Seeing a diver who has run out of gas is extremely frightening. You are aware you are looking at someone who is moments away from drowning. Actually being that person must be beyond terrifying and out the other side.
As DIR divers, this is something we like to avoid, so I thought I'd post the DIR take on gas management - how much gas we reserve for safety. Some people say leave the bottom with 50 bar. Some people say hit the surface with 50 bar. DIR divers work out how much they need for any given depth, and keep that in reserve.
There are two concepts to consider:
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 usable 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 with a single 12 litre cylinder, where we can ascend directly to the surface; and a 45 metre dive in twin 12's, where we have to ascend to 21M to do a gas switch.
To calculate min deco, we make several assumptions.
Firstly, we assume an ascent rate of 9 metres 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.
Finally, we assume one minute at the bottom sorting out whatever has gone wrong and getting moving
30 Metre Dive
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.
8 minutes of Gas for 2 divers = 16 minutes of gas required
Using the SAC rate at 30L/min we can work out the consumption rate at depth.
Depth consumption Rate = Average depth X SAC
In this case the I would call the average depth is 10m, or 2BAR.
Therefore, Depth consumption rate = 2 Bar X 30 Litres = 60 L/min.
What we really need to know is how much gas is needed.
The number of minutes of gas required x depth consumption rate = Total litres gas required
So, 16 mins X 60 L/min = 960 Litres of Gas Required
We said the diver in this case had a single 12 litre cylinder.
Minimum gas in BAR = the total litres of gas required divided by the size of the cylinder.
In this case: 960L divided by 12L = 80 BAR is Minimum Gas.
So, on a 30 metre dive, with both divers using a single 12, we know we need to leave the bottom before we hit 80 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 a single 12, and are going to be picked up by the boat, this gives us 220 - 80 = 140BAR 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 in twin 12's 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.
First, calculate the ascent profile:
The first stop is at 80% of the ATAs; in this case is 34 metres, but we deal in multiple of 3 metres, so we will call it 33 metres. So the rate of ascent is 9m/min to the first stop, then slowed to 3m/min to the next gas source - 21m in this case.
1 minute at 45 resolving issue
2 minutes to get from 45 to 33 (more than 9 metres per min so has to be 2)
1 minute to get from 33 to 30
1 minute to get from 30 to 27
1 minute to get from 27 to 24
1 minute to get from 24 to 21
1 minute to allow for gas switch
This gives us an 8 minute ascent until we are on the next gas source.
We have the same assumption regarding the SAC rate = 30L/min per diver.
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…
8 minutes of Gas for 2 divers = 16 minutes of gas required
Depth consumption Rate = 4.3 Bar X 30 L/min = 129 L/min
16mins X 129L/min = 2064 Litres of Gas Required.
2064L divided by 2 X 12L cylinders = 86 BAR - always rounded up: 90 BAR is the Minimum Gas
Assuming a 220 bar fill, and again using the gas rule of all available gas, this means we have 130 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.
As the depths increase, the time spent at 1.5 x 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 1.5.
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. 本帖最后由 孤風 于 14-10-2013 02:58 AM 编辑
发表于 17-4-2013 09:46 AM
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） tech pass 是不是很难的？ 
希望今年年尾就能上课， 到时候多多关照 
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