Gas matching in action

Recently, a reader asked us for help with a gas matching problem. The problem was based on the following values:

  • Diver A is using steel 72s filled to 2,500 psi.
  • Diver B is using stell 85s filled to 3,600 psi.

As the question was posed using “Imperial” values (technically “US customary units”), we will work this through using psi and cubic feet. Were we using metric units, there is a good chance we could do all the math in our heads.

We will also round numbers to a single decimal point for simplicity.

Start by getting real

If you want to achieve the highest degree of accuracy, you must know what your cylinders’ actual capacity is. This is usually not what divers assume it to be. For example:

  • The classic Pressed Steel “72” had an actual capacity of 71.2 cubic feet at 2,475 psi.
  • The Faber “85” has an actual capacity closer to 83 cubic feet at 2,640 psi.

These are the values we will use in this example.

Baseline multipliers

There are many different ways to calculate turn pressures for divers using dissimilar cylinders. The most precise (and most cumbersome) method is to use baseline multiplier values. While many divers choose to use one of the easier methods, it’s important to understand at least the theory underlying baseline multipliers.

Most instructors calculate non-metric baseline multiplier values by dividing a cylinder’s working pressure in psi by capacity in cubic feet. This means:

  • For steel “72s,” the baseline multiplier value is 2,475 psi divided by 71.2 or 34.8 psi per cubic foot.
  • For steel “85s,” the baseline multiplier value is 2,640 psi divided by 83 or 31.8 psi per cubic foot.

Regardless of the method used, the underlying approach to gas matching is largely the same. The steps involved include:

  • Identify the controlling diver.
  • Determine the controlling diver’s usable gas.
  • Convert usable gas from pressure to volume.
  • Deduct an equivalent volume from the other diver’s starting pressure.

For simplicity, we will use single-tank values. The turn pressure will still work out the same.

1 Identify the controlling diver

  • Diver A has roughly 71.8 cubic feet (2,500 psi divided by 34.8).
  • Diver B has 113.2 cubic feet (3,600 psi divided by 31.8).

This makes Diver A the controlling diver (like you even needed to do the math to figure out this one).

2 Identify the controlling diver’s useable gas

Diver A’s starting pressure of 2,500 psi is not easily divisible by 3. Therefore:

  • Round Diver A’s starting pressure down to a value that is easily divisible by 3. In other words, 2,400 psi.
  • Divide 2,400 psi by 3 to get a usable gas value of 800 psi.
  • Deduct 800 psi from Diver A’s actual starting pressure of 2,500 psi.
  • This will make Diver A’s turn pressure 1,900 psi.

It’s important to remember to deduct the usable gas value from Diver A’s actual starting pressure and not the number he rounded down to.

3 Convert pressure to volume

Divide Diver A’s useable gas pressure (800 psi) by his baseline multiplier (34.8). This works out to 23 cubic feet.

4 Convert this value for the larger cylinder

Multiply 23 cubic feet by Diver B’s baseline multiplier (31.8). This works out to 731.4 psi. As most pressure gauges are calibrated in 100-psi increments, we’ll round this down to 700 psi. This is Diver B’s usable gas.

5 Determine Diver B’s turn pressure

Deduct Diver B’s usable gas pressure of 700 psi from his starting pressure of 3,600 psi. This gives a turn pressure of 2,900 psi.

This leaves us with turn pressures of 1,900 for Diver A and 2,900 for Diver B. So, should Diver B suddenly lose all his gas, Diver A should theoretically have just enough gas to get get them both out. But will he?

Wait…there’s more!

The biggest flaw in gas matching is that it assumes each diver’s RMV will remain constant both while entering normally and exiting while sharing gas. It won’t. It would not be unusual for each diver’s RMV to as much as double when exiting under this much stress. This means that even the most precise gas matching only gets divers halfway home.

This would also be true if divers had identical tanks and starting pressures. Fortunately, several things may work in the divers’ favor:

  • It’s unlikely a total gas loss by one of the divers would happen at their maximum point of penetration. The closer divers are to the exit, the greater their chance of making it.
  • The affected diver is unlikely to suffer an instantaneous loss of all of his or her gas. Even in the case of severe valve or manifold damage, it will take a while for a set of manifolded doubles to completely drain. And, if the affected diver can isolate quickly enough, he or she may be able to save close to half his or her gas…if not more.
  • If the divers have had to battle flow going in, but ride it coming out, this further increases their odds.

The bottom line is, despite these factors which may work in your favor, it’s foolish to push Thirds.

Learn more

  • Many cave divers maintain there are situations in which any sort of gas matching is unnecessary. Examples include teams of three and when all team members are using sidemount or CCRs. There is also the issue of when the disparity be dissimilar cylinders is so minor that gas matching may not be necessary. Find out why.
  • Among the biggest problems when using baseline multipliers is the fact you pretty much need a calculator to do so accurately. But if you do this out of the water, the disparity between air and water temperature can affect the accuracy of your calculations. There are easier ways to do gas matching you can do in the water without a calculator. Find out what they are.
  • Gas matching isn’t just an issue among buddies. There is also what happens when diving sidemount cylinders with different starting volumes. Learn why.
error: Content is protected !!