The amount of gas removed from a cylinder for drysuit inflation is marginal. But, for those math guys or those who would like to have a better understanding of gas management and gas use, some of the following may be considered.
The formula for compression compensation is as follows:
SECF = [CVAS - (CVAS/[(Depth/33) + 1])] X [(Depth/33) + 1} X 2
Where SECF is the Surface Equivalent Cubic Feet and CVAS is Compressible Volume At Surface and the X 2 is a safety factor for wasted gas as divers make minor buoyancy adjustments.
For example, if a 160 lb. diver wearing a drysuit and properly balanced single AL80 tank system were to require 26 pounds of ballast near the surface to make an ocean dive, the compressible components of a diver's rig might be 0.40 cubic feet. If that diver were to descend to 100 feet, the math would be as follows:
SECF = {0.40cf - (0.40cf/[(100/33) + 1])} X {(100/33) + 1} X 2
SECF = {0.40cf - (0.40cf/4.03)} X 4.03 X 2
SECF = {0.40cf - .0992} X 4.03 X 2
SECF = 0.3008 X 4.03 X 2
SECF = 2.42 cf or about 2.5 cubic feet of gas.
How many psi is that in an AL 80?
80 cubic feet/3000 psi working pressure of the cylinder = .026 cubic feet per psi X 100 psi gauge increment = 2.6 cubic feet of gas per every 100 psig read on the diver's SPG.
In this case, a skilled diver inflating a drysuit to compensate for a 100 foot dive would need to use about 100 psi from his cylinders to inflate his drysuit.
If you want to be really anal-retentive about gas calculations, there are several points of interest to consider beyond how much gas will be required for drysuit inflation.
The first is Non-Reliable Gas.
Because the analog pressure gauges used by divers are "slow to start", meaning they are less accurate in the beginning of their scales from 0 - 100 psig and only moderately accurate over the remainder of their scales, the last 100 psig left in the cylinder may be referred to as non-reliable gas and not considered available for diving purposes. For example, if your SPG reads your gauge at 2900 psi, you may decide to call that 2800 psi for planning purposes.
The second consideration is Locked Gas.
During a dive, as we descend deeper, our SPG's read pressure values from our cylinders in comparison to atmospheric pressure and not ambient pressure. At any depth, a scuba unit can only be drained to a pressure that is equal to the surrounding pressure. At a depth of 200 feet, for example, approximately 100 psi in a tank will not be extracted from that cylinder. It is "locked" by the pressure exerted upon it. As a diver ascends, gas will become available with decreasing pressure and "unlock".
The third consideration is pressure loss due to Gas Cooling.
The General Gas Law states that, with a volume held constant, the absolute pressure exerted by a gas will vary directly with the absolute temperature.
The equation may be written as: P2 = [P1 X T2]/T1
Divers can expect to see a pressure drop after their cylinders have been filled and are cooling down from the molecular excitement that occurred during the filling process. If gas planning happens at this stage, there may be less gas in the cylinder for the dive than the diver realizes. Cooler water temperatures that are vastly below the air temperature, for example, 80°F air and 53°F water may also reduce available gas due to cooling.
For additional safety, a diver may wish to remove 100 psi due to non-reliable gas, 100 psi due to locked gas at depth, and finally add in a 10% loss due to cooling.
What does all this mean to the diver? Well, most divers will determine rock bottom calculation to be a minimum of 500 psig. If we looked at the worst case scenario of using about 100 psi for suit inflation, 100 psi for non-reliable gas, 100 psi for locked gas and up to 10% of a tank's pressure dropping due to cooling, all of this voodoo can easily be absorbed into the rock bottom calculation of a minimum of 500 psi rock bottom or minimum gas for all dives. For those with more aggressive profiles or who prefer much greater safety factors built into gas management, all these factors can be removed into a 500 psi chunk that is not calculated and all minimum gas and available gas planning can begin using 2500 psig from a 3000 psig cylinder. For the truly anal, all these can be calculated precisely and worked into the available gas planning.
When inflating drysuits with back gas, the gas lost is minimal and unnoticed during the dive. On a day when your breathing rate is excellent your drysuit skill might be off, while on a day when you rarely add any gas to your drysuit, your breathing rate may be less perfect.
Most divers can expect to use an average of 2 cubic feet of gas for open water suit inflation during dives to all recreational depths and up to 4 cubic feet for technical dives. Deeper divers will need greater inflation capacity. Those inflating with argon bottles may need to move to larger bottles than 6cf once venturing to 200 feet and beyond.
As was stated, undergarments can change this average, as well as drysuit inflation skill, and how much or how little gas a diver prefers to have in his or her drysuit. Personally, I like mine tight for streamlining and better control of my feet, and I tend to use very little gas. Other divers prefer to be warmer, wear thicker underwear and hold more gas in the drysuit for insulation.
