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).
Atmospheres
The atmosphere (atm) is a unit of pressure being equal to 101,325 pascals. It is based on the average atmospheric pressure at sea level on Earth.