Millimeter 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 $133.322$ pascals. It is denoted by the symbol "mm".

Although not an SI unit, the millimeter of mercury is still routinely used in medicine, meteorology and many other scientific fields.

One millimeter of mercury is approximately 1 Torr, or 1/760 of standard atmospheric pressure. The two units are not exactly equal; however, the difference (less than 0.000 015%) is negligible for most practical uses.

History and definition
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. For this reason it became customary to measure pressure in "millimeters (or inches) of mercury".

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.

The density 13595.1 kg/m3 chosen for this definition is the approximate density of mercury at 0 °C, and 9.80665 m/s2 is standard gravity. The use of an actual column of mercury to measure pressure normally requires correction for the density of mercury at the actual temperature and the sometimes marked variation of gravity with location, and may be further corrected to take account of the density of the measured air, water or other fluid.

Relation to the torr
The precision of modern transducers is often insufficient to show the difference between the torr and the millimetre of mercury, about one part in seven million.

Use in medicine and physiology
In medicine, pressure is still generally measured in millimeters of mercury. These measurements are in general given relative to the current atmospheric pressure: for example, a blood pressure of 120 mmHg, when the current atmospheric pressure is 760 mmHg, means 880 mmHg relative to perfect vacuum.

Routine pressure measurements in medicine include:
 * Blood pressure, measured with a sphygmomanometer
 * Intraocular pressure, with a tonometer
 * Cerebrospinal fluid pressure
 * Intracranial pressure
 * Intramuscular pressure (compartment syndrome)
 * Central venous pressure
 * Pulmonary artery catheterization
 * Mechanical ventilation

In physiology manometric units are used to measure Starling forces.