Cave divers use some unique equipment, and they also use some special terminology – not just to discuss the features found inside cave and cavern environments, but to discuss the gas that’s used during cave diving. You’ll find this article helpful if you’re brand new to diving, if you are considering cave diver training, or if you’re wondering just what your cave diver friends are talking about when discussing the gas they use.
Penetration Gas
Penetration gas is the portion of the gas that a diver uses to enter a cave and conduct exploration. It doesn’t matter what’s in the gas mix – it’s always going to be referred to by this name. A cave diver’s penetration gas should never be more than one-third of the total gas supply or starting gas volume, and as you’ll discover when cave diving, penetration gas supply is determined by the person who has the lowest volume starting out. This helps to ensure that no one runs out of air during the dive.
Controlling Diver
The controlling diver is the diver who starts out with the lowest gas volume. This may or may not be considered the same as having the lowest starting gas pressure; different tank sizes may be in use so gas volume is used rather than gas pressure.
Turnaround Point
You might think of a turnaround point as a specific destination, because in most discussions, it is. When talking about cave diving though, the turnaround point is the dividing point on the gas gauge between the penetration gas and the exit gas.
Exit Gas
The exit gas is technically considered to comprise two thirds of the total gas volume or two thirds the breathing gas supply. Exit gas is determined by subtracting penetration gas volume from starting gas volume, and it must always exceed that number by a factor of no less than two to one. This is an important safety measure that is meant to provide a sufficient air reserve for dealing with unforeseen circumstances.
Cave Diving Procedures Using Starting Volumes
Now it’s time to put the entire puzzle together. You’ll learn and practice the following procedures during your cave diving courses, if you decide to move forward with cave diving.
All divers check their beginning gas pressure and the controlling diver is determined. This is the team member who has the lowest starting gas pressure; there can be more than one controlling diver, and in some cases everyone on the team may be a “controlling diver.”
Next, penetration gas volume is determined by dividing the controlling diver’s starting gas pressure by three. If the starting gas pressure number is one that’s not easily divisible by three, it is rounded down to the next lowest number that can be easily divided by three. The first third represents penetration gas, and the remaining two thirds represent the exit gas.
Finally, the turnaround point is determined for each member – not just for the controlling diver. All divers deduct the controlling diver’s penetration gas value from their own starting pressures. Whoever reaches the turnaround point first during the dive determines turnaround.
Double-check to be sure calculations are correct for an additional margin of safety, and never give in to the temptation to pass the turnaround point. Following these rules for gas management while cave diving increase your ability to enjoy a fun, safe exploration and come out of the cave accident-free.