Millimetre of mercury
| millimetre of mercury | |
|---|---|
| Unit of | Pressure |
| Symbol | mmHg, mm Hg |
| Conversions | |
| 1 mmHg in ... | ... is equal to ... |
| SI units | 133.3224 Pa |
| English Engineering units | 0.01933678 lbf/in2 |
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 currently defined as exactly 133.322387415 pascals.[1] It is denoted mmHg[2] or mm Hg.[3]
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 (101325/760 ≈ 133.322368421053 pascals). Although the two units are not equal, the relative 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 specific weight 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.[citation needed]
The density 13595.1 kg/m3 chosen for this definition is the approximate density of mercury at 0 °C (32 °F), and 9.80665m/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.[4]
Each millimetre of mercury can be divided into 1000 micrometres of mercury, denoted μmHg or simply microns.[5]
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 difference between these two units is about one part in seven million or 0.000015%.[6] By the same factor, a millitorr is slightly less than a micrometre of mercury.
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
In physiology manometric units are used to measure Starling forces.
| Pascal | Bar | Technical atmosphere | Standard atmosphere | Torr | Pound per square inch | |
|---|---|---|---|---|---|---|
| (Pa) | (bar) | (at) | (atm) | (Torr) | (lbf/in2) | |
| 1 Pa | — | 1 Pa = 10−5 bar | 1 Pa = 1.0197×10−5 at | 1 Pa = 9.8692×10−6 atm | 1 Pa = 7.5006×10−3 Torr | 1 Pa = 0.000145037737730 lbf/in2 |
| 1 bar | 105 | — | = 1.0197 | = 0.98692 | = 750.06 | = 14.503773773022 |
| 1 at | 98066.5 | 0.980665 | — | 0.9678411053541 | 735.5592401 | 14.2233433071203 |
| 1 atm | ≡ 101325 | ≡ 1.01325 | 1.0332 | — | 760 | 14.6959487755142 |
| 1 Torr | 133.322368421 | 0.001333224 | 0.00135951 | 1/760 ≈ 0.001315789 | — | 0.019336775 |
| 1 lbf/in2 | 6894.757293168 | 0.068947573 | 0.070306958 | 0.068045964 | 51.714932572 | — |
See also
References
- ^ 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 2021-06-04, retrieved 2021-12-16
- ^ "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.
- ^ Hoffman, Dorothy; Singh, Bawa; Thomas, John H. (1998). Handbook of vacuum science and technology (PDF). San Diego, CA: Academic Press. p. 171. ISBN 978-0-12-352065-4. OCLC 162128757.
- ^ "Pressure Units". National Physical Laboratory (NPL). Archived from the original on 28 January 2015. Retrieved 16 September 2020.