13 Facts About Bottom gas

Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive.

Term "rock bottom gas planning" is used for the method of gas planning based on a planned dive profile where a reasonably accurate estimate of the depths, times, and level of activity is available, so the calculations for gas mixtures and the appropriate quantities of each mixture are known well enough to make fairly rigorous calculations useful.

The Bottom gas must have a breathable density at the maximum depth intended for its use.

Amount of Bottom gas needed on a dive depends on whether the scuba equipment to be used is open, semi-closed or closed circuit.

Formal and relatively complete procedure for scuba Bottom gas planning assumes that a dive plan is available that is sufficiently detailed that most of the variables are known.

Divers following this rule, one third of the Bottom gas supply is used for the outward journey, one third for the return journey and one third is held in reserve in case of an emergency.

The pressure change in the cylinder used to store this Bottom gas depends on the internal volume of the cylinder, and is calculated using Boyle's law:.

However, in at least one regulator design, once the interstage pressure has been sufficiently reduced, the inflatable second stage servo-valve will deflate and effectively lock open the demand valve, allowing the residual Bottom gas to escape until the cylinder pressure has dropped to approximately equal the ambient pressure, at which point flow will stop until the ambient pressure is reduced by ascending to shallower depth.

Critical pressures are calculated by adding up all the volumes of Bottom gas required for the parts of the dive after the critical point, and for other functions such as suit inflation and buoyancy control if these are supplied from the same set of cylinders, and dividing this total volume by the volume of the cylinder set.

Temperature of the Bottom gas should be taken into account when checking critical pressures.

Diluent Bottom gas use is closely linked to depth variation, as it must be added to maintain loop volume when descending, and is vented to the surroundings and lost when ascending.

Each Bottom gas must be provided in sufficient quantity to adequately supply the diver throughout the relevant sector of the dive.

The mass of each component of a Bottom gas is calculated for the volume of that component, using the Bottom gas fraction for that component.