| 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 |
Amillimetre of mercury is amanometricunit ofpressure, formerly defined as the extra pressure generated by a column ofmercury onemillimetre high. Currently, it is defined as exactly133.322387415 pascals, or approximately[a] 1 torr =1/760 atmosphere =101325/760 pascals.[1][2] It is denotedmmHg[3] ormm Hg.[4][2]
Although not anSI unit, 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 literature indexed 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.
The millimetre of mercury is defined as the pressure exerted by a column of mercury 1 millimetre high with adensity of13595.1 kg/m3 (approximate density at 0 °C or 32 °F) atstandard gravity (9.80665 m/s2), i.e. precisely133.322387415pascals.
The use of an actual column of mercury for precise measurement of pressure requires corrections for theactual gravity at given location (±0.44%) and the density of mercury at the actual temperature (−0.45% at 25 °C or 77 °F).Precision may be further improved by taking account of the density of the fluid whose pressure is being measured.[7][clarification needed][verification needed]
Atorr is a similar unit defined as exactly1/760 of astandard atmosphere (1 atm =101325 Pa), i.e.133.322368421… pascals.
The torr is about one part in seven million or0.000015% smaller than the millimetre of mercury;[8] such difference is negligible for most practical uses.
Each millimetre of mercury can be divided into 1000micrometres of mercury, denoted μmHg or simply microns.[9]
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 philosopherAnaximenes ofMiletus 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 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 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 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 weight of 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 of13595.1 kg/m3 when the acceleration due to gravity is exactly9.80665 m/s2.
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:
In physiologymanometric units 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 =10−5 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 | — |
