Millimetre of mercury

The millimetre of mercury (mmHg) is a pressure unit defined by the pressure exerted by a one-millimetre mercury column under specified conditions. It is widely used for blood pressure and vacuum measurements.

Hydrostatic pressure: Pressure in a fluid caused by the weight of fluid above a point. A mercury column produces pressure through the weight of the fluid above its base.
Blood pressure: The pressure exerted by circulating blood on the walls of blood vessels. Clinical blood pressure is conventionally reported in millimetres of mercury.
Evangelista Torricelli: An Italian physicist who demonstrated atmospheric pressure with a mercury tube in the seventeenth century. His experiment established the mercury-column principle behind the unit's historical use.
Centimetre of water: A pressure unit based on the pressure exerted by a one-centimetre water column. Water-column units use the same hydrostatic idea but a much less dense fluid.
Mercury column: A vertical body of liquid mercury whose height can indicate pressure. The unit's defining one-millimetre height is measured in this column.
Sphygmomanometer: An instrument for measuring blood pressure, traditionally using an inflatable cuff. Its pressure scale has historically been calibrated in millimetres of mercury.
Mercury barometer: A barometer that balances atmospheric pressure against the height of a mercury column. Its scale made mercury-column heights a practical way to express pressure.
Bar (unit): A metric unit of pressure equal to 100,000 pascals. The bar is a convenient decimal pressure unit without a liquid-column definition.
Gravitational acceleration: The acceleration imparted to objects by a gravitational field. The column's pressure depends on the strength of gravity at the measurement location.
Vacuum engineering: The design and operation of systems that maintain pressures below atmospheric pressure. Vacuum gauges often express residual pressure in millimetres of mercury.

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