Re: Is there a conversion for pressure level read in mercury vs water?

Since pressure in units like mm of mercury or feet of water is expressing a weight of liquid above a unit area, all you need to convert between them is the densities of the two liquids. So if, at the conditions you are measuring, the density of water was 1 g/cm3 and the density of mercury was 12 g/cm3, then 1 mmHg would be 12 mmH2O. That was just an example; you'd have to look up the actual densities.

If you want to be more precise, it gets more complicated, because the densities of the liquids change with temperature (and, to a lesser extent, with pressure). So people use a temperature reading and tables of densities to figure out what the density of mercury and/or water are at their conditions. Often, you see "standard" values defined so that a number does not become a different pressure if somebody comes up with a more accurate density of mercury or water.

Scientists today avoid expressing pressures in height of water almost entirely (except maybe people who deal with water and sewer systems); heights of mercury are still used some. But where possible it is best to use real pressure units of force per unit area; the SI unit is the pascal (Pa). Of course if you are making a measurement with a mercury manometer (or a water manometer, though I don't know of any scientific uses of water manometers), you need to use the density of the liquid and the acceleration of gravity to convert that to a real pressure unit.

You can find more information on units of measurement and conversions under: http://physics.nist.gov/cuu/Units/index.html

Allan Harvey
"Don't blame the government for what I say, or vice-versa."