Atm revisited

An atm, or atmosphere, is a unit for measuring pressure, that is used a great deal by technical divers. The total atmospheric pressure at sea level is defined as 1 atmosphere (atm), also called an "ata", an atmosphere absolute. At sea level, the fraction of oxygen in air is 0.21or 21%, so the partial pressure of oxygen, PO2 (total pressure x gas fraction = gas partial pressure) is 0.21atm. 1 atm = 1.013 bars (another common unit of pressure). For physiological purposes atms and bars are more or less interchangeable. Another pressure unit with great utility is the kilopascal, a metric unit denoted kPa. A kPa is 1/100 of a bar -- very close to 1/100 atm. In time most physiological pressures will be defined in kPa. Some other common pressure units are as follows:

1 atm = 14.7 Psig

1 fsw = 1/33 atm (feet of seawater)

1 ffw = 1/34 atm ( feet of fresh water)

1 msw = 1/10 bar (meters of seawater)

1 msw = 3.2568 fsw

1 kPa = 1/100 bar