{{short description|Manometric unit of pressure}} {{Infobox unit | bgcolor = | name = millimetre of mercury | image = | caption = | standard = | quantity = [[Pressure]] | symbol = mmHg | symbol2 = mm Hg | namedafter = | extralabel = | extradata = | units1 = SI units | inunits1 = {{convert|1|mmHg|Pa|disp=out|sigfig=6}} | units2 = [[English Engineering units]] | inunits2 = {{convert|1|mmHg|lbf/in2|disp=out|sigfig=7}} }} [[File:Barometer by Charles Frodsham, view 1, London, date unknown, mahogany, mercury, metal, glass - Shugborough Hall - Staffordshire, England - DSC00449.jpg|thumb|[[Mercury barometer]]]]
A '''millimetre of mercury''' is a [[Pressure measurement#Liquid column (manometer)|manometric]] [[Unit of measurement|unit]] of [[pressure]], formerly defined as the extra pressure generated by a column of [[mercury (element)|mercury]] one [[millimetre]] high. Currently, it is defined as exactly {{val|133.322387415|ul=pascals}}, or approximately{{efn|Although the two units are not equal, the [[relative difference]] (less than {{gaps|0.000|015%}}) is negligible for most practical uses.}} 1 [[torr]] = {{sfrac|760}} [[standard atmosphere (unit)|atmosphere]] = {{sfrac|{{val|101325}}|760}} pascals.<ref>{{cite book |title=BS 350: Part 1: 1974 – Conversion factors and tables |date=1974 |publisher=[[British Standards Institution]] |page=49}}</ref><ref name="EEC-80-181">[https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A31980L0181 Council 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/EEC] of the [[European Economic Community]]</ref><!-- as UK was member of the EEC in 1980, this should supersede the definition of 1974---> It is denoted '''mmHg'''<ref>{{SIbrochure8th|page=127}}</ref> or '''mm Hg'''.<ref name="AMA_MOS">{{cite web |publisher=[[American Medical Association]] |title=AMA Manual of Style Online |url=http://www.amamanualofstyle.com/ |access-date=2018-02-24 }}</ref><ref name="EEC-80-181" />
Although not an [[International System of Units|SI]] unit, the millimetre of mercury is still often encountered in some fields; for example, it is still widely used in [[medicine]], as demonstrated for example in the [[medical literature]] indexed in [[PubMed]].<ref name="NCBI-NLM-PubMed-mm-Hg">{{cite web |author=National Center for Biotechnology Information of the National Library of Medicine of the United States |url=https://pubmed.ncbi.nlm.nih.gov/?term=mm+Hg&schema=alltitle |website=PubMed |title=Articles demonstrating pressures in mm Hg |access-date=2023-09-02}}</ref> For example, the U.S. and European guidelines on [[hypertension]], in using millimeters of mercury for [[blood pressure]],<ref name="pmid_31145710">{{cite journal |last1=Gijón-Conde |first1=T |last2=Sánchez-Martínez |first2=M |last3=Graciani |first3=A |last4=Cruz |first4=JJ |last5=López-García |first5=E |last6=Ortolá |first6=R |last7=Rodríguez-Artalejo |first7=F |last8=Banegas |first8=JR |title=Impact of the European and American guidelines on hypertension prevalence, treatment, and cardiometabolic goals. |journal=Journal of Hypertension |date=July 2019 |volume=37 |issue=7 |pages=1393–1400 |doi=10.1097/HJH.0000000000002065 |pmid=31145710|s2cid=86674318 |hdl=10486/717819 |hdl-access=free }}</ref> are reflecting the fact (common basic knowledge among health care professionals) that this is the usual unit of blood pressure in clinical medicine.<!--An article from 2008 addressing the question of why is at PMID: 18270053 DOI: 10.1109/MEMB.2007.911401; a check of a typical (widely used, well-regarded) textbook on physiology 101, at its chapters on cardiovascular and hemodynamic basics, finds that it simply shows blood pressure being measured in mm Hg without explicitly explaining why that unit is the usual one for BP, but it certainly is, as any physician or nurse knows.-->
==Definition== The millimetre of mercury is defined as the pressure exerted by a column of [[mercury (element)|mercury]] 1 [[millimetre]] high with a [[density]] of {{val|13595.1|u=kg/m3}} (approximate density at {{conv|0|°C|disp=or}}) at [[standard gravity]] ({{val|9.80665|u=m/s2}}), i.e. precisely {{val|133.322387415}} [[Pascal (unit)|pascal]]s.
: 1 mmHg = {{nowrap|1=1 mm × {{val|13595.1|u=kg/m3}} × {{val|9.80665|u=m/s2}}}} = {{val|133.322387415|u=Pa}} (exactly)
The use of an actual column of mercury for precise measurement of pressure requires corrections for the [[Earth's gravity variation|actual gravity]] at given location (±0.44%) and the density of mercury at the actual temperature (−0.45% at {{conv|25|°C|disp=or}}). If the top the column - the surface whose height is being measured - is exposed to some other fluid, that other fluid's temperature-dependent density must also be accounted for.<ref>{{cite book|last1=Kaye|first1=G.W.C.|last2=Laby|first2=T.H.|title=Tables of Physical and Chemical Constants|date=1986|publisher=Longman|isbn=0582463548|pages=22–23|edition=XV}}</ref>{{verification needed|date=February 2025}}
A [[torr]] is a similar unit defined as exactly {{sfrac|760}} of a [[standard atmosphere (unit)|standard atmosphere]] ({{val|1|u=atm}} = {{val|101325|u=Pa}}), i.e. {{val|133.322368421|end=…|u=pascals}}.
: 1 Torr = {{sfrac|760}} atm = {{sfrac|{{val|101325}}|760}} Pa = {{val|133.322368421|end=…|u=Pa}}
The torr is about one part in seven million or {{val|0.000015|u=%}} smaller than the millimetre of mercury;<ref>{{cite web|title=Pressure Units|url=http://www.npl.co.uk/reference/faqs/pressure-units|archive-url=https://web.archive.org/web/20150128141543/http://www.npl.co.uk/reference/faqs/pressure-units|archive-date=28 January 2015|publisher=National Physical Laboratory (NPL)|access-date=16 September 2020}}</ref> such difference is negligible for most practical uses.
Each millimetre of mercury can be divided into 1000 [[micrometre]]s of mercury, denoted μmHg or simply microns.<ref>{{cite book|url=http://dmf.unicatt.it/~gavioli/corsi/MSFM/ref/Vuoto+Camere/Books/Handbook_of_Vacuum_Science_and_Technology.pdf|page=171| last1=Hoffman | first1=Dorothy |last2=Singh|first2=Bawa|last3=Thomas|first3=John H.| title=Handbook of vacuum science and technology | publisher=Academic Press | publication-place=San Diego, CA | year=1998 | isbn=978-0-12-352065-4 | oclc=162128757}}</ref>
==History== {{Further|Pressure head}} For 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 philosopher [[Anaximenes of Miletus|Anaximenes]] of [[Miletus]] claimed 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 Torricelli]] conducted 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 for [[Galileo]], was that air was weightless and it is vacuum that provided force, as in a siphon. The discovery helped bring Torricelli to the conclusion:
{{cquote|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 as [[Torricelli's experiment]], was essentially the first documented pressure gauge.
[[Blaise Pascal]] went 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 to [[Mercury poisoning|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 [[Gravity of Earth|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 {{gaps|13|595.1|u=kg/m<sup>3</sup>}} when the acceleration due to gravity is exactly {{gaps|9.806|65|u=m/s<sup>2</sup>}}.
==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 catheter]]ization * [[Mechanical ventilation]]
In physiology [[manometric]] units are used to measure [[Starling equation|Starling forces]].
{{pressure Units}}
==See also== * [[Bar (unit)]] * [[Centimetre or millimetre of water]] * [[Inch of mercury]] * [[Inch of water]] * [[Pound per square inch]] * [[Torr]]
==Notes== {{Notelist}}
==References== {{reflist}}
[[Category:Units of pressure]] [[Category:Mercury (element)]]