Don’t we all know the question of how long a scuba tank lasts? We usually answer with a casual “It depends…” and have the depth and our breathing in mind. And quite rightly so: these are precisely the two things we need to knoe in order to be able to calculate our consumption and plan dives accordingly.
The value we want to know is the SAC rate, the surface air consumption. It indicates how many liters of breathing gas you consume per minute. While an adult breathes around 6-8 liters per minute at rest on the surface, this can be much higher under stress. And of course you use much more under water: you breathe under higher pressure and therefore need much more gas to fill the same (lung) volume.
If you are not calculating in liters, this can be done in the cuft-version aswell. But it is up to the ones eho use these units to explain that….
Why is the SAC so important?
Your own SAC rate forms the basis for three crucial questions when diving:
1. situation comparison: How much does my gas consumption increase in stressful or strenuous situations compared to relaxed dives? An experienced diver can have his consumption at rest at about 12-18 liters/minute, while the same diver suddenly consumes 20-30 liters/minute during stress or physical exertion.
2. dive planning: How much breathing gas do I need to take with me for a planned dive? Without knowing your own SAC, realistic gas planning is impossible.
3. reserve calculation: How high must my gas reserve be in order to return safely to the surface in an emergency? This consideration leads directly to the concept of “rock bottom minimum gas”.
How do you calculate the SAC rate?
The Surface Air consumption is the volume of gas that someone consumes, adjusted to sea level. For example, if you breathe 15 times per minute, inhaling and exhaling one liter each time, this would be an SAC of 15 liters (a value that is realistic for diving).
How can you determine your own SAC rate while diving? For this we need a few values from your last dive:
- Consumption: How much bar did you use? e.g. 150 bar from a 10l bottle – 1500barl
- Average depth: We breathe under pressure, every liter is compressed… To be able to compare this, we convert it to the surface. We can do this with the average depth of the dive. Example: Average depth 12m – 2.2 bar
- Time: We want a value per minute, so we need to divide our consumption by the dive time.
Gas consumption: 1500 barl
divided by
Pressure (average depth): 2.2 bar
divided by
Minutes: 45 minutes
150barl/2.2bar/45 min = 15.15 l/min – That would be the SAC rate in our example.
Too much calculating? Then just use the SAC calculator 🙂
