Page 47
Doc. 13301-Tec-RevB (2022-05-25)
Technical Modes
Operating Instructions
9.6. SAC calculations
Surface Air Consumption (SAC) is the
rate of change
of tank pressure
, normalized as if at 1 atmosphere of
pressure� The units are either psi/minute or bar/minute�
The Perdix 2 calculates SAC averaged over the last two
minutes� The data from the first 30 seconds of a dive
are discarded to ignore the extra gas that is typically
used during this time (inflating BCD, wing, or dry suit)�
SAC vs RMV
Since SAC is simply based on rate of tank pressure
change, the calculations do not need to know the
tank size� However, this means that the SAC is NOT
transferable to tanks of a different size�
Contrast this to respiratory minute volume (RMV)
which is the volume of gas your lungs experience
per minute, measured in Cuft/min or L/min� The
RMV describes your personal breathing rate, and is
therefore independent of tank size�
Why SAC instead of RMV?
Since RMV has the desirable property of being
transferable between tanks of different sizes, it
seems to be the better choice on which to base GTR
calculations� However, the main drawback of using
RMV is that it requires setting up tank size correctly for
each tank� Such setup is easy to forget and is also easy
to setup incorrectly�
SAC has the great property of not requiring any setup,
making it the simplest and most reliable choice� The
drawback is that it is not transferable between tanks of
different sizes�
SAC Formula
The SAC is calculated as follows:
The time samples are taken 2 minutes apart, and
P
amb,ATA
is the average ambient pressure (i�e� depth) over this
time frame�
Since the Perdix 2 displays and logs SAC, the formula
for calculating RMV from SAC is useful� Knowing
your RMV can help with planning dives using tanks of
various sizes�
Calculating RMV from SAC - Imperial units
In the imperial system, tank sizes are described using
two values; capacity in Cuft at a rated pressure in psi�
For example, a common tank size is 80 Cuft at 3000
psi�
To convert SAC in [psi/minute] to RMV in [Cuft/
minute], calculate how many Cuft are stored per psi,
then multiply this by the SAC to get RMV�
For example, a SAC of 23 psi/min with an 80 Cuft
3000 psi tank would be an RMV of (23 x (80/3000))
= 0�61 Cuft/min�
Calculating RMV from SAC - Metric units
In the metric system, tank sizes are described using
a single number, the tank’s physical size in liters [L]�
This is how much gas could be stored at a pressure of
1 bar, so effectively the units of tank size are [L/bar]�
This makes converting SAC to RMV easy� When using
metric units, simply multiply the SAC by tank size�
For example, a SAC of 2�1 bar/min with a 10 L tank
would be an RMV of (2�1 x 10) = 21 L/min�
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