Recreational Gas Management and Planning

One thing we try to emphasize in our courses is the importance of proper gas planning at all levels.  In technical diving this normally means allowing for enough gas for a team to finish the dive with one team member suffering a complete catastrophic loss of gas.

We normally apply the same logic to recreational no decompression limits diving and focus that the amount of gas reserve we need to maintain is dependent upon the depth rather than air consumption rates.  The gas planning currently used by most divers for gas planning (low on air at either 50 bar/700 psi or 70 bar/1000 psi) is not really an appropriate method for most dives while use of the ‘rule of thirds’ can lead to many other problems.

Most divers have a Surface Air Consumption rate (or SAC rate) of around 15-25 litres per minute, this is a regular breathing rate – in case of an out of gas emergency or panicked diver this is normally around the 30 l/min mark.  Given this bit of basic information it’s possible to work out the amount of gas we’d need to get from a specified depth to the surface.

Sadly we don’t normally work in litres when underwater so it’s good to use a unit we’re all familiar with; pressure.  A typical recreational aluminum scuba tank, the S80 or 12l tank, has an internal volume of around 10.5 to 11 litres, based on 30 l/min consumption rate this would correspond to a pressure of around 2.85 bar of gas per minute at the surface (30 l/min / 10.5 l = 2.856 bar/min).

We now need to allow for the fact that in a worst case scenario there would be two of breathing from the same tank – normally a team member who is out of gas.  In this case the breathing rate doubles from 2.85 bar/min to 5.7 bar/min.  With this information we can work out the gas required to ascent from any depth by using the pressure at the average depth during the ascent (i.e. half the depth) and the 5.7 bar/min consumption rate.

The final steps we need to consider is we can’t suddenly ascend as soon as our team member is out of gas; typically we are looking at up to 30 seconds to donate gas and a minute to stabilise the situation before starting the ascent – i.e. another 1.5 minutes at the bottom before ascending.  It’s also a good idea to factor in a safety stop at 5m, it’s not required for most dives but allowing at least 1 minute at 5m at the end of the dive is highly recommended – the full 3 minute stop is better again, however in our emergency planning we’ll just allow for a 1 minute stop for now (it’s easy to extend in a real situation if we still have gas available!).

With all the information above we can now work out some gas volumes for dives, lets say we want to dive to 30m and need to know or reserve – we can calculate it as follows:

Maximum depth = 30m

Pressure at maximum depth = 4 bar

Average depth for Ascent = 15m (30m / 2)

Pressure at average depth = 2.5 bar

Ascent time = 3 minutes (allowing for 10m/min ascent rate)

Safety stop = 1 minute at 5m

Gas used:

At bottom = 34 bar (1.5 min x 4 bar x 5.7 bar/min)

In ascent =  43 bar (3 min x 2.5 bar x 5.7 bar/min)

At safety stop = 9 bar (1 min x 1.5 bar x 5.7 bar/min)

TOTAL = 86 Bar

The first thing most people notice is this is more than the standard 50 bar or 70 bar many divers follow – what does this mean?  Well, in short it means you really didn’t have enough gas to safely ascent to the surface if your buddy ran out of gas, sadly this is something most divers are doing regularly without realizing it.

We can also try the number for a shallower dive, say 12m:

Maximum depth = 12m

Pressure at maximum depth = 2.2 bar

Average depth for Ascent = 6m (12m / 2)

Pressure at average depth = 1.6 bar

Ascent time = 1.2 minutes (allowing for 10m/min ascent rate)

Safety stop = 1 minute at 5m

Gas used:

At bottom = 19 bar (1.5 min x 2.2 bar x 5.7 bar/min)

In ascent =  11 bar (1.2 min x 1.6 bar x 5.7 bar/min)

At safety stop = 9 bar (1 min x 1.5 bar x 5.7 bar/min)

TOTAL = 38 Bar

This time we need less reserve than the normal 50 bar most divers work too.

This seems a little complex to calculate for every dive or change in depth, however if we look at this for different depths we see a pattern start to emerge in the ratio between depth and the required pressure reserves:

Depth	10m	15m	20m	25m	30m	35m	40m
Average Depth	5m	8m	10m	13m	15m	18m	20m
Ascent time (10m/min)	1.0 min	1.5 min	2.0 min	2.5 min	3.0 min	3.5 min	4.0 min
Donate Gas (1.5 minutes)	17 bar	21 bar	26 bar	30 bar	34 bar	38 bar	43 bar
Ascent Gas Use	9 bar	15 bar	23 bar	32 bar	43 bar	55 bar	68 bar
Safety Stop (1 minute)	9 bar	9 bar	9 bar	9 bar	9 bar	9 bar	9 bar
Total Gas Required	34 bar	45 bar	57 bar	71 bar	86 bar	102 bar	120 bar
Depth to Pressure Ratio	1:3.42	1:2.99	1:2.85	1:2.82	1:2.85	1:2.91	1:2.99

You can see that based on the above information the depth to pressure ratio for non-stop recreational diving limits is generally within a 1:3 pressure to depth ratio when using metric units, i.e. if you multiply your depth in metres by 3 that is the minimum gas reserve you should maintain in bar.

This is where our depth times 3 gas reserve that we use for recreational non-stop in courses and guided dives is derived from, in other words if we're at 30m any divers at 90 bar of pressure (30 x 3) should signal 'low on gas' to the guide, if we are at 15m the low on gas pressure would be 45 bar.  If we have divers using PSI PSI we use 50 times the depth in metres or 500 psi per 10m of depth.

This is the basis of gas planning we use for all our recreational courses, you’ll find that when comparing the reserves above to the standard recreational table non-stop limits a lot of dives in the 20-30m range would result in gas reserve becoming the limiting factor for the dive duration, not the non-stop limit time.  However you’ll also see as you ascent towards the end of the dive the gas reserve required also reduces, allowing the dive to be continued while still maintaining an adequate safety reserve of gas to finish the dive.

This is the basis of our suggestion that any dives to 30m or deeper should be done in either twin tanks/sidemount configuration or down with an additional slung tank to provide the team an adequate gas safety reserve.

Please note this is based on recreational non-stop diving only and doesn’t apply to overhead environments or decompression diving.  We’d also suggest staying above these minimum reserves rather than working directly to them, particularly for the shallower depths and when using rented or unserviced equipment.