Respiratory Minute Volume (RMV) is the amount of air you breathe measured in cubic feet per minute. You’ll often hear someone saying their SAC rate is .6. That’s actually incorrect. Surface Air Consumption (SAC) is a measure of PSI per minute breathed. Someone might have, for instance, a SAC rate of 12psi/min. SAC is specific to the cylinder size it is determined on.
RMV, on the other hand, can be used with any cylinder, regardless of size since it is a measure of volume, not pressure. When determining RMV, you must know your cylinder’s rated pressure. Rated pressures are typically stamped around the top portion of the cylinder. However, aluminum 80s, for example, actually only hold 77.4 cubic feet of gas. For ease of reference, they have come to be called 80s. And low pressure steel cylinders are rated to 10% over the number stamped on the cylinder. You will see “2400” stamped on LP cylinders, but they are actually rated to 2640 PSI.
Following are rated pressures for common cylinder sizes:
80 – 3100
100 – 3300
Low Pressure cylinders (LP) – 2640
High Pressure cylinders (HP) – 3442
Steel 72s – See numbers stamped on cylinder. Will vary dependent on when cylinder was manufactured.
Three different RMVs are typically calculated – Average, Resting, and Working. Average RMV is calculated by descending to a set depth (33 fsw or 34 ffw works best), noting starting pressure, swimming at a comfortable pace at the same depth for 5 minutes, and noting ending pressure. Enter the numbers into this form to calculate average RMV. Resting RMV is calculated by descending to a set depth (again 33 fsw or 34 ffw), noting starting pressure, hovering at that depth for 5 minutes, and noting ending pressure. In my courses, I actually have my students do this at 15 or 20 ffw during a 5 minute safety stop. Working RMV is calculated by descending to a set depth, finding an unmovable structure (a big rock works great), noting starting pressure, swimming as fast as you can while trying to move it for 2 minutes, and noting ending pressure. Once you have your 3 RMVs calculated, you can use them to safely plan your dives.
This form is to be used to get an estimate of your gas consumption rate. It is only as accurate as the method used. Use the RMV calculated using this form at your own risk to plan your dives. Regardless of RMV and dive plan, all divers should monitor cylinder pressure regularly throughout the dive and begin ascent at a predetermined pressure as calculated during dive planning. In other words, if you get injured or die, it’s not my fault!