Most of us were taught in our open water class that gas planning consists of ?be back on the boat with 500 psi.? The reason behind this is simple:submersible pressure gauges (SPGs) are notoriously inaccurate at the low end and the theory is that 500 psi is a ?safe? margin for error, thus being on the surface with this amount means you still have some gas in your tank.
Unfortunately, this rule of thumb is completely useless when it comes to dive planning. The trouble is that it doesn?t answer the question, ?At what tank pressure do I need to leave the bottom??Think about it for a second. Why do I care about how much gas I have left on the surface? There’s an unlimited supply of it up there! What I really care about is how much gas I have underwater and how much I need to make it back to that unlimited supply without getting injured or drowning!
Consider this realistic emergency situation:
You are happily diving along at 100 ft when a team-mate comes up to you, panicked and completely out of gas. Being a good team-mate, you donate your regulator to him and sort out the situation. Both of you are diving typical single aluminum 80s. You look at your pressure gauge and see that you have 800 psi remaining. Is that enough gas to get you and your buddy safely to the surface? How do you know? What are your options? Do you have enough gas to start swimming for the anchor line, should you begin a safe, controlled ascent now, or do you need to bolt for the surface as fast as you can?
To answer this question, before entering the water we must calculate something called Rock Bottom. Rock Bottom is the minimum amount of gas required to get two divers safely to the surface from the maximum depth including a controlled ascent (maximum of 30 ft/min ascent rate) and safety/decompression stops. With proper planning, just because one diver is out of gas is no reason to blow to the surface or eliminate safety stops!
Rock bottom must never be exceeded! It is the absolute minimum gas reserve that is required to safely ascend with an out of gas team-mate. If a team member reaches rock bottom, the dive is over then and there, without question or debate. Remember, the gas on your back isn’t just your gas, it is also your team?s gas and conversely, the gas on your team-mate’s back, is also your gas.
Just a disclaimer here: you might die if you do this my way, and my description of Rock Bottom may or may not bear any resemblance to GUE’s version of Rock Bottom or Minimum Gas Reserve.
Calculating Rock Bottom
Calculating Rock Bottom for any given dive is really not that difficult, but there are a few pieces of information that must be collected before beginning.
- Actual: capacity of our tank in cubic feet
- Working: pressure of our tank in psi
- Planned: maximum depth of the dive
- Stressed: SCR (surface consumption rate) for each diver
In this example, we?ll be using a typical aluminum 80 tank which has an actual capacity of 77.4 ft3 and a working pressure of 3000 psi (it holds 77.4 ft3 of gas when pressurized to 3000 psi). For the sake of easier math, we?ll round the actually capacity to 80 ft3. In real life scenarios, you probably shouldn’t round up like that. We?ll also assume a dive with a maximum depth of 100 ft. If you don?t know your actual stressed SCR, 1 ft3/min per person is a reasonably conservative number for most divers and helps assist and encourage planning by making the math easier.Exact numbers are not necessary here; just close, but conservative approximations.
The first step is to break up the ascent into sections.For a 100 ft dive, given an ascent rate of 30 ft/min plus a 3 minute safety stop at 15 ft, this dive will have 4 sections:
- 1 minute to handle the emergency at 100 ft
- 3 minutes to ascend from 100 ft — 15 ft at 30 ft/min
- 3 minutes at 15 ft
- 1 minute from 15 ft to the surface
So, it will take 8 minutes to reach the surface.
Next, convert the depth ranges to depth averages in ATA (absolute atmospheres), where one ATA is equal to 33 fsw (feet of sea water).
- 100 fsw/33 msw = 4 ATA
- 100 fsw to 15 fsw = average of ~60 fsw = ~ 3 ATA. Because of the linear ascent rate of 30 ft/min, the average depth can be calculated with the following steps:
- 100-15 = 85 (starting depth minus target depth)
- 85 / 2 = 42.5 (divide by 2 to get average)
- 42.5 + 15 = 57.5 (~60)(average depth plus target depth = true average depth)
- 15 fsw ~1.5 ATA
- 15 fsw to surface = ~1 ATA
Now multiply depth * SCR * time to convert SCR to ft3 at depth.
- 4 ATA* 1 ft3/min * 1 minutes= 4 ft3
- 3 ATA* 1 ft3/min * 3 minutes = 9 ft3
- 1.5 ATA* 1 ft3/min * 3 minutes= 4.5 ft3
- 1 ATA*1 ft3/min * 1 minute= 1 ft3
Add this all together and we see that one diver will require 18.5 ft3of gas to reach the surface with a stressed breathing rate.For two divers to reach the surface using this ascent profile, it will require twice that amount, or 37 ft3 of gas.That is almost half the volume of an aluminum 80!
The last step is to convert ft3 to PSI.This is done with the following formula:
v/wv * wp
where v is the volume of gas (37 ft3), wv is the working volume of the tank (80 ft3) and wp is the working pressure of the tank (3000 psi).
(37 ft3/80 ft3 ) * 3000 psi = 1387 psi (~1400)
Rock bottom on a 100 ft dive with an aluminum 80 is 1400 psi.
Each time you calculate a new rock bottom (if the tank or depth has changed), write it down in your dive log or wet-notes so you have it for easy reference the next time.
So, was 800 psi in an Al80 enough to get you and your out of gas team-mate back to the surface safely with a 3 minute safety stop, under stressed conditions? Probably not. In fact, you should have started your ascent 600 psi ago, long before you had an out of gas team-mate!It looks like you don’t have the gas to make it to the anchor line, so you better start ascending immediately. Your team might be able to make a controlled ascent, but chances are that you’ll end up having to increase your ascent rate beyond a safe limit or omit a recommended safety stop as a result of not spending a few minutes planning your gas supply on the surface.
With proper gas planning and a little discipline to stick to the plan, you will minimize the chances of having to choose risking a decompression injury over drowning. It also helps with peace of mind knowing that you have enough gas available to handle the situation.