A topic I discuss with all of my cave diving and technical students is volume matching. If you’ve had any kind of cave or technical training hopefully this will be a review for you and maybe help clarify things. If this is new to you, then please read carefully because this is very important information and something you should institute in your own dive planning.

Whenever you plan any kind of overhead dive, whether one with an actual overhead or a virtual overhead, your dive team needs to plan gas management according to volume. Not everyone has the same size cylinders. When you’re diving with someone who has different size cylinders than you, volume needs to be accounted for. Simply planning gas management based on pressure isn’t enough.

Let’s look at an example of a dive team planning based on pressure alone. Two divers, one with LP85s and one with LP95s plan a cave dive together. Their starting pressures are 3600 psi each, so they plan on a turn pressure of 2400 psi. * LP85s have a tank factor of 6.4 (85/2640 x 2 x 100 = 6.4). This means that 1200 psi of LP85s is 76.8 cubic feet (cf). LP95s have a tank factor of 7.2 (95/2640 x 2 x 100 = 7.2). This means 1200 psi of LP95s is 86.4 cf. This is a difference of almost 10 cf!

So what’s the big deal? Let’s say both divers reach their turn pressure of 2400 psi at the same time and suddenly the diver using LP95s has a catastrophic gas loss and has to air share with his buddy during the exit. The diver in LP85s has 2400 psi, or 153.6 cf available. However, he needs 163.2 cf (76.8 cf + 86.4 cf). He is almost 10 cf short of the gas necessary to get back to the surface.

What are the chances of having a catastrophic gas loss at maximum penetration? Yes, they are pretty slim, but most divers will experience an increase in gas consumption rate when any kind of incident happens during the dive so even if the incident was sooner or later, the diver in LP85s would probably not have enough for his buddy. However, if the diver in LP95s were to adjust his turn pressure to volume match with his buddy then this wouldn’t be an issue.

Because LP85s are the smaller of the cylinders they are the limiting factor. With a penetration pressure of 1200 psi that gives 76.8 cf. What we need to do is find out how much pressure that equates to in the LP95s. This is pretty simple. Take the 76.8 cf and divide by the tank factor of 7.2 and multiply by 100 (76.8/7.2 x 100). This equals 1067 psi. So the diver in LP95s should not breathe any more than 1067 psi if the diver in LP85s breathes 1200 psi.

Let’s look at another example. This time we have divers with LP95s and LP108s doing a dive together. The LP95s are the limiting cylinder because they are smaller. If both sets of cylinders are filled to 3600 psi and we use 1200 psi as penetration that means we have 86.4 cf in the LP95s. The tank factor for the LP108s is 8.2 (108/2640 x 2 x 100). So we take the 86.4 and divide by 8.2 then multiply by 100. This gives us 1054 psi. So the diver in LP95s turns at 2400 psi and the diver in LP108s turns at 2550 psi. Pretty simple, right?

It can be when you’re sitting on your couch with a calculator and some scrap paper. But you don’t have that luxury in the water and we all know that once you get in the water your cylinder pressures are going to adjust depending on the water temperature. While we can have a general idea of our turn pressures before the dive, we need to fine tune it once in the water. How do we do this without a calculator?

It’s pretty simple. I use what I call the Rule of 10. For every rated cubic foot difference multiply by 10. Let’s look at the 85s and 95s again. There is a rated difference of 10 cf. Multiply this by 10 and you get 100. So with the 85s being the limiting cylinders, we determine penetration pressure based on the pressure in the 85s and subtract 100 from that to determine the penetration pressure for the 95s. If the 85s have 3500 psi and we use 1100 psi as the penetration pressure, then we use 1000 psi as the penetration pressure for the 95s.

But wait! Didn’t we determine that the difference between 85s and 95s is 133 psi (see above)? Yes, but since we aren’t diving to full 1/3s and since submersible pressure gauges are not very accurate, that extra 33 psi isn’t going to make much of a difference. In a set of 85s 1100 psi is 70.4 cf and in a set of 95s 1000 psi is 72 cf. In addition, both sets are starting at 3500 so we have an additional 200 psi or 12.8 cf in the 85s that becomes part of the emergency reserve (3500 psi – 1100 psi penetration = 2400 psi; 2400 psi – 1100 psi exit = 1300 psi that is left for emergency reserves or 83.2 cf which is 11.2 cf more than 1000 psi in the 95s).

With 95s and 108s, the difference is 13 cf or 130 psi. Round up to 200 psi and if the diver in 95s penetrates 1100 psi then the diver in 108s penetrations 900 psi. That is equal to 79.2 cf for the 95s and 73.8 cf for the 108s. And if both divers have a starting pressure of 3600 psi and both use the same pressure that leaves 1400 psi for emergency reserves in the 95s and 1800 psi for emergency reserves in the 108s, or 100.8 cf in the 95s and 147.6 cf in the 108s. Seems like a lot, but no one ever died from having too much gas.

What about dive teams diving low pressure cylinders and high pressure cylinders? While the Rule of 10 can be used for a team diving all HP cylinders, we can’t just use the Rule of 10 in a mixed team without some additional information. We need to convert the HP to LP cylinders or vice versa. The two most common HP cylinders are HP100s and HP130s. HP100s have a tank factor of 5.8 (100/3442 x 2 x 100). This is equivalent to LP77s (77/2640 x 2 x 100). HP130 have a tank factor of 7.6 (130/3442 x 2 x 100). This is equivalent to LP100s (100/2640 x 2 x 100). # So when you have mixed teams just convert the HP cylinders to either LP77s or LP100s and use the Rule of 10 to figure out your turn pressures.

One last point I’d like to make is that gas consumption does not need to be considered when volume matching. Volume matching accounts for gas consumption. As long as your team is volume matching then whoever has the greater gas consumption will reach his turn pressure first and turn the dive. You don’t have to account for it in your calculations. So getting bigger cylinders because your gas consumption rate is high is not going to help unless your entire dive team gets bigger cylinders. You have to ensure your buddy has enough reserve gas to get YOU back to the surface and if you’re diving LP120s and your buddy is diving LP85s and you both start at 3600 psi and turn at 2400 psi that will NOT happen. Besides the point that bigger cylinders require more gas in your wing which creates more drag and increases your gas consumption even more. But that’s the subject of another article.

If you have any questions about volume matching or gas management please contact me.

- I do not condone planning dives to full 1/3s for any dive. Doing so is not conservative enough. Even Sheck Exley stated diving with a gas plan of 1/3s is not conservative enough.

# It is true LP100s don’t exist, but that doesn’t matter here.