Amillimetre of mercuryis amanometricunitofpressure,formerly defined as the extra pressure generated by a column ofmercuryonemillimetrehigh, and currently defined as exactly133.322387415pascals[1]or exactly133.322pascals.[2]It is denotedmmHg[3]ormm Hg.[4][2]
| millimetre of mercury | |
|---|---|
| Unit of | Pressure |
| Symbol | mmHg, mm Hg |
| Conversions | |
| 1 mmHgin... | ... is equal to... |
| SI units | 133.322 Pa |
| English Engineering units | 0.01933678 lbf/in2 |
Although not anSIunit, the millimetre of mercury is still often encountered in some fields; for example, it is still widely used inmedicine,as demonstrated for example in themedical literatureindexed inPubMed.[5]For example, the U.S. and European guidelines onhypertension,in using millimeters of mercury forblood pressure,[6]are reflecting the fact (common basic knowledge among health care professionals) that this is the usual unit of blood pressure in clinical medicine.
One millimetre of mercury is approximately 1torr,which is1/760of standardatmospheric pressure(101325/760≈133.322368pascals). Although the two units are not equal, therelative difference(less than0.000015%) is negligible for most practical uses.
History
editFor much of human history, the pressure of gases like air was ignored, denied, or taken for granted, but as early as the 6th century BC, Greek philosopherAnaximenesofMiletusclaimed that all things are made of air that is simply changed by varying levels of pressure. He could observe water evaporating, changing to a gas, and felt that this applied even to solid matter. More condensed air made colder, heavier objects, and expanded air made lighter, hotter objects. This was akin to how gases become less dense when warmer and more dense when cooler.
In the 17th century,Evangelista Torricelliconducted experiments with mercury that allowed him to measure the presence of air. He would dip a glass tube, closed at one end, into a bowl of mercury and raise the closed end up out of it, keeping the open end submerged. The weight of the mercury would pull it down, leaving a partial vacuum at the far end. This validated his belief that air/gas has mass, creating pressure on things around it. Previously, the more popular conclusion, even forGalileo,was that air was weightless and it is vacuum that provided force, as in a siphon. The discovery helped bring Torricelli to the conclusion:
We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight.
This test, known asTorricelli's experiment,was essentially the first documented pressure gauge.
Blaise Pascalwent farther, having his brother-in-law try the experiment at different altitudes on a mountain, and finding indeed that the farther down in the ocean of atmosphere, the higher the pressure.
Mercury manometers were the first accurate pressure gauges. They are less used today due tomercury'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 thespecific weightof mercury depends on temperature andsurface 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 of13595.1kg/m3when the acceleration due to gravity is exactly9.80665m/s2.[citation needed]
The density13595.1kg/m3chosen for this definition is the approximate density of mercury at 0 °C (32 °F), and9.80665m/s2isstandard 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 significant variation of gravity with location, and may be further corrected to take account of the density of the measured air, water or other fluid.[7]
Each millimetre of mercury can be divided into 1000micrometresof mercury, denotedμmHgor simplymicrons.[8]
Relation to the torr
editThe 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 or0.000015%.[9]By the same factor, a millitorr is slightly less than a micrometre of mercury.
Use in medicine and physiology
editIn 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 asphygmomanometer
- Intraocular pressure, with atonometer
- Cerebrospinal fluidpressure
- Intracranial pressure
- Intramuscular pressure (compartment syndrome)
- Central venous pressure
- Pulmonary artery catheterization
- Mechanical ventilation
In physiologymanometricunits are used to measureStarling forces.
| Pascal | Bar | Technical atmosphere | Standard atmosphere | Torr | Pound per square inch | |
|---|---|---|---|---|---|---|
| (Pa) | (bar) | (at) | (atm) | (Torr) | (lbf/in2) | |
| 1 Pa | — | 1 Pa =10−5bar | 1 Pa =1.0197×10−5at | 1 Pa =9.8692×10−6atm | 1 Pa =7.5006×10−3Torr | 1 Pa =0.000145037737730lbf/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
editReferences
edit- ^BS 350: Part 1: 1974 – Conversion factors and tables.British Standards Institution.1974. p. 49.
- ^abCouncil Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to units of measurement and on the repeal of Directive 71/354/EECof theEuropean Economic Community
- ^International Bureau of Weights and Measures(2006),The International System of Units (SI)(PDF)(8th ed.), p. 127,ISBN92-822-2213-6,archived(PDF)from the original on 2021-06-04,retrieved2021-12-16
- ^"AMA Manual of Style Online".American Medical Association.Retrieved2018-02-24.
- ^National Center for Biotechnology Information of the National Library of Medicine of the United States."Articles demonstrating pressures in mm Hg".PubMed.Retrieved2023-09-02.
- ^Gijón-Conde, T; Sánchez-Martínez, M; Graciani, A; Cruz, JJ; López-García, E; Ortolá, R; Rodríguez-Artalejo, F; Banegas, JR (July 2019). "Impact of the European and American guidelines on hypertension prevalence, treatment, and cardiometabolic goals".Journal of Hypertension.37(7): 1393–1400.doi:10.1097/HJH.0000000000002065.PMID31145710.S2CID86674318.
- ^Kaye, G.W.C.; Laby, T.H. (1986).Tables of Physical and Chemical Constants(XV ed.). Longman. pp. 22–23.ISBN0582463548.
- ^Hoffman, Dorothy; Singh, Bawa; Thomas, John H. (1998).Handbook of vacuum science and technology(PDF).San Diego, CA: Academic Press. p. 171.ISBN978-0-12-352065-4.OCLC162128757.
- ^"Pressure Units".National Physical Laboratory (NPL). Archived fromthe originalon 28 January 2015.Retrieved16 September2020.