mm of water
The millimeter of water (mmH₂O) is a unit of pressure exerted by a 1 millimeter column of water at 4 degrees Celsius under standard gravity.
Inches of water
Inches of water (inH₂O) is a pressure unit from the Imperial system. An inch of water is defined as the pressure from a one inch column of water at 4°C (39.2°F) (which is the temperature at which water is densest).