What is the difference between the units 'torr' and 'mm of Hg'?
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
Updated on August 01, 2022Comments
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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?
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Admin about 8 yearsIn 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”.
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DSinghvi over 8 yearsthat means 760 mm Hg is not exactly equal to 1 atm
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DSinghvi over 8 yearsAlso is 1 mm of Hg an exactly defined quantity
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katahdin over 8 years@DSinghvi: Yes to both questions.
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katahdin over 8 yearsAs I said before, the difference between the values is so negligible, you can just assume that 1 torr = 1 mm Hg
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MaxW almost 7 yearsVery very few chemistry experiments are ever done to 0.000015% precision. Something like trying to measure Avogadro's Constant would be an exception.
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katahdin about 6 yearsNote 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.
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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.