atmospheric pressure.
It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity. Conversion to metric units depends on the density of mercury, and hence its temperature; typical conversion factors are: [1]
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 °C (32 °F) and standard gravity (g n = 9.806 65 m/s 2). [2] It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure.