Underwater Recovery

Underwater Recovery

Dive Gasses:

Air:  78 % Nitrogen - 21 % oxygen

a good underwater gas for depths of surface to about 125 FSW (feet of sea-water) but usable to 200 FSW.

NITROX: 32%-50% Oxygen.

a custom blend of Nitrogen and Oxygen usually with higher concentrations of Oxygen, which permit longer bottom times and shorter decompression times than Air. This mix has a maximum MOD (maximum operating depth) much shallower than air and usually under 110 feet FSW. This is an excellent gas for deco (decompression).

TRI-MIX: a specialty gas mix usually consisting of 10% - 40% Helium, Oxygen in the range of 15% and the balance Nitrogen. This is a gas that is useful for deepest dives and should be used over 200 FSW. The Helium in this mix has a higher heat transfer capability and therefore produces thermal issues with divers.

Pressure and it's effects:

Water Pressure: The deeper in water the greater the pressure (or weight of the water) in PSIA (pounds per square inch absolute)  At 33 FSW the pressure is twice the surface pressure of 14.7 PSIA ot 29.4 PSIA. At 66 FSW the pressure is 44.1 PSIA and at 99 FSW it is 58.8 PSIA. This continues to increment in 14.7 PSIA steps with each 33 FSW.

Gas Consumption: the regulator gives a diver gas at the same pressure as the ambient water pressure. Therefore at 33 FSW it takes twice the amount of gas to fill the divers lungs as it did on the surface, halving (50%) the length of time his gas supply will last underwater. At 66FSW the gas supply will last 1/3 rd as long as on the surfase and 1/4 at 99 FSW. The denominator continues to increment every 33 FSW and so is 1/5 at 132 FSW etc.

Decompression: The deeper a diver dives the faster Nitrogen is assimilated into the blood. The partial pressure of gas continues to increase with depth and time, until the diver can not return, directly to the surface. Incremental stops of 30 FSW, 20 FSW and 10 FSW may be required, depending on depth and legnth of dive. For a first dive in 24 hours the following table applies:

US Navy Dive tables

For dives exceeding these limits, a diver must 'stage' at shallow depths with additional gas supplies before returning to the surface.

Information provided is for general information and must not be used as reference data for diving.