Friday, 13 April 2007

DIR Minimum Gas - Explained

Minimum Gas
The DIR concept of “minimum gas” is the unbreakable rule by which we plan our dives. Minimum gas represents the minimum amount of gas carried by each diver that will allow the diver to get themselves, and another diver, to the next available gas source. For a short shallow dive, this might mean a direct ascent to the surface. For a longer dive with mandatory decompression stops, this might mean the ascent to a depth where it becomes appropriate to switch to another gas. Minimum gas is an absolute. It is never violated during the bottom portion of the dive, and once minimum gas is reached, the only option is to call the dive and ascend immediately.

Gas Rules
This is how we plan our gas to be used. Let’s say our minimum gas is 70 Bar in our twinset, and we start the dive with 220 bar. That means we have 150 bar of usable gas. With that 150 bar, we then apply a gas rule. That rule might be thirds (when the dive MUST return to a starting point), halves (when it would be useful for the diver to return to the start point but not essential) or all useable gas (where the diver can just ascend from wherever they are and be picked up). The “All useable gas” rule is often applied in UK waters where the boat can just come and get us, but we might apply halves if the skipper wants us to return to a shotline. The beauty of the gas rules is that if we get back to our starting point early, and ahead of minimum gas, we can just recalculate our halves, or thirds, and go off again.

Calculating Minimum Gas

The fundamantal principle. At all times, a DIR diver enough gas to get themselves, and a team mate, to the next available gas source.

OK, so in order to know how much gas we have available for the dive, and in order to know our safety factor, we have to be able to calculate minimum gas. I’m going to run through two examples, a thirty metre dive where we can ascend directly to the surface, and a 45 metre dive, where we have to ascend to 21M to do a gas switch

30 Metre Dive

To calculate min deco, we make several assumptions.

• Firstly, we assume an ascent rate of 9M per minute to 50% of the depth, then 3 Metres per minute after that.
• Secondly, we assume a SAC rate of 30 Litres per minute for both divers. This might seem high to some people. But, in an incident, SAC rate can hit 100. Even if it then settles down, it’s going to average out significantly higher than your normal rate
• We assume one minute at the bottom sorting out whatever has gone wrong and getting moving

So, here we are at thirty metres, and all hell breaks loose so two divers have to ascend directly to the surface on one diver’s backgas.

50% of 30m = 15 metres
So, from the bottom

1 Minute resolving Issue
2 Minutes to get from 30M to 15M (plan on 9m/min but most people achieve around 7m/min)
1 Minute to get from 15M to 12M
1 Minute to get from 12M to 9M
1 Minute to get from 9M to 6M
1 Minute to get from 6M to 3M
1 Minute to get from 3M to the surface

This means a 8 minute ascent time.

Now we need to know the average depth. For the dive above, I would call the average depth 10 Metres or 2 Bar

8 minutes of Gas for 2 divers = 16 minutes of Gas required
Depth consumption Rate = 2 Bar X 30 Litres = 60 Litres Per minute

16 X 60 = 960 Litres of Gas Required

Therefore minimum gas for this dive would be 960 litres of gas, or 40 bar in a set of twin 12 litre cylinders.

So, on a 30 metre dive, with both divers using twin 12s, we know we need to leave the bottom before we hit 40 bar to ensure that if there is a total loss of gas for one diver, we can safely and slowly ascend to the surface. This might be far more conservative than other measures due to the slow ascent rate, but it is very comforting to know we can ascend at this rate and still know we have enough gas.

If we have a 220 bar fill in the twinset, and are going to be picked up by the boat, this gives us 160 bar to use during the dive which is nice.

Some people might find the maths a little baffling, but others might just see it immediately. Once you run a few of them through your head you can do it simply enough. We would do this before we hit the water if it was a known depth, or at the bottom of the shotline if it was different than expected.

Now, these figures are always the same, so you could work out the min gas required for each depth, or write them on a slate. However, this defeats the true strength of the calculation, and it is strongly encouraged that DIR divers learn to do this in their heads. Here’s the reason. Let’s say you reach the seabed, and the depth is shallower or deeper than you planned, you can just recalculate minimum gas on the spot and make the most of the gas you have with you without risking safety. That last factor is the real winner for me, and the calculation is simple enough after a while.

Let’s take the calculation a little deeper, shall we say 45 metres?

45 Metre Dive

On this dive, we are at 45M and we are carrying a stage of 50%. Remember that Minimum gas is supposed to get you and another dive to the next available gas source, which in this case is 21M where we would switch to the 50%.

We use the same SAC of 30 Litres Per minute.

80% of the ATAs (or 75% depth) in this case is 34 metres, but for simplicity we deal in multiples of 3 metres, so we will call it 33 metres.

This, then is the ascent profile

1 minute at 45 metres resolving issue
2 minutes to get from 45 to 33 (its more than 9 metres so has to be 2 minutes travel)(upto 80% = 9mtr/min)
1 minute to get from 33 to 27 (80% to 21m = 6mtr/min)
1 minute to get from 27 to 21
1 minute to allow for gas switch

This gives us an 6 minute ascent to the gas switch

Now we need to know the average depth. For the dive above, I would call the average depth 33 Metres or 4.3 Bar.

The calculation, then…

6 minutes of Gas for 2 divers = 12 minutes of Gas required
Depth consumption Rate = 4.3 Bar X 30 Litres = 129 Litres Per minute

12 X 129 = 1548 Litres of Gas Required.

In a set of twin 12s this would equate to 65 bar. Again, we will round this up to be 70 bar. Assuming a 220 bar fill, and again using the gas rule of all available gas, this means we have 150 bar of gas to play with before we have to leave the bottom.

So that’s minimum gas a la DIR.

Remember that you “could” always get up a lot faster than this, but why not plan for a nice controlled, slow ascent, rather than planning for a race up to the surface.

In addition, the numbers here are exact, but in the case of a different bottom depth, it would be advisable to use easier numbers e.g. 4.5 instead of 4.3 (working this through min gas would still be 70bar rounded up).

Finally, as the depths increase, the time spent at 1.5x your normal SAC during the ascent is not realistic and also becomes penalising and so those DIR divers doing Tech 2+ depths use a different average SAC rate than 1.5x

Finally, just a reminder of the Golden Rule

At all times during a dive, we carry enough gas to get ourselves, and a buddy or team mate, to the next available gas source. This rule is never violated.

Hope this has been useful to someone


(updated due to standards change by Wilbo)