How to calculate the gas required for an extended safety stop?

Understanding the Basics of Extended Safety Stop Gas Planning

To calculate the gas required for an extended safety stop, you need to determine your Surface Air Consumption (SAC) rate, then apply it to the total time you plan to spend at the safety stop depth, while also factoring in a substantial reserve for emergencies. The core formula is: Gas Required = (SAC Rate x Depth Pressure x Time) + Reserve. An extended safety stop, typically performed at 5-6 meters (15-20 feet) for 3 to 5 minutes or longer after deeper dives, is a critical procedure for off-gassing excess nitrogen, but it consumes breathing gas that must be meticulously planned for, especially when diving with a single cylinder. This isn’t just a simple math problem; it’s a fundamental aspect of dive safety that integrates physiology, physics, and pragmatic contingency planning.

Step 1: Calculating Your Personal Surface Air Consumption (SAC) Rate

Your SAC rate is the cornerstone of all gas planning. It’s the amount of gas you breathe per minute at the surface, expressed in bar per minute (if using a metric pressure gauge) or cubic feet per minute (PSI/min). There’s no universal “average” – your SAC rate is unique to you and is influenced by fitness, comfort in the water, workload, and thermal stress. To calculate it, you need to conduct a simple test during a relaxed, shallow dive.

How to Conduct a SAC Rate Test:

1. Swim at a steady, relaxed pace on a shallow reef or in a calm bay at a constant depth (e.g., 10 meters / 33 feet is ideal).
2. Note your starting pressure (e.g., 200 bar) and the exact time.
3. Swim for a precise duration, say 10 minutes, maintaining the same depth and effort.
4. Note your ending pressure (e.g., 170 bar).
5. Calculate the gas used: 200 bar – 170 bar = 30 bar.
6. Calculate the gas used per minute: 30 bar / 10 min = 3 bar/min.
7. Now, adjust this for surface pressure. The pressure at 10 meters is 2 bar/ata. So, your SAC Rate = 3 bar/min ÷ 2 ata = 1.5 bar per minute.

This 1.5 bar/min figure is your baseline. You should re-calculate this periodically and use a higher, more conservative rate for planning stressful or challenging dives. A nervous diver or someone fighting a current might have a SAC rate of 2.0 bar/min or higher.

Step 2: Applying Depth and Time to Your SAC Rate

Now, let’s plan for a specific scenario. You’re finishing a dive and plan to perform a 5-minute extended safety stop at 5 meters (15 feet). The absolute pressure at 5 meters is 1.5 bar/ata. Using the conservative SAC rate of 2.0 bar/min we calculated earlier:

Gas Consumption at Depth = SAC Rate x Pressure
2.0 bar/min x 1.5 ata = 3.0 bar per minute.

For a 5-minute safety stop:
3.0 bar/min x 5 minutes = 15 bar.

So, for the safety stop alone, you will consume approximately 15 bar of gas from your cylinder. But this is only the beginning of the calculation. This is the minimum required under ideal, calm conditions.

Step 3: The Non-Negotiable Reserve: Planning for the Unexpected

This is the most critical part of the calculation and where many recreational divers fall short. You must plan for things to go wrong. What if your buddy has an equipment failure and needs to share your air? What if you accidentally descend a few meters during the stop, increasing your breathing rate and depth pressure? What if you need to extend the stop because you’re feeling fatigued? A proper reserve is not a suggestion; it’s a requirement for safe diving.

A common and robust method is the Rule of Thirds: use one-third of your gas for the descent and swim out, one-third for the return and ascent, and keep one-third in reserve. For gas planning focused on the safety stop, a more direct approach is to double or even triple your calculated consumption.

Let’s apply a conservative reserve to our example:

• Calculated gas for 5-min stop: 15 bar
• Reserve for buddy (doubling the consumption): +15 bar
• Contingency for stress/extended time: +10 bar
• Total Gas to Allocate for Safety Stop: 40 bar

This means you must begin your ascent to the safety stop with at least 40 bar more than your absolute minimum reserve pressure (which itself might be 50 bar). So, if your plan is to surface with 50 bar, you need to start your ascent at 90 bar (50 + 40) to comfortably and safely execute the extended stop. This kind of conservative buffering is what separates adequate planning from truly safe planning. For those using smaller secondary systems like a 1l scuba tank for emergency air, calculating the usable gas from that unit with the same rigor is essential, as its capacity is significantly less than a primary cylinder.

Practical Gas Planning Table for Common Scenarios

The following table provides estimated gas requirements for a single diver using a conservative SAC rate of 2.0 bar/min. Remember, these figures include a reserve for a calm emergency and should be increased further for more demanding conditions.

*A 10-minute stop might be used in decompression diving or for added conservatism on a long, deep recreational dive.

Advanced Considerations: Decompression and Rich Mixes

For divers venturing into decompression diving or using enriched air nitrox (EANx), the calculation becomes more complex but follows the same principles. If you are conducting a mandatory decompression stop (not just a safety stop), you are legally obligated to have enough gas to complete it. This involves planning for worst-case scenarios, like a complete gas failure at maximum depth, requiring a controlled ascent and staged decompression on a backup gas supply.

When using nitrox with a higher oxygen percentage (e.g., EAN36 or EAN40), you can sometimes use a shallower stop depth because the oxygen partial pressure is higher, enhancing off-gassing. However, your gas volume calculation remains primarily about the cylinder pressure, not the mix. The SAC rate in bar/minute is largely independent of the gas mixture; you are still moving the same volume of gas, even if its composition is different. The major benefit of nitrox here is physiological, not in terms of raw gas volume needed for the stop itself.

Integrating Gas Planning into Your Overall Dive Plan

Your safety stop gas calculation is not an isolated number. It must be seamlessly integrated into your overall dive plan. This includes your turn pressure—the pressure at which you must end the dive and begin your ascent to ensure you have enough gas for the stop and reserve. Using our previous example, if your total gas supply is 200 bar, and you need to surface with 50 bar, your usable gas is 150 bar. If you allocate 40 bar for the ascent and safety stop, then the gas for the main part of your dive is 110 bar. Your turn pressure would be 200 bar – 110 bar = 90 bar, which aligns perfectly with our earlier calculation. Discussing and agreeing upon this turn pressure with your buddy before the dive is a fundamental safe diving practice. This holistic approach ensures that the extended safety stop, a procedure designed to increase safety, does not become the reason you end up in a low-air emergency.