Recreational Gas Planning

Most divers encounter gas-management rules long before anyone explains where the numbers come from. "Be back on the boat with 500 PSI" is a familiar instruction in recreational dive briefings. I've also heard divers insist, "I've paid for this dive, I'm going to use all the gas." The first establishes an expectation for how much gas should remain when the dive is over. The second treats anything left in the cylinder as something the diver has paid for but failed to use. Neither statement, by itself, tells you how much gas you need to complete the dive safely.

Returning to the boat with 500 PSI may be perfectly reasonable, but that number tells you nothing about when you should begin your ascent. Getting two divers to the surface from one hundred feet requires a different amount of gas than ascending alone from thirty. The requirement changes with depth, cylinder capacity, breathing rate, and the time needed to manage an emergency.

The idea of using every last bit of gas presents a different problem. Gas remaining in your cylinder is not wasted simply because you did not breathe it. Some of that gas has a specific purpose: keeping you and your buddy alive if something goes wrong. Once that reserve has been calculated, the rest becomes available for the dive. Using all of that usable gas can be an entirely reasonable strategy. Consuming the emergency reserve simply to extend the dive is not.

Start With the Dive You Are Actually Planning

A shallow reef dive with an unrestricted ascent is fundamentally different from a dive that takes you away from a preferred exit point. Both require an emergency gas reserve, but they may require different strategies for allocating the gas available above that reserve.

In ordinary open water, the surface may be directly accessible throughout the dive. If conditions deteriorate or a teammate encounters a problem, the team can begin ascending without retracing its route. That flexibility reduces the gas needed to leave the environment.

Other dives involve a return journey even though a direct ascent remains possible. You might be following a shoreline, exploring a reef from a moored boat, or swimming away from a descent line that you intend to use on the way back. Current, visibility, navigation, and the effort required to reach that exit affect the gas needed for the return.

The strategy follows from those circumstances. A direct-ascent dive and a return-to-exit dive can use the same cylinder and Minimum Gas Reserve yet require different turn pressures. Choosing the strategy is therefore a diving decision supported by mathematics, not a mathematical rule applied without regard to the environment.

Know Your Gas Consumption

Respiratory Minute Volume, or RMV, describes how much breathing gas you consume per minute at the surface, expressed as a volume such as cubic feet per minute. Using volume allows the same consumption rate to be applied to different cylinders.

Surface Air Consumption, or SAC, is often expressed as a pressure change per minute. It can be useful for reviewing dives, but PSI per minute depends on cylinder capacity. A diver breathing at the same rate will show a different pressure-consumption rate when switching to a different cylinder. RMV avoids that limitation by using gas volume rather than gauge pressure.

Depth determines how quickly that volume is consumed from the cylinder because each breath requires more gas at greater ambient pressure. Workload also matters. Swimming against current, managing unfamiliar equipment, or assisting another diver can increase consumption considerably.

Use an RMV that reflects the work you expect rather than automatically choosing the lowest number from your logbook. An easy reef dive may provide a useful baseline, but it does not necessarily represent your breathing rate during a strenuous return or emergency.

Minimum Gas Reserve Comes First

Minimum Gas Reserve, or MGR, is the amount of gas needed to manage a credible emergency from the most demanding point of the planned dive. For ordinary recreational open-water diving, the scenario is typically a buddy losing their gas supply at depth and both divers making a controlled ascent while sharing one cylinder.

The calculation accounts for two divers consuming gas, ambient pressure, time spent resolving the problem, and the ascent itself. A deeper dive generally requires more emergency gas because the divers are breathing at greater pressure and must travel farther to reach the surface.

This is why I do not favor carrying a universal 500 or 1,000 PSI reserve from one dive to the next. The meaningful question is whether that pressure represents sufficient gas for the emergency you have planned to manage.

For example, the separate MGR guide calculates approximately 1,150 PSI for its illustrated 100-foot dive using an AL80 and the stated consumption and ascent assumptions. That number comes from the requirements of the dive rather than a familiar gauge reading.

With MGR established, we can determine how much gas remains available for the ordinary portion of the dive.

From Minimum Gas to Usable Gas

Usable gas is the difference between starting pressure and the calculated Minimum Gas Reserve. For the following examples, assume an AL80 starts at 3,000 PSI and that a separate calculation has established an MGR of 800 PSI.

\[ \begin{aligned} \text{Usable Gas} &= \text{Starting Pressure} - \text{MGR}\\[6pt] &= 3000 - 800\\ &= \boxed{2200\ \text{PSI}} \end{aligned} \]

We now have 2,200 PSI to allocate according to the dive plan, while 800 PSI remains set aside for the defined emergency. The next decision is how much of that usable gas may be consumed before the team turns or begins ascending.

Two Ways to Allocate Usable Gas

All Usable Gas

All usable gas is the simplest approach when a direct ascent is available and no return swim is required before surfacing. The gas above MGR is available for the ordinary dive, with the calculated emergency reserve protected.

Using our example:

\[ \begin{aligned} \text{Usable Gas} &= 3000 - 800 = 2200\ \text{PSI}\\[6pt] \text{Ascent Pressure} &= 3000 - 2200\\ &= \boxed{800\ \text{PSI}} \end{aligned} \]

That is the minimum-pressure boundary established by the example, not an instruction to continue swimming until the gauge reads 800 PSI regardless of the conditions. Gas required to complete the intended exit must be accounted for, and the team must begin its ascent while it can still do so within the plan.

Consider a relatively shallow site where the team can begin ascending directly from any point. Once an appropriate emergency reserve has been calculated, there may be little reason to reserve another substantial portion of the cylinder for a return journey that the dive does not require.

Additional unused gas is not automatically evidence of better planning. The objective is to protect the gas needed for the actual circumstances rather than shorten the dive simply because a particular allocation sounds conservative.

Rule of Halves

Halves becomes useful when the dive includes a return journey. The usable gas is divided into two portions, one for traveling away from the exit and another for returning. Minimum Gas Reserve remains separate.

\[ \begin{aligned} \text{Usable Gas} &= 3000 - 800 = 2200\ \text{PSI}\\[6pt] \text{Half of Usable Gas} &= \frac{2200}{2} = 1100\ \text{PSI}\\[6pt] \text{Turn Pressure} &= 3000 - 1100\\ &= \boxed{1900\ \text{PSI}} \end{aligned} \]

At 1,900 PSI, the diver has consumed half of the usable gas and begins the return. If the return requires approximately the same consumption as the outbound journey, the diver reaches the exit with the calculated 800 PSI emergency reserve still available.

This is useful when you prefer to return to a descent line, shore entry, or boat even though a direct ascent remains possible in an emergency. It provides an allocation for the return instead of relying on whatever gas happens to remain.

The assumption is that the journey back will not require substantially more gas than the journey out. Swimming downcurrent initially and returning against the flow can invalidate that assumption. Increased workload, deteriorating conditions, and differences in consumption between teammates can do the same.

Where the return is likely to be more demanding, the team should turn earlier or adjust the route. Halves provides a framework, not permission to ignore the conditions.

Rule of Thirds: Why It Doesn't Belong in Recreational Gas Planning

The Rule of Thirds comes from overhead diving, particularly cave diving and wreck penetration. In those environments, a diver cannot simply ascend when something goes wrong. The team must return along its penetration route before reaching open water, potentially while sharing gas.

The traditional rule allocates one third of the starting supply for penetration, one third for the exit, and the final third as an emergency reserve. That reserve serves the purpose of Minimum Gas Reserve, although allocating a third does not, by itself, establish that the available gas is sufficient for every emergency.

For a cylinder starting at 3,000 PSI:

\[ \begin{aligned} \text{One Third} &= \frac{3000}{3} = 1000\ \text{PSI}\\[6pt] \text{Turn Pressure} &= 3000 - 1000\\ &= \boxed{2000\ \text{PSI}}\\[6pt] \text{Reserve After Return} &= \boxed{1000\ \text{PSI}} \end{aligned} \]

This is a different gas-management problem from recreational open-water diving. In recreational diving, we can calculate the emergency gas needed for two divers to ascend from the maximum planned depth. That becomes our MGR, rather than relying on a predetermined fraction of the cylinder.

If the dive permits a direct ascent, we can use the gas available above MGR without allocating additional gas for a return journey. If we intend to swim away from an exit and return to it, the Rule of Halves divides that usable gas between the outbound and return portions, with MGR remaining protected.

Halves therefore provides the out-and-back allocation appropriate to recreational diving, while MGR provides the calculated emergency reserve. There is no need to import the Rule of Thirds from cave and wreck penetration when the dive does not present the problem that rule was developed to address.

The Team Has One Gas Plan

Gas planning is a team responsibility, even though each diver carries their own cylinder. Both divers need to understand when the team will turn and whether sufficient gas is available to deal with a failure affecting either person.

Two divers starting with 3,000 PSI do not necessarily carry the same gas volume. Different cylinder capacities and breathing rates can create substantial differences in available gas and how quickly it is consumed. For this reason, the team should establish each diver's applicable limits and agree to act when the first reaches their turn condition.

More substantial differences in cylinder capacity or consumption may require gas matching rather than relying on equal pressure readings. Whatever the configuration, gas checks should be part of ordinary communication throughout the dive, not something performed only when a computer alarms.

Conditions Can Change the Plan

Gas planning happens before entering the water, but gas management continues throughout the dive. Cold, current, difficult navigation, equipment problems, or an unexpected task can increase consumption and make the original assumptions less reliable.

Depth changes those assumptions as well. Greater ambient pressure increases gas consumption, and an emergency ascent from deeper water requires additional time and gas. A reserve calculated for one maximum depth should not be reused unchanged for a deeper dive.

When actual conditions differ from the plan, adjust before reaching the original turn pressure. That might mean turning earlier, shortening the route, beginning an ascent, or ending the dive. The arithmetic gives you a framework for recognizing when those decisions are necessary.

Make the Numbers Part of the Briefing

Before entering the water, both divers should know their starting pressures, calculated MGRs, agreed gas-management strategy, and applicable turn pressures. They should also understand the route, planned depth, exit options, and conditions that would trigger an early return or ascent.

A turn pressure is a decision point associated with the intended dive. MGR is the emergency gas that the plan protects. Under an all-usable-gas allocation, those numbers may be identical, but they represent different concepts.

The advantage of doing the calculations beforehand is that you do not have to solve them underwater. You monitor the remaining gas, compare it with the plan, and recognize when conditions require a change. Once you understand how consumption, reserve, route, and turn pressure work together, gas management becomes a deliberate part of the dive rather than an exercise in watching the gauge and hoping the numbers work out.