Millimeter of mercury
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 and now defined as precisely 387415 133.322pascals. It is denoted by the symbol mmHg or mm Hg.
Although not an SI unit, the millimetre of mercury is still routinely used in medicine, meteorology, aviation, and many other scientific fields.
One millimetre of mercury is approximately 1 Torr, which is 1/760 of standard atmospheric pressure (325101/ = 368421053 pascals). The two units are not exactly equal; however, the 133.322relative difference (less than 0.000015%) 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.
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 "millimetre of mercury" as the pressure exerted at the base of a column of mercury 1 millimetre 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 (32 °F), 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. The torr is about one part in seven million or 0.000015% less.
Use in medicine and physiology
In medicine, pressure is still generally measured in millimetres 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
|Pascal||Bar||Technical atmosphere||Standard atmosphere||Torr||Pounds per square inch|
|1 Pa||≡ 1 N/m2||10−5||×10−5 1.0197||×10−6 9.8692||×10−3 7.5006||0.000 145 037 737 730|
|1 bar||105||≡ 100 kPa
≡ 106 dyn/cm2
|1.0197||92 0.986||750.06||14.503 773 773 022|
|1 at||066.5 98||665 0.980||≡ 1 kgf/cm2||0.967 841 105 354 1||735.559 240 1||14.223 343 307 120 3|
|1 atm||325 101||25 1.013||1.0332||1||760||14.695 948 775 514 2|
|1 Torr||133.322 368 421||0.001 333 224||0.001 359 51||1/ ≈ 0.001 315 789||1 Torr
≈ 1 mmHg
|0.019 336 775|
|1 lbf/in2||6894.757 293 168||0.068 947 573||0.070 306 958||0.068 045 964||51.714 932 572||≡ 1 lbf/in2|
- BS 350: Part 1: 1974 – Conversion factors and tables. British Standards Institution. 1974. p. 49.
- International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 127, ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14
- "AMA Manual of Style Online". American Medical Association. Retrieved 2018-02-24.
- Kaye, G.W.C.; Laby, T.H. (1986). Tables of Physical and Chemical Constants (XV ed.). Longman. pp. 22–23. ISBN 0582463548.
- "Pressure Units". National Physical Laboratory (NPL). Retrieved 25 January 2015.