APPC Appendix C 89
Appendix C
SAC AND RMV RATES
SAC Rate (in PSI per minute at surface) and RMV Rate
(in cubic feet per minute at surface) calculations
STEP 1
STEP 2
STEP 3
STEP 4
STEP 5
Determine PSI Used per minute:
PSI Used ÷ Time in minutes = PSI Used
per minute
Determine Pressure at Depth (in ATA):
(Depth ÷ 33) + 1 = Atmospheres Absolute
Pressure at Depth (or Depth ATA)
Determine SAC Rate:
PSI Used per minute ÷ Depth ATA = SAC
Rate
Determine Cylinder Baseline:
Rated cubic feet ÷ Rated PSI =
Cylinder Baseline (cubic feet per PSI)
Determine RMV Rate:
SAC Rate x Cylinder Baseline = RMV
Rate (cubic feet per minute at surface)
Determine Cylinder Volume:
Cylinder Baseline x PSI in cylinder = cubic feet of gas in
cylinder
Figure C-1 Surface Air Consumption (SAC) Rate
and Respiratory Minute Volume (RMV) Rate imperial
formulas.
COURSE NOTES
Use this area for note-taking during your course.
(Figure C-1) An air consumption rate is the speed at which
a diver uses air. Air consumption rates are usually given in
terms of how much air a diver breathes in one minute on the
surface (at one atmosphere of pressure). Knowing your air
consumption rate is useful in scuba diving for dive planning,
gauging comfort/stress, and identifying gear problems. Divers
typically express air consumption using SAC Rates and RMV
Rates.
A Surface Air Consumption Rate, or SAC Rate, is a
measurement of the amount of air a diver uses in one minute
on the surface. SAC Rates are given in units of pressure; either
in psi (imperial, pounds per a square inch) or bar (metric).
Because SAC Rates are given in terms of cylinder pressure, and
not in terms of volume of air, SAC Rates are cylinder specific.
• 500 psi air in a standard 80 cubic foot cylinder corresponds
to 13 cubic feet of air whereas . . .
• 500 psi of air in a low pressure 130 cubic foot cylinder
corresponds to 27 cubic feet of air. And so . . .
• A diver who breathes 8 cubic feet of air/minute will have a
SAC Rate of 300 psi/minute when diving with a standard
aluminum 80 cubic foot cylinder but a SAC Rate of 147
psi/minute when diving with a low pressure 130 cubic foot
cylinder.
Because SAC Rates are not transferable between tanks
of different sizes, divers usually begins air consumption
calculations using their RMV Rate, which is independent of
cylinder size. The diver then converts their RMV rate to a SAC
Rate based on the volume and working pressure of the cylinder
they plan to use on a particular dive.
A Respiratory Minute Volume Rate, or RMV Rate, is a
measurement of the volume of breathing gas that a diver
consumes in one minute on the surface. RMV Rates are
expressed in either cubic feet per a minute (imperial) or liters
per a minute (metric). Unlike a SAC Rate, an RMV Rate can be
used for calculations with cylinders of any volume.
• A diver who breathes 8 cubic feet of air a minute will always
breathe 8 cubic feet of air a minute regardless of the size of
the cylinder that the air is stored in.
For this reason, most divers remember their air consumption
rates in RMV Rate format. Gas planning is typically worked
through in RMV Rate format, and then converted to either psi
or bar based on the type of cylinder to be used.