Surface Air Consumption (SAC) Rate Calculator
Disclaimer: I take no responsibility for the accuracy of the
calculations and data below. Always verify against other sources.
This calculator can be used to find the Surface Air Consumption (SAC)
in litres per minute based on the provided average diving depth in metres,
length of time at that depth in minutes, how may bar of breathing gas was used,
and the size of the cylinder used in litres.
Just enter values for "Time at Depth", "Average Depth", "Gas Used",
and "Cylinder Size" and click on the "Submit" button.
Diving for 30 minutes at an Average Depth of 20 metres with Gas Used of 150 bar from a Cylinder Size of 12.2 litres
your Surface Air Consumption (SAC) rate rounding up is:
20.4 litres per minute
Disclaimer:
This calculator provides information about gas use in scuba diving.
UNDER NO CIRCUMSTANCES is there any implication
that any gas mixture is safe or even life supporting at any depth.
If you have not recived training in scuba diving, you should not attempt
to use any of these results. Improper use of gasses in diving
can result in injury or DEATH.
Explanation
When planning technical dives it's essential to calculate if there is
sufficient breathing gas to complete the dive. Gas management planning
strategies vary, and there are many combinations and permutations of
dive scenarios that determine exactly which strategy will be adopted.
In order to calculate the quantity of breathing gas required, we need
to be able to estimate our gas consumption rate under various diving conditions.
Typically divers will work out their consumption rates for during the descent phase,
working on the bottom, the ascent phase, and resting at decompression
and/or safety stops.
This is done by recording the Average Depth in metres for the part of the dive
under scrutiny, the cylinder pressure in bar at the start of the period, the cylinder
pressure in bar at the end of the period, the Cylinder Size in litres based on its
water capacity, and the Time at Depth for the duration of the part of the dive. By
subtracting the starting cylinder pressure from the end cylinder pressure we get
the Gas Used in bar for this part of the dive.
From this information we can calulate the breathing gas consumption rate at depth
and convert it to an equivalent consumption rate at the surface
 surface air consumption (SAC) rate  which we can then use in planning
subsequent dives at various depths.
Formula
To calculate the equivalent air depth (EAD) using Nitrox for a specific FO_{2}
and planned diving depth, the following formulas are used:
Metric  metres
SAC (lpm) = ((Gas Used * Cylinder Size) / ((Average Depth / 10) + 1)) / Time at Depth
In which Gas Used in bar is the cylinder pressure at the start of the period,
minus the cylinder pressure at the end of the period, Cylinder Size is the
water capacity of the cylinder in litres, Average Depth is the average depth for
the period of the dive in metres, Time at Depth is the duration
of the period of the dive in mintues, and SAC is the surface air consumption rate
in litres per minute.
For example, if we use 150 bar of gas from a cylinder with a water capacity size
of 12.2 litres, at an average depth of 20 metres for a time at depth of 30 minutes,
the SAC (lpm) is (((150 * 12.2) / ((20 / 10) + 1)) / 30) = 20.4 litres per minute.
Last modified:
Friday, 18 February 2011
