In its 2022 article on gas planning, DAN Europe calculates a square dive at 30 meters with a buddy running out of air. Result: about 50 bar from an AL80 tank is used just during the ascent of two divers. The author therefore advises keeping rather 80 to 100 bar.
This figure challenges the 50 bar reserve that many divers learned during training. However, it does not make it wrong; it mainly shows that it depends on the depth and the distance to the exit, not forgetting your own way of breathing when effort or stress come into play.
This update revisits reading the pressure gauge, the reserve, the rule of thirds, and consumption calculation. Unverified dates and some shortcuts have been removed from the old version. The rest has been cross-checked.
How to read a diving pressure gauge?
The pressure gauge displays the pressure of the gas remaining in your tank, in bar in Europe and psi in the United States. A tank filled to 200 bar contains 200 times its volume of air at surface pressure. So you read a quantity of gas, never a remaining dive time.
On a mechanical model, a needle moves over a graduated dial, often up to 300 or 400 bar. The red zone marks the low end of the scale. Its exact threshold varies by manufacturer, and it does not replace the reserve decided at the briefing. Before entering the water, open the valve, breathe a few times on the regulator, and watch the needle. If it drops with each breath then rises, the valve is probably not fully open.
A single reading tells almost nothing.
Thus, what matters is the difference between two readings separated by a few minutes, because it gives you your consumption rate at that depth and allows you to anticipate when you will reach the return pressure. Get into the habit of checking at regular intervals. Announce your pressure when the guide asks for it.
At what pressure should you ascend?
In France, the most common convention sets a reserve of 50 bar at the end of the dive, with a halfway pressure checkpoint. DAN Europe reminds that these 50 bar are enough for a shallow dive close to the exit point. The farther or deeper you go, the more you need to keep.
Why? Because the reserve must cover the ascent, the safety stop, and a problem to solve on the way. In DAN Europe’s example at 30 meters, with a flow rate of 15 liters per minute and a buddy to rescue, the gas consumed reaches 552 liters. That is about 50 bar in an 11.3-liter AL80. The author then adds a margin of 50 to 100%, as stress increases breathing, arriving at a reserve of 80 to 100 bar for this two-person dive profile.
The reserve figure is set at the briefing, not underwater en route.
But the halfway pressure remains a useful marker. On a tank starting at 200 bar, it falls to 100 bar. The gestures to announce it are not the same in all schools or countries. Check them with your guide before entering the water. For what level? A supervised beginner follows the instructor’s thresholds, and the instructor gives the signal to return.
The rule of thirds, for whom and in which cases?
The rule of thirds divides the tank into three parts. One third is for the outbound leg, one third for the return, and the last third remains as a reserve for delay or emergency. DAN described it in 2010 in Alert Diver. It mainly applies to dives where you return the same way.
This is the case for caves, penetrated wrecks, or shore dives with a long return trip. Take a tank at 210 bar. One third is 70 bar. You turn around when the gauge shows 140 bar, then you must reach the exit with at least 70 bar. With a start at 200 bar, the calculation yields less round numbers, and we generally round up.
However, the rule only takes into account your own gas.
If your buddy breathes faster than you, their third does not necessarily cover a two-person return on a single tank, especially if the problem occurs at the point farthest from the exit. That is why technical dives, such as trimix diving, add a minimum gas calculation for each team member. In recreational diving without a ceiling, the rule remains a cautious framework. Other methods exist.
How to calculate your air consumption?
Your surface consumption, noted SAC or RMV, is measured in bar or liters per minute. At a stable depth, note the pressure, swim for ten minutes, then reread the pressure gauge. Then divide the drop by the time, then by the absolute pressure at the moment, as detailed by DAN in a 2010 article dedicated to this measurement method.
The absolute pressure equals the depth divided by 10, plus 1. At 10 meters, you are at 2 bar, so each breath takes twice as much gas as on the surface. DAN’s example is simple. The diver loses 40 bar in 10 minutes at 10 meters, which is 4 bar per minute. Divided by 2, this gives 2 bar per minute at the surface. With a 12-liter tank, that amounts to 24 liters per minute.
The table below starts from a surface flow rate of 20 liters per minute, chosen for the example. The figures are indicative, and your own flow rate changes with effort, cold, and stress.
| Depth | Absolute Pressure | Liters per Minute | Bar per Minute (12 L tank) |
|---|---|---|---|
| Surface | 1 bar | 20 | 1.7 |
| 10 m | 2 bar | 40 | 3.3 |
| 20 m | 3 bar | 60 | 5 |
| 30 m | 4 bar | 80 | 6.7 |
| 40 m | 5 bar | 100 | 8.3 |
And the duration calculation follows.
A 12-liter tank at 200 bar contains 2,400 liters. If you keep 50 bar, or 600 liters, you have 1,800 left at 60 liters per minute at 20 meters. That gives about thirty minutes at the bottom, not counting descent or ascent. Your dive computer can do this calculation live if it receives the tank pressure.
Why does the pressure drop faster than expected?
Three causes come up. Effort and stress accelerate breathing. Cold water cools the tank and lowers the pressure reading. A leak on a fitting, hose, or a free-flowing regulator empties the tank without you breathing more. Each is detected with more frequent readings.
Cold often misleads beginners because the pressure of a gas follows its absolute temperature. A tank filled to 200 bar at 30 °C and cooled in water at 14 °C shows about 190 bar, according to gas laws. You haven’t breathed any gas; it just contracted. That’s why the starting pressure is read once the tank is at water temperature.
However, a rapid and continuous needle drop has nothing to do with cold.
In this case, warn your buddy, end the dive, and ascend according to the procedure learned in training. A leaking regulator signals equipment failure. Pre-dive checks remain your best filter. Maintenance of the dive regulator and hose is done by a qualified workshop following the manufacturer’s intervals. On a site with current, like some reefs in the Red Sea in Egypt, the effort of kicking against the current can significantly increase your consumption compared to a calm dive. Your pressure gauge will show this faster than any prediction.
Mechanical pressure gauge, console, or transmitter: which to choose?
The mechanical pressure gauge, connected by a hose to the first stage, works without a battery and can be read at a glance. The console groups this gauge with a compass or depth gauge. The wireless transmitter screws onto the regulator and sends the pressure to a wrist computer, which calculates the remaining autonomy.
Each solution has its limits. A hose wears out and sometimes gets caught in equipment. A transmitter depends on a battery and radio link, and it can lose signal for a few seconds. Many divers therefore keep a mechanical pressure gauge as a backup with a transmitter. In short, the best option depends on your practice and where you can have your equipment repaired if a part fails during a trip far from any specialized workshop.
None of these instruments decides the reserve for you.
But the fundamental question remains the volume of your dive tank and your actual consumption. A more precise display changes nothing if you don’t read it. Keeping 80 bar instead of 50 costs you a few minutes at the bottom, in exchange for a real margin to ascend as a pair.
FAQ
What reserve should be kept in recreational diving?
The French convention often sets 50 bar at the end of the dive. DAN Europe shows that at 30 meters, with a buddy to rescue, you should rather aim for 80 to 100 bar. The chosen figure is set during the briefing with your guide.
What does half-pressure mean?
It is half of the starting pressure, i.e., 100 bar for a tank filled to 200 bar. This marker is often used to signal the guide that it is time to think about returning. The gesture used varies according to the schools, check it before the dive.
How do I calculate my air consumption?
Note the pressure at a stable depth, swim for ten minutes, then read again. Divide the drop by the time, then by the absolute pressure (depth divided by 10, plus 1). Multiply by the tank volume to get liters per minute.
Why did my pressure drop even before the dive?
A tank filled hot loses pressure as it cools in the water. A tank at 200 bar at 30 °C shows about 190 bar in water at 14 °C. If the needle continues to drop without you breathing, look for a leak.
Sources
- Let’s talk about… Gas Planning (DAN Europe, Alert Diver, 2022)
- Estimating Your Air Consumption (Divers Alert Network, Alert Diver, 2010)
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