scuba diving gas management

Surface Air Consumption

Surface Air Consumption (SAC) is a useful measurement of air consumption at surface pressure, which enables the diver to calculate their predicted air consumption at any depth on a subsequent dive.  This allows the issue of gas duration to be factored into their overall dive and contingency planning.

1)  Calculating your Surface Air Consumption

In order to calculate your SAC, you will need to know your gas consumption for a given time, at a given depth.  Simply spend 10 minutes undertaking normal activity at a set depth and monitor your SPG to see how many BAR of gas your have consumed.

It is important to be aware that calculating your SAC based on normal activity levels may not produce a result that reflects your cardio-vascular state during an emergency.  Air consumption during high workloads, extreme stress or panic may be up to 10x that of normal or resting SAC.  Consequently, many prudent divers establish both their normal SAC and also an elevated SAC.  The elevated SAC is calculated in the same way as the normal SAC, except the diver should stress their cardio-vascular system at the time they monitor their consumption.  This typically means a period of intense fin kicking.

To calculate your SAC, use the following formulae:

 (Tank Volume in litres x BAR Used)  /  Time 
 ATA (pressure)

- Example SAC Calculation:

Tank Volume (11L) x BAR used (100) = 1100L (gas used)

Gas Used (1100L) / Time (10 Mins) = 110Lpm (Litre per minute)

Lpm (110) / 3 ATA (20m) = 36.6Lpm SAC

2)  Calculating Gas Consumption at a Given Depth

Once a diver knows their gas consumption at the surface, it is easy to use this figure to predict their gas consumption at any given depth.  Simply multiply the SAC by the ATA pressure at depth.

36.6Lpm SAC x 2 ATA (10m)      =          73Lpm

36.6Lpm SAC x 3 ATA (20m)      =          110Lpm

36.6Lpm SAC x 4 ATA (30m)      =          147Lpm

 3)  Calculating the Gas Contents of your Cylinder

It is important to know the gas contents of your cylinder.  To calculate this, simply multiply the cylinder size (litres) against the pressure of gas it contains (BAR).  Be sure to remember that a proportion (normally 50 BAR) is held in reserve.

For an AL80 (11L) cylinder:

11L cylinder x 200 BAR              =          2200 Litres total gas

11L cylinder x 150 BAR              =          1650 Litres total available gas, less reserve

11L cylinder x  50  BAR               =          550   Litres reserve

4)  Calculating the Duration of Available Gas

In order to determine the time duration of available gas within a cylinder, the diver must divide the Available Gas by their Gas Consumption at a given depth.

1650L  /  110Lpm                       =          15 minutes

550L  /    110Lpm                        =          5 minutes

Rock Bottom Gas Management

Entry level divers are taught to preserve a statutory minimum gas reserve.  This is typically 50BAR or 500psi.   Rock Bottom Gas Management provides the diver with a more precise understanding of the minimum gas reserves that they need to maintain for open-water (not overhead) diving.

This minimum gas reserve is determined as the minimum amount of gas for 2 divers to ascend safely whilst sharing gas.

Calculating Rock Bottom Gas Reserves

In order to calculate the Rock Bottom Gas, it is necessary to apply SAC to the various ‘stages’ on an ascent.  It can be broken down, as follows;

  • Allow 1 minute for orientation/problem solving at the bottom depth.
  • Allow for the ascent (10m per minute) to the first stop (safety stop).
  • Allow for completion of the stop (3 mins for a typical safety stop / longer if doing deco).
  • All for the ascent (3m per minute) from the stop to the surface.

For each of these stages, apply the calculation: Time (mins) x Depth (ata) x SAC  (Lpm) x 2

  • The time is calculated by dividing distance (m) by ascent rate (mpm).
  • The depth is an average for that stage.  For ascents, it is the mid-point from the start of the ascent to the end of that ascent.  i.e. 30m to 5m average depth  is 17.5m (30m-5 / 2 + 5m= 17.5m).
  • To convert depth into ATA, simply divide the depth (m) by 10 and add 1.
  • The SAC is calculated as per the first section.  Use the highest SAC of the two divers or an average SAC of the two divers.  Alternatively, both divers could calculate an individual rock bottom and then add them together.

Example Rock Bottom calculation (for a dive to 30m):

Orientation                    1 min x 4 ata x 20Lpm x 2                         =          160L

Ascent to Stop              2.5 min x 2.75 ata x 20Lpm x 2             =           275L

Safety Stop                     3 min x 1.5 ata x 20Lpm x 2                     =          180L

Ascent to Surface         1.5 min x 1.3 ata x 20Lpm x2                 =          39L

Total Gas Requirement =          653L

To calculate the BAR (pressure) needed as a reserve, simply  divide the Gas Requirement by the volume of the cylinder used.  Gas Requirement / Cylinder Volume = BAR (pressure) reserve

654L / 11L (AL80 cylinder) = 59.5 BAR

The final set in calculating Rock Bottom gas reserve is to add a 20 BAR cushion.

59.5 BAR + 20 BAR = 79.5 BAR

Thus, the minimum reserve for 2 divers (buddy team) with a maximum SAC of 20Lpm, doing a dive to 30m depth with no decompression obligation, should be 79.5 BAR per diver.

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