DIR : Minimum Gas

Introduction

During your fundamental SCUBA training, you will have been taught about the buddy system and always returning to the surface with a safe volume / pressure remaining. The reason for this is to cover issues that might arise during the dive.

All agencies will universally agree that having some level of reserves is mandated, however there is a differing level of definition and importancy put on it however.




This blog-post is to discuss the DIR philosophy on reserves, known as Minimum Gas, historically called Rock Bottom.

Why Do I Need A Reserve?

The purpose of the reserve is for ensuring that you still have breathing gas available for "unintended" situations. These "situations"could be minor things such as suffering a bad cramp on the way back to the shot line meaning:

• You spend a few minutes stretching it out and 
• Swimming slower while the residual pain fades

Or, it could be a major situation such as getting lost inside a wreck (not that you went inside a wreck without proper training of course!).

This reserve is to cover a multitude of issues.




Typically BSAC state 50 bar as your minimum, although discussions using the rule of thirds is also explored. When I did my PADI training in 2007 I don't recall any true gas planning discussion although there was mention of coming back with a good volume left, of the order 500 psi (because I learned to dive in Florida). It wasn't until I started training with GUE on the Fundamentals course in 2010 that I encountered true gas planning as part of the curriculum.

The DIR view on reserves

Compared to the rather vague guidance from PADI and BSAC, the DIR diving system defines this reserve quite explicitly and it is thus :

Minimum Gas, or rock bottom, is the volume of gas required to get two divers from their deepest point, whilst also having their longest "decompression" obligation, from depth, to the surface, whilst sharing air and completing all safety / decompression stops on the way.

For info : when you move into the technical diving arena when using DIR principles Minimum Gas is re-defined to get you to your "next source of gas", i.e. the deco bottle. If the gas had to get you all the way to the surface, it would be a huge volume and quickly you would find technical diving impossible!!

Hypothetical Scenario

To try and explain what the DIR definition means in practice, lets use a fairly straight-foward scenario. Lets say my numpty buddy loses all of his gas because he has swam into the door frame of a wreck and has smashed both of his 1st stage regulators, he will come swimming over to me, I'll give him my spare regulator and we will head for the surface.




On the way we will have our first deep stop at 50% of our max depth for one minute. We will then have an additional 1 minute stop every 3m on the way up (other diving philosophies use the standard 3 minute / 6m safety stop) and then we will hit the surface and he can breathe the air.




From the point at which he starts breathing my spare regulator, to the point at which he breathes the atmosphere - I need that amount of gas as my reserve, that's my minimum gas that I will partition as a reserve when I enter the water.

How Do I Work Out My Reserve Volume?

To work out this volume of gas, you first need to know the rate at which your buddy and yourself consume your gas. This is often referred to as your SAC rate (surface air consumption) or RMV (respiratory minute volume). If you don't know this, a safe estimate is to use 25 lpm (litres per minute).

The next bit is simple maths, if you know the rate of gas consumption (say 25 lpm) and you know your depth (say 35m) and you know how long you are at depth (say 20 minutes) you can calculate the gas volume requirement.




Using the numbers just stated in this paragraph, the total gas volume is :

     25 (lpm) x 4.5 (35m in ATM) x 20 (mins) = 2,250 l

But I need the gas needed to get up, not stay down...

Now that you know the process for calculating the gas required to stay at a certain depth for a certain amount of time, its just a case of repeating it, but for smaller durations and for different depths to work out the gas we need to come up. What we are going to do is "plan a dive", but the dive is getting us from the bottom to the top.





All of this is two divers on the same gas supply, we also need to make some assumptions about the divers:

• Diver 1 gas consumption rate = 25 lpm
• Diver 2 gas consumption rate = 50 lpm (doubled due to stress - easily achieved, I personally have seen my own gas consumption rate hit 100 lpm when under stress)

If we were doing a 30m dive then the "ascent profile" could be described as:

• 3 min @ 30m (problem solving and launching an SMB)
• 1 min @ 15m (deep stop)
• 1 min @ 12m (1st stop 3m shallower for 1 minute)
• 1 min @ 9m (next stop 3m shallower for 1 minute)
• 1 min @ 6m (next stop 3m shallower for 1 minute)
• 1 min @ 3m (final stop 3m shallower for 1 minute)

We will ignore the travel time and gas consumed between the depths such as 9m to 6m because the gas consumed in this transition is going to be pretty small and fairly insignificant (we are not looking for an answer to the nearest litre in practice), but what we can't ignore is the ascent from the bottom, 30m, to 15m - that is going to take up a while to do that move, and we are at a reasonable depth, which means we will consume a non-insignificant amount of gas.






Assuming we ascend at 9m per minute, its still going to take us the best part of 2 minutes.

But how do you calculate the gas requirement for a changing depth? The easiest way to work this out is to average it and then use the gas calculation under the assumption that all the time is spent at that depth (to simulate the ascent).

The way I do it is to say "where am I after half the time it will take me to perform the move" - its going to take us about two minutes to move up to 15m, so half of this would be one minute. After one minute we will have ascended to 21m (because we are moving up at 9m/min from 30m). That's the depth we will use to make our "travelling gas" calculation (we are creating an artificial stop here).

So knowing this we can update our accent profile described above to include this, therefore the new profile is:

• 3 min @ 30m (problem solving)
• 2 min @ 21m (artificial stop)
• 1 min @ 15m (deep stop)
• 1 min @ 12m
• 1 min @   9m
• 1 min @   6m
• 1 min @   3m

Now we can use our time x depth x sac formula to calculate the has required at each of these stops. (the 75 lpm is the combined consumption rate of the unpanicked diver, plus the donating diver).

• 30m = 3 (mins) x 4.0 (ATM) x 75 (lpm) = 900 l
• 21m = 2 (mins) x 3.1 (ATM) x 75 (lpm) = 465 l
• 15m = 1 (mins) x 2.5 (ATM) x 75 (lpm) = 188 l
• 12m = 1 (mins) x 2.2 (ATM) x 75 (lpm) = 165 l
•   9m = 1 (mins) x 1.9 (ATM) x 75 (lpm) = 143 l
•   6m = 1 (mins) x 1.6 (ATM) x 75 (lpm) = 120 l
•   3m = 1 (mins) x 1.3 (ATM) x 75 (lpm) =   98 l

Now when you add up the gas required at each depth above it comes to a total of 2,079 litres. That sound like a lot doesn't it? That's almost 190 bar in a single 12l or 85 bar in twin-12s.


Sayy what?!

So if you were to go on a 30m dive with your buddy, and you both breathe 25 lpm (but your buddy's breathing rate doubles to 50 when under stress), you would jump in and once your SPG reaches around 200 bar you'd need to start heading up! If you didn't and waited until you passed that pressure, THEN your buddy loses his gas supply, you won't make it to the surface as you'll have consumed part of your reserve that you needed.

In practice, the above calculation does demonstrate a significant level of conservatism in that the "victim" has an elevated breathing rate for the entire ascent. In practice that would come down. I've also used two pretty high breathing rates, those could be applicable for new divers, but for more experienced divers it's overly cautious.

Using the exact same profile, but for two divers with a SAC of 16 LPM and 18 LPM, with the latter diver the victim (subjected to the doubled SAC) the volume requirement becomes:

• 30m = 3 x 4.0 (ATM) x 52 = 624 l
• 21m = 2 x 3.1 (ATM) x 52 = 322 l
• 15m = 1 x 2.5 (ATM) x 52 = 130 l
• 12m = 1 x 2.2 (ATM) x 52 = 115 l
•   9m = 1 x 1.9 (ATM) x 52 = 99 l
•   6m = 1 x 1.6 (ATM) x 52 = 83 l
•   3m = 1 x 1.3 (ATM) x 52 = 68 l

Total: 1,441 (120 bar in single 12, or 60 bar in twin-12s).


For information - thats the minimum gas I enforce when my buddy and I are diving, if I'm feeling "dived up" then I might round that down to 50 bar (more comfortable in water, lower SAC rate), but if I'm returning to diving after a long spell away from it, I will enforce the previous (85-90 bar) reserve (out-of-touch with diving, less comfortable in the water resulting in a higher SAC).

Conclusion

Hopefully from this article you can see the importance of having a robust reserve calculated, and sometimes the "just make sure you are back on the surface with 50 bar" might not be enough. You should be able to follow the maths I did in this article for using your own consumption to have a tailored reserve just for you.

I hope this has been an interesting read, any questions, let me know!