Question

What is a unit of pressure exactly equivalent to one mmHg?

Answer 1

This would be the `color(blue)("torr"`.

Explanation:

It is conventional to know the basic units of pressure and be able to interconvert between them. The "`"torr"`" is named after the Italian physicist `sfcolor(red)("Evangelista Torricelli"`, who invented the barometer in 1643.

Here are the pressure conversions worth knowing:

`1` `"atm"` `= 760` `"mm Hg"` `= 760` `"torr"` `= 1.01325` `"bar"` `= 101.325` `"kPa"` `= 101325` `"Pa"` `= 14.7` `"psi"`

Answer 2

How about `1/760*atm`?

Explanation:

We know that `1*atm` will support a column of mercury that is `760*mm` high. A mercury column is thus a good visual gauge for pressures at `1*atm`, or pressures below `1*atm`.

You raise or try to raise the mercury column over `760*mm` you will probably get mercury all over the laboratory or the classroom. You can convert this to (preferably for chemists) `"atmospheres"`, or `"kilopascals"`, or even `"pounds per square inch"`, knowing the relationship between `"atmospheres"` and the other units. You do not use `mm*Hg` for pressure OVER atmospheric.