The Dive Otter

Rock Bottom Gas Calculations

Go HERE or HERE for deep dive into understanding the concept | or Video from ISE or FlowState Divers


You are going to need to understand your consumption rate (also known as SAC or RMV) first before you dive into basic gas planning. So figure that out first. Then come back here.

Why Rock Bottom and not something else?

At the most basic level, there are 3 methods of gas planning...and 1 additional that is a neuance of the third.
1. Back on the boat with 500psi - Let's just wing it and see what happens
2. Rule of Thirds - Only appropriate when in an overhead environment
3. Rock Bottom or Minimum Reserve Gas - Let's make some assumptions and do some math and have a plan before we splash.
3b. Rule of Halves - Starts with Rock Bottom or Minimum Reserve Gas...but takes into account that you must return to some specific point to surface instead of immediately ascending when you hit your turn pressure. Such as in a shore dive.

So what is "Rock Bottom" or "Minimum Reserve Gas"?

It is the minimum amount of gas you think you will need after a catestrophic event at the deapest part of the dive. Think of it as "the worst case, but realistic scenario"

To put this into practice you need to make a few assumptions for yourself:

1. Are you going to include your buddy in this catestrophic event? This will effectively double the minimum amount of gas you need because they are now sharing your tank.
2. Are you going to do the standard safety stop? This will add 3min of air consumption to your minimum amount of gas
3. Are you going to assume 1 minute at depth to figure out what the hell is going on? You should, maybe more.
4. Are you going to assume your air consumption will significantly rise in catestrophic event? You should, because it will.
5. Use at least 500 PSI because of the way regulators and SPGs work. (Below ~200PSI they both can be inconsistant/unreliable)

What do I need to know to calculate it?

0. Your RMV - Let's assume 0.75 cuft/min
1. How deep is the deepest part of your dive? - Let's assume 100 ft for our example
2. How long of an ascent from that depth? - If we use 10ft per min then Time To Surface (TTS) is 10 minutes
(NOTE: 10ft per min is a way to add conservativeness to the calculation while keeping the math simple. You could instead use an increased RMV of say 1.5 cuft/min and then surface at 30/min. But dividing a depth by 30 is not as easy as dividing that depth by 10.)
3. Are you going to take a 3min safety stop? No. As a recreational diver, the "safety stop" is not mandatory because there is no decompression obligation, and I'm surfacing at 10ft per min. If I was surfacing at 30ft/min...then yes.
4. Are you adding 1 min at the bottom to sort things out? - Yes
5. Are you including your buddy in this catestrophic event? - Yes

The Math Calculation is...
( [cuft needed for 1 min at depth] + [cuft needed to surface from depth] + [cuft needed for safety stop] ) * [number of people using the tank]) = Rock Bottom

Put this all together

The Quick and Dirty Rock Bottom for the above example is...

cuft needed for 1 min at depth = (RMV 0.75 * 2.5atm @ 100ft) * 1min = 3 cuft
cuft needed to surface from depth = (RMV 0.75 * 2.5atm @ avg depth of 50ft) * 10min = 20 cuft
cuft needed for safety stop = 0....not taking it
Total = (3 + 20 + 0) * 2 divers = ~46 cuft of Rock Bottom gas

And the cool part about using RMV (cuft/min) instead of SAC (psi/min) you can calculate your Rock Bottom once and then apply this to ANY tank you happen to be using at the time.
That is done by taking the Volume of the tank dividing by Rock Bottom and then dividing that by the fill pressure of that tank
On an AL80, 46 cuft is 1,700 PSI (80cuft tank / 46 = 1.74 then 3,000 PSI / 1.74 = ~1,700)
*technicaly an AL80 doesn't have 80cuft of gas volume, it has 77.4...but we use 80 for ease of math

If you put this all into an excel document or your wet notes you don't need to do these calculations on the fly when the dive plan changes....

Can I use Rock Bottom and RMV for more stuff?

Yes - You can also use it to figure out if you have enough air to do the dive being planned. Let's try it...

Dive Plan: Using an Aluminum 80 tank with 3,000 PSI, dive to 100ft and tool around for 20 minutes

Is this doable with your specific RMV of 0.75cuft/min?
1. Descending to 100ft takes 2.22min at 45ft per min and an average depth consumption rate of 1.75 (RMV @ 50ft) for a total of 4 cuft of gas
2. Tooling around at 100ft for 20min takes 60 cuft (depth consumption rate of 3.0 which is RMV of 0.75 at 100ft * 20 min)
3. Our Rock Bottom at 100ft (calculated above) is 42 cuft
Total Gas Needed: 4 + 60 + 42 = 106 cuft
Our Aluminum 80, which has 77.4 cuft of gas, does not have enough gas to safely do this dive using our specific RMV.

So take the same dive plan as the above and work backwards to figure out how much time you can spend at 100ft using the dive table above...
Step 1. Find 100ft on the far left column
Step 2. Go across to the right until you reach the last green box (2943)
Step 3. Go up until you get to Bottom Time row
Answer: Roughtly 10 minutes

But does using "Rock Bottom" gas planning make sense for shallow reef/quarry dives where a safety stop isn't required and nobody is telling me to be back on the boat with 500 PSI remaining?

Strictly But we can figure out at what PSI we should start surfacing.
1. My Aluminum 80 has 77.4 cuft of volume
2. My RMV is 0.75cuft/min at the surface
3. My dive will be to 40ft
4. Rock Bottom for 40ft: 12.05 cuft
5. Rock Bottom PSI for 40ft on an AL80: 77.4cuft tank / 12.05 cuft need = 6.42 then 3,000 psi / 6.42 = 467 PSI (but we never reserve less than 500 PSI)
6. Available Gas for Time: 77.4cuft total usable volume - 12.05 cuft = 65.35 cuft
7. My Depth Consumption Rate @ 40ft is 1.66cuft/min (RMV 0.75 @ 40ft)
Maximum Bottom Time for 40ft: 65.35 cuft / 1.66cuft = ~39 minutes

But wait...there's more...Just take the table image above and look for a 40ft dive and go across until you get to the last green box. It's 2438 and 30min. I could have just estimated "Between 30 and 40 minutes" and started my ascent at 500 PSI.