SAC and RMV
SAC stands for Surface Air Consumption and RMV for Respiratory Minute Volume. Your SAC rate is to measure the amount of gas consumed in bar while you are breathing for one minute on the surface. While the RMV measure the amount of gas consumed in liters per minute on the surface. Note, in our examples we will use the metric system, meter, bar and l/min.
Keep in mind, that your SAC rate is cylinder specific. If your SAC rate was based on the use of a 12 liter cylinder but you change for example to a 15 liter, than you need to recalculate your SAC rate.
The benefit of knowing how to calculate SAC and RMV rate is that you can use these values during your dive (gas) planning. This is as well for a recreational as for a technical diver an important aspect because running out of gas is one of the most common issues in diving incidents.
Time needed: 4 minutes.
How to calculate SAC and RMV
- Check consumed gas
Check the amount of gas you consumed in bar during the dive or period of measurement, start pressure minus end pressure.
- Note the average or maximum depth
For a complete dive you can use the average depth. For a period of measurement at a constant depth use this depth, both depths are in ATA.
- Determine the time
Use the total dive time for a complete dive or the time, i.e. 10 minutes for a period of measurement at a constant depth.
- Calculate total bar used / total dive time = bar per minute used
i.e. 200 - 150 bar / 10 min. = 5.0 bar per minute used
- Calculate average or maximum depth in ATA
(depth / 10 m) + 1 = Atmospheres Absolute (ATA)
i.e. (20 m / 10 m) + 1 = 3 ATA
- Calculate SAC
SAC = (total bar used / total dive time) / (average or maximum depth in ATA)
i.e. (200 - 150 bar / 10 min.) / (20 m / 10 m) + 1
i.e. SAC = 5 / 3 = 1.67 bar/min.
- Calculate RMV
RMV = SAC x Cylinder size in liter, i.e. 10 liter, 12 liter, 15 liter
i.e. RMV = 1.67 x 12 = 20 l/min.
To find the answer, determine how many liters you consume at 30 m, 18 l/min x 4 ATA = 72 l/min. Calculate your total gas, 12 x 120 bar = 1440 liter. 1440 / 72 = 20 minutes. The answer is 20 minutes.
It is better to determine your SAC and RMV over multiple dives and under different conditions. i.e. current, warm and cold water, limited and good visibility, effort, at rest, etc. With this approach you will get a more accurate average SAC and RMV rate.
There is no such thing as a best SAC or RMV rate however you see that there is an average RMV of 20 l/min at work and around 15 l/min at rest. But don´t take these numbers as a fixed value, calculate yours and work with it.
SAC stands for Surface Air Consumption and RMV stands for Respiratory Minute Volume.
We wish you a good SAC and RMV calculation, last but not least safe and awesome diving!