Cylinder Matching is comparing divers with two different volumes of cylinders and matching their usable gas to each other. The diver with the lowest volume is the regulating diver. You may even take it further by using the diver with the highest SAC/RMV rate against the lower volume cylinder for more added safety.
It is here that the more detailed dive planning starts. But it really is not all that more difficult than what we have looked at already. Not all divers have the same RMV/SAC rate. Nor do all divers wear the same size scuba cylinders. A lady diver may only need a 63 cu.ft cylinder to do the same dive that a male diver does wearing a 80 cu.ft cylinder. The question is what if something goes wrong? Will the diver wearing the smaller cylinder have sufficient air to get both dive team members safely back to the surface? We need to plan our dives in a conservative manner. Depth, time and air are the limiting factors, when the air is gone the dive need be over. We need to plan our dives to the weakest link. Who has the smaller cylinder? Who has the higher RMV/SAC rate? Can both these divers plan a dive together and safely execute it, considering the possible situations of what can go wrong?
What we are going to show here now is how to consider divers of differing RMV/SAC rates and wearing different size cylinders can safely plan and execute a dive together, it is called Cylinder matching.
Consider this, overhead environment divers always plan their dives based on rules of third or sixths. We can see how this is a simple way of planning dives considering what might happen. Basically it is dividing your air supply into thirds. Divers use the first third to penetrate the overhead environment and the remaining two-thirds to exit and use as a reserve for those unplanned events that may occur. Even this is still a guideline and these divers will match their cylinders.
Example: 1.A diver has 3000 psig starting air pressure; He would turn to exit the dive at 2000 psig;
2. If the same diver had 2800 psig starting air pressure; he would turn the dive to exit at 1900 psig. You should have noticed that it was required to first round DOWN to a number evenly divisible by 3 to calculate the usable air for the dive and then to subtract that from the actual starting air pressure. This allows for a more conservative and safer dive plan. If you do not have the air to breathe you cannot count on it. With AIR ROUND DOWN
Now if two divers of different cylinder sizes and RMV/SAC rates were making a deep dive or a dive into an overhead environment or any dive for that matter, it would be in their best interest to plan for the difference in their RMV/SAC rates and cylinders. Just working on rules of thirds may not be good enough.
Let’s see what I mean: Example, Diver A is using a double set of LP 98 cu.ft cylinders filled to 2640 psig. Diver B is using a set of double 120 E series cylinders filled to 3500 psig. Since Diver A appears to have the smaller volume cylinders we will calculate his thirds:
2640 rounded down to 2400 divided by 3 = 800 psig. Turn pressure at 1840 or 1900 psig
So we see that Diver A has 800 psi of available air for the dive, converting this to cu.ft you would see that he can use 59.2 cu.ft of air. Remember he is using doubles so his baseline is multiplied by 2. His total air or gas by volume is 196 cu.ft of air of which 59.2 Cu.ft of usable air is just under a third.
Now how much air can Diver B use?
Diver B has a baseline of 6.9 cu.ft when using double 120 E series cylinders. We will use this to compare his cylinders back to Diver A’s using some simple math. Basically we are going to compare Diver A’s usable air to the baseline of Diver B to get his usable air and turn pressure.
(Diver A usable Air cu.ft / Diver B baseline) x 100 = Diver B Usable Air
(59.2 / 6.9) x 100 = 857.97 psig or 800 psig usable
Therefore Diver B’s turn pressure is: 3500 – 800 = 2700 psig