What is the difference between the units 'torr' and 'mm of Hg'?

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The two units torr, and mm of Hg were the same until they were redefined.

The torr was named after the Italian Evangelista Torricelli. 1 atmosphere is $101325\ \mathrm{Pa}$. The torr is defined as $1/760$ of an atmosphere. This is equal to $133.322\overline{368421052631578947}~\mathrm{Pa}$, which periodically infinitely repeats.

The mm of Hg was defined as the pressure generated by a column of mercury one millimeter high. Since the pressure of mercury depends on temperature and gravity, it is now redefined as

$$\mathrm{0.001\ mL \cdot 13595.1~mg \cdot 9.80665\ m/s^2 = 133.322387415 ~Pa}$$

This has caused a difference between the units of $0.000015~\%$, so unless you require extremely high precision, you can assume that $1\ \mathrm{torr} = 1\ \mathrm{mmHg}$

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DSinghvi
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DSinghvi

Updated on August 01, 2022

Comments

  • DSinghvi
    DSinghvi over 1 year

    According to present definition torr and mm of Hg differ though slightly. I am still confused over it. Can someone write an explicit answer differentiating between them and the reason for differentiation as earlier they were considered same. The explanation is probably related to the changes is pressure at different heights so I want the exact definitions of these units that was used earlier and what modifications have been made?

    • Admin
      Admin about 8 years
      In addition to the confusion of the two different units, the unit names and unit symbols are sometimes mixed up. The name of one unit is “torr” and its symbol is “Torr”. The name of the other unit is “conventional millimetre of mercury” and its symbol is “mmHg”.
  • DSinghvi
    DSinghvi over 8 years
    that means 760 mm Hg is not exactly equal to 1 atm
  • DSinghvi
    DSinghvi over 8 years
    Also is 1 mm of Hg an exactly defined quantity
  • katahdin
    katahdin over 8 years
    @DSinghvi: Yes to both questions.
  • katahdin
    katahdin over 8 years
    As I said before, the difference between the values is so negligible, you can just assume that 1 torr = 1 mm Hg
  • MaxW
    MaxW almost 7 years
    Very very few chemistry experiments are ever done to 0.000015% precision. Something like trying to measure Avogadro's Constant would be an exception.
  • katahdin
    katahdin about 6 years
    Note that the error in assuming 1 torr = 1 mm Hg is probably spectacularly smaller than the local variation of g depending on altitude, latitude and longitude, or the error introduced by local atmospheric densities.
  • Oscar Lanzi
    Oscar Lanzi about 6 years
    $0.001 mL$? Maybe $0.001 m$ for the first factor in the definition of mm Hg? Also the factors in both definitions are rendered to only six significant figures therefore please report the SI values of to that accuracy.