Millimetre of mercury
A millimeter of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high and now defined as precisely 133.322387415 pascals. It is denoted by the symbol "mmHg".
Although not an SI unit, the millimeter of mercury is still routinely used in medicine, meteorology, aviation, and many other scientific fields.
One millimeter of mercury is approximately 1 torr, which is 1/760 of standard atmospheric pressure (101325/760 = 133.322368421 pascals). The two units are not exactly equal; however, the relative difference (less than 0.000015%) is negligible for most practical uses.
Mercury manometers were the first accurate pressure gauges; they are less used today due to mercury's toxicity, the mercury column's sensitivity to temperature and local gravity, and the greater convenience of other instrumentation. They displayed the pressure difference between two fluids as a vertical difference between the mercury levels in two connected reservoirs.
An actual mercury column reading may be converted to more fundamental units of pressure by multiplying the difference in height between two mercury levels by the density of mercury and the local gravitational acceleration. Because the density of mercury depends on temperature and surface gravity, both of which vary with local conditions, specific standard values for these two parameters were adopted. This resulted in defining a "millimeter of mercury" as the pressure exerted at the base of a column of mercury 1 millimeter high with a precise density of 13595.1 kg/m3 when the acceleration due to gravity is exactly 9.80665 m/s2.
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