Dissimilar Cylinder Volumes for Cave Diving

Knowing your gas limits is something that all cave divers should be aware of.

Knowing your gas limits is something that all cave divers should be aware of.

When diving in a cave, there are several limiting factors that you should be aware of. The most important of these is the gas supply. Without an adequate supply of gas, the chances of your survival in the event of an emergency is reduced significantly. This means that you need to workout the amount of gas each person is carrying and their corresponding turn pressures. This is easy if you are all using the same type of cylinders filled with the same amount of gas, however if you are using different sizes then you need to find a way to balance this. The following method allows you to do this and find out which diver has the controlling gas supply (the least amount of gas) and the turn pressures for the other divers in the group. As all cave dives are based on the rule of thirds, we will be working on the principle that one third of the gas is to get us to the destination, one third is to get us back, and the final third is for any unlikely emergencies.

Metric or Imperial

I’ve said it before and I will say it again, metric calculations are so much easier than imperial. For a start, metric cylinders are labelled by the volume of water they would hold this means that a 12 litre cylinder holds 12 litres of water. The imperial system, on the other hand, rates the cylinder as the amount of gas it holds in cubic feet at its DOT/CTC pressure rating, meaning that a A80 will hold 80ft3 of gas at a pressure of 3000 psi. LP72 is 72ft3 at a pressure of 2640 psi. HP95 is 95ft3 at a pressure of 3500 psi.

Calculate the Baseline

Regardless of these differences it is possible to calculate a baseline value for the gas in each type of cylinder. For imperial it is how many cubic feet per 100 psi, and for metric it is 1 litre per 1 bar. We can then use this to work out the volume of usable gas in each cylinder and from that which volume is the controlling volume.

Metric Example

Let’s assume that we are diving in the metric world and both divers are diving with twin cylinders. If they are not diving with twins just ignore the multiply by 2 part.

As the baseline for metric is just one litre per bar we can take the number of litres in the cylinder and use that as the baseline. The divers are using doubles so we just multiply that by 2.

Diver 1 has double 15 litre cylinders with 210 bar.  15 x 2 = 30

Diver 2 has double 12 litre cylinder with 250 bar. 12 x 2 = 24

Each diver should only use one third of their gas (rule of thirds – one third to get there, one third to get back, one third in reserve). Not that when we divide the pressure in the cylinder by 3, to find the thirds value, we must round down to the nearest 10 bar, this gives another safety margin.

Diver 1 Diver 2
Baseline 30 24
Thirds 210 / 3 = 70 bar 250 / 3 = 80 bar
Usable Gas 70 x 30 = 2100 litres 80 x 24 = 1920 litres

We can see from the table that Diver 2 has the lowest volume of gas so that means that we regard this volume as the “controlling volume.”

The last thing we have to do is convert the controlling gas volume into a usable gas pressure for each diver in the team.

As it is Diver 2’s volume he will be able to use 80 bar. For Diver 1 we must work out how much he can use. We take the controlling volume and divide it by his baseline value. 1920 / 30 = 64 bar.

Diver 1’s usable pressure will be no more than 64 bar. Giving a turn pressure of 146 bar.
Diver 2’s usable pressure will be no more than 80 bar. Giving a turn pressure of 170 bar.

Imperial Example

Calculating the baseline for imperial is a little more complicated but it is not difficult. We take the cubic feet of gas in the cylinder and divide it by the pressure of the gas, and then multiply the answer by 100.

Diver 1 is using a single HP 80ft3 cylinder filled with 3500 psi. (80 /3500) x 100 = 2.2 (2.28567)

Diver 2 is using a single LP 98ft3 cylinder filled with 2640 psi (98/2640) x 100 = 3.7 (3.71212121)

Remember to round your answers down.

Now that we have the baselines for the divers if we wanted to do this for doubles we would just double these answers.

Each diver should only use one third of their gas (rule of thirds – one third to get there, one third to get back, one third in reserve). Not that when we divide the pressure in the cylinder by 3, to find the thirds value, we must round down to the nearest 100 psi, this gives another safety margin.

Diver 1 Diver 2
Baseline  2.2  3.7
Thirds 3500 / 3 = 1100 psi 2640 / 3 = 700 psi
Usable Gas  (1100 / 100) x 2.2 = 24.2ft3  (700/100) x 3.7 = 25.9ft3

We can see that Diver 1 has the controlling gas supply as his is the smallest. That means he will use his thirds value, 1100 psi. However, we now need to convert the controlling gas volume into a usable pressure for the other diver(s) – Diver 2. We do this by taking the controlling gas volume and diving it by the baseline for Diver 2 and then multiplying it by 100.

(24.2 / 3.7) x 100 = 620 psi.

Diver 1’s usable pressure is no more that 1100 psi and a turn pressure of 2400 psi.
Diver 2’s usable pressure is no more than 620 psi and a turn pressure of 2020 psi.

Caveat

While it should be understood that the planned volume usage is a constant it is important to remember the consumption rates will not be. Whichever diver reaches their turn pressure first the dive should be ended. Don’t be embarrassed about thumbing the dive because you have reached the turn pressure, it is better to exit the cave and be safe than to push on an risk everyone’s lives because of your pride.

 
First and foremost let me state that you must receive formal training from one of the many dive organisations before attempting a dive. Without formal training you risk injuring yourself. Please get training before attempting anything you have read or seen in this post. I will not be held responsible for you attempting to use the methods contained in this post. The following post is for illustration purposes only.

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