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Absolute pressure is the entire or total pressure. For example, the atmospheric pressure (bar/atm) plus the water pressure. For recreational and technical divers, bar and atm are said to be equal, whereas ata accounts for the atmospheric pressure pushing down on the water's surface.

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

Depth (msw) ÷ 10 m + 1 atm = Pressure (ata)

Use the formula to determine the pressure at 15 msw.

15 msw ÷ 10 m + 1 atm = 2.5 bar/ata

Imperial

Depth (fsw) ÷ 33 ft. + 1 atm = Pressure (ata)

Use the formula to determine the pressure at 49 fsw.

49 fsw ÷ 33 ft. + 1 atm = 2.5 bar/ata

Table 3: Salt Water
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.

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