Calculating Absolute Pressure and Gauge Pressure
Absolute pressure is the entire or total pressure. For example, the atmospheric pressure (bar/atm) plus the water pressure. For recreational divers, bar and atm are equal.
Gauge pressure is a measurement that ignores the atmospheric pressure (atm—pressure of the air pushing down). At sea level with no added pressure, gauge pressure is zero, and underwater, your submersible pressure gauge will show 10 meters/33 feet at a depth of 10 meters/33 feet.
What is the gauge and absolute pressures at a depth of 12 meters/39 feet in salt water?
Metric
Gauge pressure (g) 1.20 atmospheres gauge and it will read 12 meters.
Absolute pressure: 1.20 bar + 1 atm = 2.20 bar/ata
Imperial
Gauge pressure (g) 1.20 atmospheres gauge and it will read 39 feet.
Absolute pressure: 1.20 bar + 1 atm = 2.20 bar/ata
To calculate absolute pressure at depth in salt water, use the following formula. (Note: msw = meters of sea water, fsw = feet of sea water)
Metric
Pressure (ata) = Depth (msw) ÷ 10 + 1 atm
Use the formula to determine the pressure at 15 msw.
Pressure (ata) = 15 ÷ 10 + 1 atm
P = 2.5 bar/ata
Imperial
Pressure (ata) = Depth (fsw) ÷ 33 + 1 atm
Use the formula to determine the pressure at 49 fsw.
Pressure (ata) = 49 ÷ 33 + 1 atm
P = 2.5 bar/ata
| Depth (Gauge Pressure) | Pressure (Absolute) |
|---|---|
| 6 m/20 ft. | 1.6 bar/ata |
| 15 m/49 ft. | 2.5 bar/ata |
| 20 m/66 ft. | 3.0 bar/ata |
| 25 m/82 ft. | 3.5 bar/ata |
| 32 m/105 ft. | 4.2 bar/ata |
Note: If you did not have to add the atmospheric pressure, then the pressure at 15 meters/49 feet would be 1.5 bar/ata. By adding the atmospheric pressure of 1 bar/atm, you get 2.5 bar/ata.