Ratio Deco at the Tech1 Level.
Firstly, a few caveats. I have put some thought into all of this so please don't skip them. I expect they will be obvious to everyone on YD, but perhaps not to the person who googles Ratio Deco and stumbles across this post.
1. This post does not represent Ratio Deco as taught by GUE, FifthD or indeed any other agency. It is my interpretation of what I have been taught. Thus, any errors are my own.
2. Ratio Deco usually forms only a small part of a larger course and requires a rounded grasp of specific skills that allow Ratio Deco to be utilised safely. Thus, before using Ratio Deco I can only recommend you seek such training. Please do not dive the profiles suggested in this post without undertaking such training. You could end up hurt or dead.
3. Ratio Deco works in conjuction with a specific set of variables. There are no rules, only guidelines. Use them at your peril.
A few myths dealt with
It's mystic sorcery and "secret" information - well it aint anymore if it ever was
It's complicated - keep reading, it's so simple it's dissapointing once you learn it!
It requires lots of training - The skills required to handle RD do take time to learn. RD itself takes about 15 minutes on Tech1
OK. Now that's out of the way, are you all sitting comfortably, good, then I'll begin....
Has anyone read AndrewG's article on Ratio Deco? The man may be a diving God, and may no more about decompression than most people on the planet, but by God he's a terrible writer. Or perhaps I am simply insulting him and it's written in a such a fashion to enocurage people to take courses and have the concepts clearly explained. The only documentation on RD I have been able to find on the web has been obfuscated by crepuscular logic and an involute style which, frankly, gets on my tits. I thought it might be useful to post some information about ratio Deco that demystifies it, and allows for a more informed debate and discussion on the topic.
So, what is Ratio Deco.
Garf's Definition - the Wordy one
Ratio Deco is defined as being a set of rules that allow an individual or team to calculate the amount of mandatory deco and ascent rates based on the average depth they have been at and the length of time they have been at the bottom.
Garf's defintion - The easy one
If we know how deep we are and how long we've been there we can work out the deco on the way up in our heads without the need of a computer.
"Ratio" refers to the fact that, for a given depth, there is a ratio between the length of time a diver spends on the bottom portion of a dive and the amount of decompression they will have to do on the way to the surface. "Deco" refers to the fact that this process is managing the ascent portion of the dive, including all deep stops, ascent rates, and shallow stops.
Other definitions
"Bottom Time" - The time that passes from the time we arrive at the bottom to the time we leave the bottom. Notice does not include the descent portion of the dive.
"Bottom Gas" - 21/35 or 18/45 Only. The two are interchangeable and the decompression required stays the same. Handy huh
"Decompression Gas" - 50% Only.
The Pre-requisites
In order to make Ratio Deco work for us, there are a number of things we need to be able to do, and a few things we must have with us.
Firstly, we need to be able to control our rate of ascent. I don't mean stop ourselves from rocketing to the surface, although that's a handy skill. I mean we need to be able to change our rate of ascent on demand. For Tech1 level diving, we need to be able to ascent at a constant 3 metres per minute where necessary, 6 metres per minute where necessary and 9 metres per minute where necessary. If we cannot do any of that, don't go near Ratio Deco.
Secondly, we need to have with us standard gases. The ratio deco I am going to explain in this post works when we use either 21/35 or 18/45 as backgas and 50% as deco gas. And that's it. Mess up the standard gases, and it all goes to ratshit. With some confidence and experience behind you, you could alter the deco to compensate, but I am a long way from that in my diving and for the sake of simplicity and safety let's just say that the standard gases are the rule. There are other standard backgases, and other standard decompression mixes, but we are not interested in them at the Tech1 level.
Thirdly, we need to understand something of the limitations. During the course of this post, I am going to explain that Ratio Deco is anot a rigid set of rules, but needs to be adjusted based on variables such as depth. For the sake of this post, I am going to say that the guidelines I give out are good to 48 metres and 40 minutes. Deeper or longer than that and the ratio we are going to use starts to become a little wooly, and needs to be adjusted until we reach the next "set point", which I shall explain in a moment.
Concept One : The Set Point: Calculating Decompression Required.
forget about Rebreathers, this is a different type of set point. The set point is the starting point for calculating Ratio Deco. At the tech1 level, we use 45Metres as the set point. As you increase in knowledge, you may use 66M as a set point and so on.
So, for Tech1, we use 45M. At this set point, we assume that there is a 1:1 Ratio between bottom time and decompression. That means for every minute we spend on the bottom, we spend a minute decompressing. There, wasn't that nice and simple. Minute on the Wreck, minute going up. easy.
Now, there are other Ratios. for example at 66 metres, the gases all change and the ratio is 2:1. for every minute on the wreck we need to do 2 minutes deco. Now I'm not going to get into that here becuase, frankly, I'm not trained to do it.
Let's go back to our 1:1 ratio at 45M. We do a 30 minute dive to 45M (average depth), we need to do 30 minutes decompression. Nice. However, what if our average depth was greater or less than 45M. Well for every 3 metres (or part) shallower than the plan, we take 5 minutes off the deco. for every 3 metres (or part) we add 5 minutes.
So, calculating the amount of decompression we need to do is very simple indeed. Look at the average depth and the time, and compare to 45 Metres.
Example 1: 38 metres for 40 minutes - We're 2 lots of three away from 45 (rounded down) so it must be 10 minutes less than 40 - 30 minutes.
Example 2: 47 metres for 25 minutes - We're above the 45M setpoint by part of a three so it must be 5 minutes more than 25 - 30 minutes.
So, you can see that it's very easy indeed to calculate the decompression in the 30 to 48 metre range. It get's a little more complicated if you go deeper as you are moving between the 1:1 Ratio at 45 metres and the 2:1 ratio at 66 metres, so the question arises of what do you do in the middle. That is a question I sort of know the answer to, but am not qualified to post, and it's outside the scope of what I want to cover in this post anyway. Let's stay at 48M or below to keep things nice and simple.
Portions of the Dive.
Let's go back to our 30 minute dive to 45 Metres. We now know that we have 30 minutes decompression to do on the way up, but how do we do it. Where do we do our stops, and how long should each stop be.
Well, the areas of the dive can be broken down as follows;
Bottom to 80% of Average ATAS = Lower Portion of Ascent.
80% of ATAs to 21M = Deep Stops
21 metres = Gas switch
21M - 9M = Intermediate Stops
6M - surface = Shallow Stops
Or, just for Janos, a pretty chart...
Now, let's deal with each portion of the ascent in turn.
Bottom Portion of Ascent
At this point, we are still ongassing. So, there's no reason to hang about with slow ascent rates. We use 9 metres per minute as it's quick enough to get us out of dodge, but slow enough so it can be brought to a stop comfortably. We want to get up to 80% of our ATAS at 9 metres per minute because if we do it any slower then we are going to bugger up our decompression by ongassing too much. It is critical at this point not to hang about. Once the decision is made to leave the bottom, we leave.
We need to get up to 80% of our ATAS before anything changes. Why 80%. Becuase it's around this area that we stop ongassing and start off gassing. Now, as with everything in DIR, it's not perfect for all oaccassions, but it's about the best standard you can apply to all situations, which is really what the entire concept of DIR is all about. so 80% of our ATAS. How the hell do we work that out. Sounds complicated. Here's the trick...
Let's say we're our average depth was 45 metres. convert that to ATAS =5.5. divide by ten = 0.55. Mutiply by 2 = 1.1. Take that off the depth in metres. 45 Metres - 1.1 ATAS = 34 metres. So, 80% of the ATAS when we have been diving to 45 Metres is 34 metres.
Let's do another example. We've been to 42 metres. Thats 5.2 ATAS. Multiply by 2 = 10.4. Call it 11. Take that off the depth in metres. 42 - 1.1 ATA (11m) = 31 metres.
Repeat that above excercise a few times and you'll find you can do it in a heartbeat without thinking about it, and it's the most complicated bit of maths we have to do in the whole excercise.
Now, just to make things a tad more complicated, we always round to the nearest three, becuase then it's a bloody doddle to do 3 metres per minute up to 21M. So if our 80% is at 34M, we'd actually go up to 33. If our 80% was actually 31 metres then we'd stop at 30M. See how that works?
So, taking our 30 minute dive to 45 metres. We leave the bottom after 30 minutes and we ascend at a rate of 9 metres per minute until we hit 33 metres. This probably takes us about 75 seconds.
Deep Stops Portion of Ascent
OK, so we hit 33 metres and we are now in the deep stops portion of the ascent. At the Tech1 level, this is simple. We do 1 minute stops every three metres. Looking at this another way, we can just do 15 seconds move, 15 seconds stop, which looking at it yet another way, could be done as a 6 metre per minute ascent, which is actually what we do. So all we have to do at this point in the dive is slow down from 9 metres per minute to 6 metres per minute, and then ascend at that rate until we hit 21 metres.
Now, this is an area where I could go on for ever more. The deep stops change if you go beyond Tech1 levels, and all of a sudden the maths gets a little more complicated with longer stops and changing ascent rates, but that's for another day. On our nice, simple 30 minute dive to 45 metres, all we have to do is slow down the ascent rate until we hit 21 metres.
21 Metres - the Gas Switch and first intermediate stop
21 metres is where the work begins. Here we have to switch the team from their back gas onto the decompression gas of 50%. We also need to put up a bag, and work out how much deco we have to do.
Let's get going. The gas switch. The procedure for that is another post, and I never would have believed this until trained to do it, but switching the entire team from backgas to decompression gas takes about 10 seconds. Backgas regulators are clipped off. Now, the roles come into play. Typically one person sends up a bag from 21 whilst another person is running the deco.
The first intermediate stop depth is 21 metres, however we stop at 21M for at least 3 minutes regardless of how much decompression is actually required. why do we do this. We do this to open the Oxygen Window. Now, I've read and know at least 4 different versions of what the words "Oxygen Window" actually mean, but all of them seem to agree that it's not a bad idea to spend at least 2-3 minutes at the depth where you switch to your decompression gas containing a higher partial pressure of O2. Obviously, several compartments are still ongassing, and also you have a limited gas supply, so you don;t want to stay there forever, but 3 minutes as a minimum for Ratio Deco as I am explaining it.
Shaping the Decompression Curve - Intermediate Stops.
Ahh, Howard it putting up the bag, I'd better work out what deco we are going to do before he finishes or he'll only take the piss. Well, we did 30 minutes at 45 metres. That means we have 30 minutes of Decompression to do. so where do we do it
Well, we need to do half our decompression in the intermediate stops, and half in the shallow stops.
Let's take our dive, 30 minutes at 45 metres. We have 30 minutes of decompression to do. that means we have to do 15 minutes at 21-9 metres (see table 2) and 15 minutes at 6-0 metres (see table 2).
Now, our intermediate stops are alwasy the same. They are
21M
18M
15M
12M
9M
That means we have five stops to do. If 50% of our deco is therefore divisible by 5, it's simple. We have 15 minutes to do in the intermediate stops. So we would do the following
21M - 3 minutes
18M - 3 minutes
15M - 3 minutes
12M - 3 minutes
9M - 3 minutes
Now how bloody simple is that.
There is an endless debate about whether you should do a linear curve as I have described above, or whether you should minimise the stops in the middle and load the start and end. This would mean you maximise the deco from the partial pressure of 50% and then start to push the gradient, and ignore the stuff in the middle. This becomes more apparant when the deco is NOT divisible by 5, becuase then the divers have a choice about where to do the stops.
Let's look at a few examples of that.
Let's say we have 18 minutes to do in the intermediate portion. I'd probably do
21M - 4 minutes
18M - 4 minutes
15M - 2 minutes
12M - 4 minutes
9M - 4 minutes
but we could also do
21M - 4 minutes
18M - 3 minutes
15M - 3 minutes
12M - 4 minutes
9M - 4 minutes
There is no right or wrong, and different people do different things. It becomes far more regimented at the Tech2 and Tech3 levels, but at the Tech1 level, you will get away with pretty much anything. Howard has put the bag up now. we'll wait until the clock hits three minutes at 21M and then move through the intermediate stops. We'll actually move a little faster so we are moving for 30 seconds and stablisiing for thirty seconds. Up we go to 6 metres.
Shaping the Decompression Curve - Shallow Stops.
Well, here we are at 6 metres. We had 30 minutes of decompression to do, 15 of which we did in the intermediate stops, and 15 of which we need to do here. We have a few options here. We could just do 15 minutes at 6 and then ascend over 5 minutes. If the surface conditions did not allow this we could do 18 mins and then a 2 minute ascent. Team foxturd take the best of both world, and would do the 15 minutes at 6 and then attempt a 5 minute ascent. But we're in no rush to get to the surface.
That was easy, wasn't it.
The simple example
45 metres. 30 minutes.
We thumb the dive and leave the bottom. We need to get to 80% of the ATAS. Let's see, thats 5.5 X 2 taken away from 45. That's means 34 metres. Let's call it 33.
We go up to 33 at 9 metres per minute. It takes just over a minute. We slow down to 23 metres per minute and it takes us another 4 minutes to get to 21 Metres. We started at 45 metres so its the 1: 1 ratio. We did 30 mins so we have 30 mins deco to do, 15 in the intermediate stops and 15 in the shallows.
We then switch gas and bag up.
15 minutes is divisible by 5, which makes the deco easy...
21M - 3 minutes
18 M - 3 Minutes
15M - 3 Minutes
12 - 3 Minutes
9M - 3 Minutes
Now we are at 6 metres. We have 15 minutes to do so we do them and then ascend as slowly as possible
The complicated example.
Here we are on the Salsette. We have been swimming around and over things so the average depth is actually 42 metres. We have been on the bottom for 28 minutes. Gareth hits minimum gas, so we thumb the dive. We leave the bottom immediately on thumbing the dive.
Up we go at 9 metres per minute. 80% of the ATAS from 42 metres is 5.2 *2 = 10.4 taken away from 42 = 11 metres, so lets call it 31 metres. Actually, we call it 30. It takes us a minute and a bit to get there.
At 30 we slow down to 3 metres per minute. It takes us a further 3 minutes to get to 21 Metres. During which time I'll have probably worked out the deco.
Now we gas switch and put a bag up. We do 3 minutes at 21M.
We did 42 metres for 28 minutes. thats 3 metres less than 45 so take off 5 minutes. We have 23 minutes deco to do. Sod that, let's do 24
Half of 24 is 12, divided by 5 is not possible so let's do the following
21 M = 3 Minutes
18 Metres = 2 Minutes
15 Metres = 2 Minutes
12 Metres = 2 Minutes
9 Metres = 3 Minutes
Up we go through the intermediate stops.
Now we are at 6 metres. We have 12 minutes to do
Let's keep things simple. 12 minutes at 6 and then ascend as slowly as possible.
Dive Over.
Now wasn't that painless?
Comparisons with Dive Models
The nearest model to ratio deco for the sake of comparison is gradient factors of 30/85
Taking our two dives,
the 45 metre /30 min dive would give us a runtime of about 66 mins (with a 3 minute ascent from 6m), as compared to a runtime of 63 mins from decoplanner.
the 42 metre / 28 min dive would give us a runtime of about 59 mins (with a 3 minute ascent from 6M), as compared to a runtime of 53 mins from decoplanner.
It will be closer or further away depending on the depths and times. Ratio deco is not perfect, it suits some dives better than others.
Summary
DIR is not about a perfect configuration, or a perfect set of standards. It's about providing a kit configuration and set of standards that can be applied everywhere. It may be a good fit in some situations, and a poorer fit in others, but there is so much advantage in having the standard that DIR divers who think through all the standards just have to accept this.
Ratio Deco, like everything else in DIR, does not provide the perfect option for all scenarios. However, it does provide an option that fits all scenarios to a greater or lesser degree. Learning where these degrees are is all part of the education process. If we dived to 54 metres, I probably wouldn't be happy with 1:1, but would start padding it out.
Whilst on the subject of padding, the question may arise where one person in the team wants to do more deco. It doesn't matter why. The most conservative person always wins. Of someone wants to extend the stops, that's what we do, no discussion, no arguments.
One interesting point is that if you always round up the deco to the next ten minutes, it all becomes ridiculously easy. Lets say we have 30 minutes deco to do. That's 15 in the intermediate stops and 15 in the shallows. I get to 18 metres and signal 3. If we follow the rule of always rounding up the deco to the next ten minutes then the team know from that one signal exactly how much deco I am planning on doing at each stop, and the total amount of deco we have to do. However, that is making the assumption that you don't mind doing a few extra minutes deco.
We do not really have to worry about carrying enough deco gas becuase using this form of ratio deo and using twin 12s means we will not be able to get into the position of not having enough deco gas unless we lose our AL80 deco bottle. If that happens, Ratio Deco also allows for a lost deco gas. We can just double the stops and deco out on the back gas. At the Tech1 level, it just works in terms of gas logistics.
Now, just on a closing note, for the love of God don't go out and do this without training in all the other elements I have mentioned. However, I hope it has been useful information and an interesting read
Dive safe everyone
Garf.
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
Garf
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
Garf
(updated due to standards change by Wilbo)