# Torr

Physical unit
Unit name Torr, millimeter mercury
Unit symbol ${\ displaystyle \ mathrm {Torr, \, mmHg}}$
Physical quantity (s) pressure
Formula symbol ${\ displaystyle p}$
dimension ${\ displaystyle {\ mathsf {M \; L ^ {- 1} \; T ^ {- 2}}}}$
In SI units ${\ displaystyle \ mathrm {1 \, Torr = {\ frac {101 \, 325} {760}} \; {\ frac {kg} {m \; s ^ {2}}}}}$ ${\ displaystyle \ qquad \ quad \ approx \ mathrm {133 {,} 322 \; Pa}}$
Named after Evangelista Torricelli
Derived from millimeter
See also: Pascal , bar , physical atmosphere , technical atmosphere , meter of water column

The Torr (unit symbol: Torr) and the millimeter mercury column (unit symbol: mmHg, sometimes: mm Hg) are identical units of measurement for pressure . The unit millimeter of mercury, sometimes written millimeter of mercury , is also called Torr for short - in honor of Torricelli , who invented the mercury barometer . The units indicate the static pressure that is generated by a mercury column 1 mm high at 0 ° C and under normal acceleration .

The millimeter of mercury is not an SI unit , but is a legal unit in the EU and Switzerland , permissible for the application area “blood pressure and pressure of other body fluids”.

## definition

The Torr is exactly connected to the unit physical atmosphere by definition via and this in turn by definition exactly with the SI unit Pascal via . That makes ${\ displaystyle \ mathrm {1 \; atm = 760 \; Torr}}$${\ displaystyle \ mathrm {1 \; atm = 101 \, 325 \; Pa}}$

${\ displaystyle \ mathrm {1 \; Torr = 1 \; mmHg = {\ frac {101 \, 325} {760}} \; Pa = 133 {,} 322 \ ldots \; Pa}}$.

So is . ${\ displaystyle \ mathrm {1 \; Pa = 7 {,} 500 \, 61 \ ldots \ cdot 10 ^ {- 3} \; Torr}}$

## Derivation

The pressure exerted by a column of mercury depends on the relationship between the density of the mercury and the gravitational acceleration . With the standard acceleration of fall , the definition of the Torr has a value of ${\ displaystyle p = \ rho \; g \; h}$${\ displaystyle \ rho}$ ${\ displaystyle g}$ ${\ textstyle g = 9 {,} 806 \, 65 \; \ mathrm {\ frac {m} {s ^ {2}}}}$

${\ displaystyle \ mathrm {\ rho = {\ frac {101 \, 325 \; Pa} {9.806 \, 65 \, m / s ^ {2} \ cdot 760 \; mm}} = 13 {,} 595 \ , 098 \, ... \; g / cm ^ {3}}}$

underlying. This value corresponds to the table value of the mercury density of 13.5951 g / cm 3 belonging to 0 ° C or the equivalent value given elsewhere within the scope of the rounding.

The conversion to the unit meter water column (unit symbol: mWS), which is no longer legally fixed, results from the exact definition of ${\ displaystyle \ mathrm {1 \; mWS = 9806 {,} 65 \; Pa}}$

${\ displaystyle \ mathrm {1 \; mWS = 73 {,} 555 \, 9 \ ldots \; Torr}}$.

## Usage and history

The unit was previously used in physics and meteorology ( air pressure ); In Germany and Austria, the Torr and conventional millimeter mercury are no longer generally permitted since January 1, 1978.

In medicine, the pressures of body fluids may continue to be given in mmHg. A blood pressure of »120 to 80« corresponds approximately to a systolic pressure of 16 kPa (or 160 mbar or hPa) and a diastolic pressure of 10.6 kPa (or 106 mbar or hPa), whereby here not the absolute but the relative Pressure (compared to air pressure) is meant.

In Switzerland, the unit cmHg (centimeter of mercury) is used for quantitative information on vacuum brakes on railways.

In the US , torr is the most common unit of pressure, alongside psi .

For the conventional millimeter mercury column, the characters mm Hg and mm QS or mmQS were used in the past.

## Individual evidence

1. Wolfgang Demtröder: Experimentalphysik 1: Mechanics and heat. Springer, 7th edition, 2015, p. 187
2. a b c d Klaus Langeheinecke (eds.), Peter Jany, Gerd Thieleke: Thermodynamics for engineers: A text and workbook for studying. Vieweg + Teubner, 8th edition, 2011, p. 315
3. ^ Richard E. Dickerson, Harry B. Gray, Marcetta Y. Darensbourg, Donald J. Darensbourg: Principles of Chemistry. de Gruyter, 2nd ed., 1988, p. 57
4. ^ Paul A. Tipler, Gene Mosca: Physics: for scientists and engineers. Springer, 7th edition, 2015, p. 378
5. Directive 2009/3
6. Swiss Unit Ordinance
7. a b DIN 1314 printing - basic terms and units. 1977
8. ^ Resolution 4 of the 10th CGPM (1954)
9. a b DIN 1301, Part 3: Units - Conversion of non-SI units , 2018
10. Hans U. v. Vogel: Chemists Calendar. Springer, 1956, p. 392
11. Unit Ordinance , 1994 (as of 2019), footnote to Section 7
12. Peter Kurzweil: The Vieweg unit lexicon: formulas and terms from physics, chemistry and technology. Vieweg, 1999, p. 40 f
13. Swiss Driving Regulations (FDV) A2016 Federal Office of Transport (FOT), July 1, 2016 (PDF; 3 MB). R 300.5 Appendix 1 Additional provisions vacuum brake