For most divers, the memory of their very first scuba certification remains vivid: equalising ears for the first time, hovering awkwardly at five metres, and being told - firmly - that dives must end with at least 50 bar (or 500 PSI) remaining in your cylinder. This simplistic rule is still a staple in open water courses worldwide, rooted not in precision gas planning, but in the acknowledged unreliability of analogue SPGs (Submersible Pressure Gauges) at low pressures.
However, as your diving matures, and especially as you move into more advanced environments, this “50-bar rule” is not only insufficient but potentially dangerous. It offers no guidance on when you should begin your ascent to safely make it back to the surface, nor does it take into account the possibility that a teammate might require your gas in an emergency.
This article delves into the nuanced and often neglected world of gas planning and ascent strategy, drawing on the script above to build a comprehensive understanding of how to plan your dives with clarity, confidence, and care. Whether you're a budding technical diver or a recreational diver seeking to develop good habits, what follows is essential knowledge.
Ending your dive when a team member hits a pre-set pressure - say 100 bar or 80 bar - is a lazy approach. It feels like planning, but it misses the critical question:
“When should I begin my ascent so that I and my teammate can both return safely, even in an emergency?”
Good diving is about being deliberate. And that means we calculate - not guess - how much gas we need to get home safely, including worst-case scenarios.
Before we can do any meaningful gas calculations, we must first understand our ascent profile. That’s right - gas planning starts not with the depth or the dive time, but with how you intend to ascend.
Most recreational agencies use a default DSAT-style ascent strategy:
- 9 metres per minute ascent to 5 metres
- 3-minute “safety stop” at 5 metres
- Ascend to surface
But experienced divers know that slowing down the ascent - especially from shallower depths - adds significant safety. A fast ascent from 5 metres can introduce microbubbles and increase the risk of decompression stress. Thus, we recommend a conservative 1-minute ascent from 5 metres to the surface.
Let’s add a final minute as contingency for deploying a DSMB (delayed surface marker buoy), managing an out-of-gas emergency, or simply pausing to assess conditions.
So, for a dive to 30 metres, your ascent profile may look like this:
- 3 minutes: 30m to 5m
- 3 minutes: safety stop
- 1 minute: ascent to surface
- 1 minute: contingency
→ Total: 8 minutes
This becomes our foundation for all calculations to follow.
The Minimum Reserve is not your private emergency stash. It’s the gas you carry for your teammate in case they suffer a complete gas failure at the deepest point of the dive. It is sacred - you must never tap into it under normal diving conditions.
To calculate this, you need:
- Emergency Surface Air Consumption Rate (SAC) – usually 30 L/min in a stress situation
- Average depth during ascent – from our profile, roughly 15 metres
- Ambient pressure at average depth – 2.5 ATA (1 ATA for surface + 1.5 ATA for 15m)
- Ascent duration – 8 minutes
- Number of divers – always 2 (you and your teammate)
Reserve = 30 L/min x 8 mins x 2.5 ATA x 2 divers = 1200 litres
Now, to express this in bar, divide by your cylinder size.
Assuming an 11.1-litre cylinder:
1200 ÷ 11.1 = ~108 bar → round to 110 bar for easier readability
So, your Minimum Reserve is 110 bar.
With a 200-bar fill and a 110-bar reserve, you are left with:
200 - 110 = 90 bar usable gas
This is the gas you can actually use for the dive itself. The rest is untouchable.
There are two broad types of dive profiles:
In these, you do not return to the starting point. Your turn pressure is simply the reserve. Once your SPG hits that number, you ascend.
Example:
- Start: 200 bar
- Reserve: 110 bar
- Dive until: 110 bar → then ascend
In these, you must return to your entry point, retracing your path. You now apply the rule of halves to the usable gas.
With 90 bar usable, you get:
90 ÷ 2 = 45 bar for outbound
So, your turn pressure becomes:
200 - 45 = 155 bar
That means once a teammate hits 155 bar, it’s time to turn and head back.
Many divers have been taught to apply the Rule of Thirds:
- 1/3 out
- 1/3 back
- 1/3 reserve
This principle works only if your reserve calculation aligns with a third of your starting gas, which it often does not - especially in deeper or longer dives.
Let’s examine our example again:
- 1/3 of 200 bar = ~65 bar
- 65 bar < 110 bar (our actual required reserve)
This creates a dangerous shortfall. If you had planned this dive using thirds, you would not have enough gas to get both you and your teammate to the surface in an emergency.
Moral of the story? Don’t use thirds unless you’ve proven it gives you the correct reserve.
It’s common for dive teams to have varying tank sizes - some divers may have 15L cylinders, others 11.1L or even 7L. This does not change the reserve calculation logic:
Always calculate gas planning based on the diver carrying the smallest volume of gas.
Why? Because that diver will be the limiting factor in any gas-sharing scenario. If you plan based on the largest cylinder, and the smallest one goes dry, your calculations are invalid - and your teammate is endangered.
All members of the team must respect the same turn pressure, even if some could technically stay longer.
The scuba industry has normalised procedures that do not hold up under scrutiny. Standardised rules like “end with 50 bar” or “follow the thirds rule” might suffice in benign conditions, but they do not equip divers to handle emergencies with clarity and margin.
Professional, responsible, and team-oriented divers calculate gas requirements based on reality, not routine. They factor ascent times, team gas sharing, stress SAC rates, and cylinder capacities into their plan. Anything less is gambling.
- Review your dive log
Look at your deeper dives. Using the above method, re-calculate your actual minimum reserve. Did you actually have enough gas for two divers to ascend from the bottom?
- Practice emergency SAC rates
In controlled conditions, simulate stress-breathing and measure your SAC. This will help you build personal accuracy into your planning.
- Educate your dive buddies
Share this knowledge with your teammates. Consistent team behaviour saves lives.
- Move beyond passive diving
Don’t rely blindly on guides, computers, or instructors. Build your awareness. Every dive should be one you understand from first breath to last bubble.
Gas planning is more than arithmetic. It's a mindset of responsibility, of anticipating risk, and of committing to the safety of your team. The difference between a lucky dive and a well-planned one is usually invisible - until something goes wrong.
And when it does, it’s not the best diver who survives - it’s the best-prepared team.
Welcome to the world of deliberate diving. Welcome to true dive mastery.
