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).
mm of mercury
The millimeter of mercury (mmHg) is pressure unit traditionally used in meteorology. It is the pressure from a 1 mm column of mercury at 0°C under normal gravity.