stumps

Scientists measure small pressures in mm of mercury, which, in a fit of stupidity, was renamed Torr. Prior to mm of mercury, inches of mercury was the standard. HVAC people work in inches of water, which works out well for the pressures that normally exist in ducting, and the differentials that occur across filters.

In all of these cases, the pressure is a measure of the ability lift a column of water, or mercury. Water is much lighter than mercury, so it takes a 34 foot high column to measure the pressure of our atmosphere, where as it only takes a 30 inch column of mercury to do the same.

-Chuck