1. Back on the boat with 500psi - No plan...just tool around until you run out of gas
2. All Usable - You can surface whenever you want
3. Rule of Halves - You prefer to return to some specific point to surface
4. Rule of Thirds - You must return to some specific point to surface
As a recreational diver you will generally use either "All Usable" or "Rule of Halves".
Turning the Dive
As part of your dive planning, or at least the pre-dive check with your buddy(ies), one of the absolute key things to discuss and agree on is when will the dive be over and we need to start heading back or ascending...known as "turning the dive". You can figure out the pressure at which you must turn the dive by using one of these gas planning calculations.All Usable
If there are no overhead concerns, be they hard (cave or wreck) or virtual (boat traffic) and you can surface at any point of the dive...then this is generally the gas plan you want. Most common for quarry or lake dives where a surface swim back to the water exit point is immaterial and there are no large boats to worry about.Example using 0.75 RMV, a max depth of 70ft and everyone is using a single Aluminum 80 tank of air...
Step 1: Determine Rock Bottom or Minimum Reserve Gas which is 24cft or 1000 PSI
Step 2: Subtract 1000 PSI from the working pressure of the AL80 tank (3000 PSI) we get 2000 PSI Total Usable Gas
Step 3: Subtract 2000 PSI Usable Gas from the working pressure of the AL80 tank (3000 PSI) we get 1000 PSI Turn Pressure
Decision: The first diver in the group to reach 1000 PSI will signal to end the dive and begin our ascent
Rule of Halves
If there are overhead concerns or you need/want to ascend at a specific point (such as an anchor or downline) but in an emergency you could surface directly...then this is generally the gas plan you want.Example using 0.75 RMV, a max depth of 70ft and everyone is using a single Aluminum 80 tank of air...
Step 1: Determine Rock Bottom or Minimum Reserve Gas which is 24cft or 1000 PSI
Step 2: Subtract 1000 PSI from the working pressure of the AL80 tank (3000 PSI) we get 2000 PSI Total Usable Gas
Step 3: Divide 2000 PSI in half (the entire point of rule of halves) we get 1000 PSI as Actual Usable Gas
Step 4: Subtract 1000 PSI Usable Gas from the working pressure of the AL80 tank (3000 PSI) we get 2000 PSI Turn Pressure
Decision: The first diver in the group to reach 2000 PSI will signal to turn the dive and we are to proceed to the agreed ascent point
Rule of Thirds
If there are overhead concerns or you need/want to ascend at a specific point (such as an anchor or downline) and in an emergency you can not surface directly...then this is generally the gas plan you want. You must not only have enough gas to ascend, you must also have enough gas to get your buddy back to the ascent point.Example using 0.75 RMV, a max depth of 70ft and everyone is using a single Aluminum 80 tank of air...
Step 1: Determine Rock Bottom or Minimum Reserve Gas which is 24cft or 1000 PSI
Step 2: Subtract 1000 PSI from the working pressure of the AL80 tank (3000 PSI) we get 2000 PSI Total Usable Gas
Step 3: Divide 2000 PSI by 3 (the entire point of rule of thirds) we get ~600 PSI as Actual Usable Gas
Step 4: Subtract 600 PSI Usable Gas from the working pressure of the AL80 tank (3000 PSI) we get 2400 PSI Turn Pressure
Decision: The first diver in the group to reach 2400 PSI will signal to turn the dive and we are to proceed to the agreed ascent point
Putting it all together
Once you know your RMV and your Rock Bottom or Minimum Reserve Gas it is super simple to calculate what your turn pressure should be. And once you have calculated this once, it is a great thing to have in a Wetnote or part of your dive kit for doing planning. It would look something like....