Gas Management and Planning

There are few basic gas management rules that most divers follow, the most common is rule of thirds, which is a third used for outward part of the dive, a third used for return journey then a third in reserve for emergencies. There are also a couple of others normally reserved for overhead environments such as rule of forths and rule of sixths. Having turn pressures is an excellent idea and practice in dive planning and dive management, however how long does that third of gas actually give you?

Enter SAC (Surface Air Consumption) or RMW (Respiratory Minute Volume), which just relate to how much air you use at the surface allowing you to plan your dive in a little/a lot more detail. I have always used the SAC terminology and being a UK diver, I will be be doing the calculations in Bar, Litres and Meters. So in this post I am going to discuss
1) How you can work out your various SAC rates
2) How you can used these in dive planning

There is a simple calculation to work out your SAC Rate; Amount of gas used divided by time then divided by ambient pressure.

So lets take lets take an example dive using a 12l cylinder, Start Pressure 210, End Pressure 105, Max Depth 20, Avg Depth 15, Dive Time 25.
Amount of gas used: 210-105=105. 105×12(cylinder size) = 1,260ltrs used.
Time: 25
Ambient Pressure, we will use average depth as we want to find the average SAC for the dive: 15 divided by ten, then add 1. So 15/10=1.5, 1.5+1=2.5
So our average SAC for this dive would be 1260/25 = 50.4, 50.4/2.5 = 20.16

If you have the information record in your dive logs, you can always work back through them working out your average SAC rate for each dive. This will give you a good starting point to understand your SAC rate. You will notice your SAC could change dramatically from one dive to the next, this could be on how hard you were working on the dive, ie drift vs swimming into a current or if you were in warm water or cold.

For those not the greatest at Maths there are plenty of free online calculators available. However if you manage to learn the formula its very easy to do rough calculations on the fly, which I will cover later.

So far we have looked at Average Dive SAC rates, but once you have got your head round that, there are actually 3 rates you could do with knowing. First is your static SAC rate, this is your SAC when basically hovering not swimming such as holding a safety stop. This one is fairly easy to measure, hold a stop for 5mins, note the depth, start and end pressures then do the maths above. 2nd Normal SAC, this is the rate for normal swimming, the trick is to choose a depth, and as per above make a note of the pressures, swim for about 5mins one way and then back the same way for 5 minutes (this takes into account any current). 3rd is your working SAC rate, ideally you want to do this in the 5m depth range for safety, find something you can hold on to, take a note of pressures, and kick hard and quick for about 3mins.

Now we know our various SAC rates we can plan out our dive in as much detail as we like. It might be as simple as working out how long you have for the whole dive, to planning you gas usage at varying stages of your dive ie Decent gas used, leg 1, 2, 3 etc of the dive, Ascent gas, Safety stop gas. If you go into this much detail you can closely monitor your dive against plan to see if you are on track. You can also use it to work out how much gas you need in an emergency. I have written a 2nd post about this here where I highlight the risk of using single cylinders on dives below 20m
https://grahamsavill.wordpress.com/2017/07/26/have-enough-gas-is-that-12l-really-suitable-for-30m-divers/

Last of is calculations on the fly, for example if you have the following Static SAC 11, Normal 14, Working 20. It can make calculations extremely easy to allow you to work out how much gas time you have left. So using the rates above 18 which is between normal and working rates allows a margin of increased workload, but it also fits nicely into a 12l tank as 1.5bar. So if you are at 10m, its 3 bar (mulitplied by ambient pressure x2) a minute, at 30m its 6bar (multiplied by 4) this makes calculating remaining air time extremely easy.

Maths can be a rather dull subject to some, but I hope this gives you an idea on how you can calculate your gas usage and how to use that for better dive planning.

As always thank you for reading.

G-SAV