Gas matching is an important part of the dive planning process and an essential component of any pre-dive safety checks. If you and your team have a bad day and end up having to share air it could make the difference between a comfortable swim out of the cave and a rather frantic race, worrying that your gas reserves might not last. Gas matching is nothing more complicated than adjusting the size of your reserve to compensate for factors that might affect it. We are basically making sure that the reserves we plan are actually large enough to support our exit in an air sharing emergency.
When do we gas match?
I am sure others could add to the list but here are a few examples for guidance.
In a team of two divers, when the diver with the largest RMV is also using the largest cylinders.
Any time when the team has to negotiate restrictions.
In a team of three or more, where there is a breathing rate difference of more than double between two divers.
When don’t we need to gas match?
In a team of three or more divers (apart from the breathing difference mentioned above)
A team of two divers using the same sized cylinders (exception as above)
What does all this mean? Let’s look at them more closely.
If a diver with a larger breathing rate uses larger cylinders (which, let’s admit, is quite common), something goes wrong at the point of furthest penetration and it is the bigger breather who needs to support the smaller breather then his larger cylinders plus the smaller breather should mean ample gas to swim home. The other way around, however, means that it is likely that the amount of gas in reserve in the smaller breather’s smaller cylinders will not support both of them out of the cave.
When negotiating restrictions, with all other things being equal, it should take the same time to travel in through the restriction as it does out. However a considerable amount of extra time might be needed when divers are sharing air through a restriction, hence the need to check that the reserves are going to be big enough.
If you have someone on the team with a very large SAC then it would be a good idea to check the numbers to ensure the reserve volume is sufficient to support them out of the cave if you have to donate gas.
For my imperial friends we have the delights of what are known as ‘dissimilar tank calculations’. I am going to leave that for a separate short article. Those who have to use these will no doubt understand!
How do we do the maths for gas matching. One way is to cheat a little and get a gas matching table. IANTD certainly have them and they are easy to use.
Another way is to do some simple maths to make sure the reserves are large enough. It mainly boils down to adjusting your turn pressure, turning sooner than would be expected, using less gas on the way in and out and keeping more than 1/3 in reserve.
If my buddy has an SAC of 24 litres per minute and mine is lower at 12 lpm I can look at a chart for mine and my buddies SAC rates and quickly read the SAC Ratio Factor (SRF). The SRF is nothing fancier than a new percentage of my starting pressure at which I need to turn. The usual 1/3 used means that turn pressure is about 66%. Looking at the SRF for mine and my buddies SACs, in this example, I need to turn earlier at 76%. So for a start pressure of 230 bar I will turn at about 175 bar, thus making sure my reserve third is a big enough volume to support my buddy if needed. I didn’t do the maths, I just flipped the chart over to check the numbers where my SRF crosses my start pressure.
If you dive with the same team then it is easy to pop the numbers down in your wet-notes, either from the a chart or from sitting down with a calculator!
Why is this important? Well, if you don’t match gas, this could happen:
Diver A has an SAC of 12 litres per minute and is diving on twin 12s charged to 200 bar
Diver B has an SAC of 15 litres per minute and is diving on twin 15s charged to 200 bar.
They will reach their turn pressure at the same time. If, at that moment Diver B has a catastrophic gas loss and needs to share gas with Diver A…..well Diver B will have used 2000 litres of gas going in. Diver A will have used 1600 litres. Diver A will need the same volume to get out, 1600 litres leaving his 1/3 in reserve at 1600 litres. Diver B still needs 2000 litres of gas so there is a 400 litre shortfall. This isn’t going to have a happy ending.
On the other hand, if the dive team match gas, and calculate that Diver A needs a reserve big enough to match the gas volume needed by his buddy…..well Diver A just needs 2000 litres as a reserve, leaving his usable gas at 2800 litres, 1400 litres for the dive in, 1400 litres for the dive home. Depending on the depth of the dive that extra 200 litres from each leg of the journey now put aside for the reserve might only be just a few minutes further in but puts the gas plan back in to the realms of being safety first.
1400 litres from the starting pressure means Diver A turning the dive at 145 bar rather than 135 bar.
All of this has been simplified slightly to fit into a short article so it does not take in to account deeper cave diving with a significant decompression obligation or any other multi-stage extended range penetration dive. That is probably better left to the classroom as part of a Deep Cave Diver or Multi-Stage Cave Diver course but the main principles are very similar in concept.