Choosing your deco mix?

[JAG]

I have used several diferrent deco mixes from 50% to 80% and has been said, you assess your dive and use a mix you believe to be correct for that dive. That said though, I'm using 50% more though due to being able to get on it quicker and saving a bit of back gas for the second dive of the day when you can't get a fill between dives. I'm also trying to get the old muscle memory type process going so that it becomes more automatic and you get used to the runtimes on similar dives so that if the all computers failed and I lost my buddy I would have an idea of deco obligations.

[Mark Chase]

Originally Posted by big_si

Hi all,

I'm just getting into the whole deco kettle of fish and am doing relatively small amounts of deco. I am going to be doing Adv. Nitrox and Deco procedures with Mark Powell this year (just trying to sort out some dates Mark and then I'll be in touch to see what we can sort out!)

I was just wandering what factors come into play when planning what deco gas you're going to use?

For example. What changes your choice between having your stage (assume one stage) filled with 50%, 80% or 100%?

Obviously there are different MODs so you can switch to your stage and accelerate your deco at different stages of the ascent but what are the benefits of each? Are there safety factors that come into play (i.e. one mix being better to use to decrease chance of a bend)?

I've played around with Nautilus for a 40m dive with 30min bottom time and got the following data using air as back/bottom gas;

Deco Gas Run time
50% 72mins
80% 67mins
100% 67mins

So very little different in run time. (Best combination I came up with if using 2 stages was 50%+80% with run time 49mins)

Which deco gas would you choose for the above example and why? What sort of dive depths / times would prompt you to use a different mix and why?

I guess this question kinda rules out the DIR input a bit as I understand that you'd be diving trimix for this depth and that might change the numbers a bit? I'm still very interested in what your answer is too though.

Thanks all for your input!

Si

This may or may not be covered on the course. The worst i will be doing is giving you questions to ask. Just please don't say "But Mark Chase said" because if he gets pissed off with all the questions its on your head not my'n



There are several ways to approach decompression gas choice.


1: Optimum decompression.

Optimum decompression is a balance between, most efficient, cleanest and least hassle.

As an example on a 40m dive for 60mins optimum decompression would be best mix @ 1.4 at max depth (28%) and 50% and 100% for deco.

60mins on the bottom = 37mins deco (Using 20/85 GF)


In terms of gas usage and gas required for my SAC this dive is comfortably within twin 12s and two 7ltr stages.


+ Sides

• Pretty much the least deco you will do on this dive without a CCR
  
• Nice PN2 gradient steps for good deco
  
• Clean deco using 100% with no inerts

- Sides

• Carrying two deco stages
  
• Deep bailout gas is not breathable before 21m (30m if you agree with 2.0pp02 for emergency)
  
• 100% for deco forces a 6m stop which can prove imposable on shot line ascents and difficult in rough sea conditions.
  
• 100% is often difficult to fill to 200+bar and the LDS may not be able to give you enough gas to complete the deco unless you use a much larger cylinder.
  
• LDS may refuse to fill a large cylinder because they are running low on 02.

Minimal equipment / balancing the deco against the dive

Whilst carrying two stages is super efficient deco it adds equipment stress, drag and the potential for error into the dive profile demanding greater skill levels and attention to detail. Its hard to screw up a gas switch if you only have one gas. Its easier to carry reels etc when one side of the body is clear. Carrying one stage in a balanced manor is much easier than carrying two especially if they are all on one side.


So on a 40m for 60min dive you may consider only one deco gas to be suitable. Your choice is now a little more focused. Do you choose a pure 100% which Will give clean deco? Well if you run the numbers through a planner it gives 50mins of deco but it puts a lot of pressure on your back gas. If the plan goes exactly as it should you will only have a reserve of 50bar. So you have to decide if this is a sensible plan.

Option B would be a deeper gas. 80% is only saving you 10 bar of back gas to you have to get radical and look at a deeper deco mix.

50% gives you 55mins deco so only 5min more than 100%. but you now have a slightly more sensible 80 bar reserve to get you and possibly your buddy from 40m to 21m in an emergency. Planned on a 30 sac X 2 at avg 30m depth for the ascent it gives you both 8mins to get to 21m which is plenty if your switched on, organized and well rehearsed.

50% Pn02 spike is good and it keeps you above 1.0pp02 for most of the ascent with the worst being a drop to 0.81 at 6m. Staying above 1.0 at all times for deco is optimum.


+ Sides

• Minimum kit stress
  
• Minimum skill set required at stage deco level
  
• Easy fills
  
• Efficient gas choice in terms of gas management
  
• A standard gas choice for a lot of divers

- Sides

• A 7ltr is a bit small for this dive so you might have to use a 10 or an AL80
  
• 18mins (50%) more deco than using 50% and 100% and inerts in the mix all the way
  
• You still have to get from 40m to 21m to switch gas in an emergency

DIC Deco

I don't know what to call this so Ill call it DIC deco. (Doing it Chasey) This is not specifically in the books and is strongly influenced by my own local diving considerations and the fact i hate deco and love redundancy. The following advice there for needs the most scrutiny.

DIC deco has three main goals. One: is to make the gas logistics of the dive work to maximize the potential of twin 12s. Two: is to minimize deco and Three: is to provide redundancy

Whilst 40m is not the best example of this idea Ill stick with it as I am assuming no trimix training as yet.

DIC deco: Best mix on the bottom 28%. 36% and 80% for deco

40m for 60mins = 40mins deco

Why?

OK now i have minimized the stress on back gas with a 83 bar reserve and i am comfortably inside doing the dive with two 7ltrs or even two Ali 40's. I have no problems if we are doing a shot line ascent because I can get on the 80% at 10m and do all my deco at 7m if I need to. I have no problems with fills as i require a much lower pressure on my 80% using two gases for deco and even if i chose Al40s i have fewer problems getting 200bar of 80 than i do getting 200bar of 100%

All good but there is the further advantage that my deep switch is at 33m. If i switched at 40m id only be on a 1.77 PP02. I now have a deep gas redundancy. I now have a 7ltr pony. I now do not have to rely on my buddy to get me out of trouble should i loose back gas. i am self sufficient and I am good for a solo dive should this occur by choice or by accident. I am also good for wreck penetration with improved levels of safety.


+ Sides

• Efficient deco with only 3mins more than the optimum 50 and 100%
  
• Gas logistics are minimized in all areas of filling, gas planning and restricted deco depths.
  
• Self sufficient rig reducing the pressure on the buddy to perform in an emergency situation and reducing the stress on you to have faith in your buddy.
  
• Smaller lighter tanks can be used for the dive reducing the hassle of diving two stages.
  
• Additional safety margin for entanglement or penetration of the wreck

-Sides

• Not quite optimum deco
  
• Inerts in the final deco mix
  
• Non standard deco gas and profile so buddy's will have to agree to it
  
• Still needs two tanks with the necessary increased skill-set preparation and risk of gas switching error.

Whilst DIC deco sounds great for a 40m dive the numbers start to fail when you get deeper unless your will ing to consider bailing out on a high PP02. The numbers work for me because in an emergency I am but this is against the advice of many experienced an well qualified divers so do it DIC at your peril.

ATB

Mark Chase

[Garf]

40 Metre Dive for 30 Metres a la DIR.

We'd be diving 21/35 Trimix with 50% in the deco bottle. If I ran the deco, we'd do something like this below....

30 minutes at 40M
2 minutes to 30 metres
1 mninute to 27 metres
1 minute to 24
1 minute to 21

now we have 25 mins of deco to do. Lt's call it 26. Half of which done in the intermeditate stops, half of which done at 6m

3 minutes at 21 including gas switch
2 @ 18
2 @ 15
3 @ 12
3 @ 9
13 @ 6

Then I'd do a 5 minute ascent from 6 metres to the surface, giving me a total runtime of about 66 minutes. This is more conservative than it needs to be but I'm in no rush to get back on the boat.

[big_si]

Thanks hugely for all your replies guys! Really appreciate it! Especially the huge answer by Mr Chase! Interesting to see the DIR approach too. Cheers Garf!

Opened my eyes to a couple of factors that I hadn't thought of due to my current reletively short bottom times. If extending this bottom time or using an 80+% mix then the drain on my backgas comes into play. At present I always have plenty of redundancy on backgas which gives me a good 2nd dive bimble if there is one.

Previously the only direction I was trying to understand it from was from the Deco efficiency and why certain mixes would be chosen. I could see why maximising the ppO2 would be ideal and could see how using 2 stages to keep the pressure gradient high would be better.
I can now also see further benefits to the CCR in terms of being able to maintain a 1.4-1.6ppO2 set point and therefore constantly use optimum mix and get really efficient deco.

I'm REALLY looking forward to doing the course now! Lovely lovely knowledge!!!

Thanks again guys! Would be greens all round if I could!

[Clare Gledhill]

I guess this question kinda rules out the DIR input a bit as I understand that you'd be diving trimix for this depth and that might change the numbers a bit? I'm still very interested in what your answer is too though.

Deco gas choices for me are made slightly easier by standard gases - but contrary to what you read on the internet we try not to leave our brains on the dock - lets look at a few options.

Take your 40 metre dive - for enough time to incur deco - lets say 25 minutes of deco a la Garf's example. The deco that he set out is fine - I'd be happy to dive it. What are the other choices available?

Well we can deco on back gas obviously - long, boring, cold and not very efficient. Beyond

A large proportion of the deco on this profile is at 6 metres - where oxygen would be a good gas to have as it is at a high partial pressure at the stop. We have done what is a fairly short dive so our O2 exposure has not been too great - 50 per cent at this 6 metre stop is giving us a PPO2 of only .8 in comparison. If I were going to take O2 on this dive I'd shift the deco slightly which would put me on O2 after about ten minutes of ascent time.

That ten minutes is the reason why 50 per cent is a good choice for this dive - for gas reserves rather than decompression issues. As Mark set out, we can switch to it at 21 metres and, if we have a problem at depth and have to get ourselves and a buddy to the next available source of gas, a 50 per cent stage will be much nearer to us time wise - so will cut down the amount of gas that we need to carry in reserve.

OK - so 50 per cent gives me a gas to switch to early on - and O2 gives me an efficient gas for the stop where I'm going to spend some time - so why not take both?

Well - can I be bothered to take two stages on a dive to save myself perhaps 3 or four minutes of deco - with all the complicating factors of taking those bottles around with me on the dive and managing them on ascent. To be honest - for a dive of this duration - no. Make the dive a lot longer - say 90 minutes and you save 40 minutes from taking the O2 bottle (apart from the fact that you would need to take two stages for deco gas anyway so may as well take two different mixes).

Most of us don't plan 90 minute bottom times though - so lets reference it in something more tenable.

30 metres for 60 minutes. A dive at this depth is not going to require me to stop at 21 metres - so pausing there to gas switch is not very sensible from a gas loading perspective.

Seven minutes will get me comfortably to 6 metres, I can cope with that in my gas planning and I will build a little in for coping with any problem that I encounter. Carrying O2 on this dive means that my deco will be completed on an optimum gas which will mean I can either get out more quickly or sit there and enjoy the additional conservatism that the gas gives me.

Does that help?

[big_si]

Originally Posted by Clare Gledhill

Deco gas choices for me are made slightly easier by standard gases - but contrary to what you read on the internet we try not to leave our brains on the dock - lets look at a few options.

Take your 40 metre dive - for enough time to incur deco - lets say 25 minutes of deco a la Garf's example. The deco that he set out is fine - I'd be happy to dive it. What are the other choices available?

Well we can deco on back gas obviously - long, boring, cold and not very efficient. Beyond

A large proportion of the deco on this profile is at 6 metres - where oxygen would be a good gas to have as it is at a high partial pressure at the stop. We have done what is a fairly short dive so our O2 exposure has not been too great - 50 per cent at this 6 metre stop is giving us a PPO2 of only .8 in comparison. If I were going to take O2 on this dive I'd shift the deco slightly which would put me on O2 after about ten minutes of ascent time.

That ten minutes is the reason why 50 per cent is a good choice for this dive - for gas reserves rather than decompression issues. As Mark set out, we can switch to it at 21 metres and, if we have a problem at depth and have to get ourselves and a buddy to the next available source of gas, a 50 per cent stage will be much nearer to us time wise - so will cut down the amount of gas that we need to carry in reserve.

OK - so 50 per cent gives me a gas to switch to early on - and O2 gives me an efficient gas for the stop where I'm going to spend some time - so why not take both?

Well - can I be bothered to take two stages on a dive to save myself perhaps 3 or four minutes of deco - with all the complicating factors of taking those bottles around with me on the dive and managing them on ascent. To be honest - for a dive of this duration - no. Make the dive a lot longer - say 90 minutes and you save 40 minutes from taking the O2 bottle (apart from the fact that you would need to take two stages for deco gas anyway so may as well take two different mixes).

Most of us don't plan 90 minute bottom times though - so lets reference it in something more tenable.

30 metres for 60 minutes. A dive at this depth is not going to require me to stop at 21 metres - so pausing there to gas switch is not very sensible from a gas loading perspective.

Seven minutes will get me comfortably to 6 metres, I can cope with that in my gas planning and I will build a little in for coping with any problem that I encounter. Carrying O2 on this dive means that my deco will be completed on an optimum gas which will mean I can either get out more quickly or sit there and enjoy the additional conservatism that the gas gives me.

Does that help?

Thanks Clare! More great info!

So as I understand it the choice of gas comes down to the balance of depth / time of a dive and maximising your ppO2 (above ppO2 of 1bar ideally) to maximise efficiency.

I know it's not easy to simplify as there are a number of variables but am I correct in summarising that this thinking would mean...

Deeper (40m-ish for me) dive for x mins would suggest opting for a 50% deco gas to provide a reachable MOD whilst having a more useful ppO2 upto about 9m.

30-ish meter dive for y mins (y a fair amount longer than x) would mean that 100% (or maybe 80%? giving 1.3ppO2 @6m) would provide a reachable MOD whilst maximising ppO2.

And then in an ideal world, for a deep depth / long bottom time combo, then the combination of 2 deco gases would be the method of choice.

S

[Clare Gledhill]

You have got hold of some of the variables there - especially in the short recreational diving that we are all doing here but it's not quite as simple as that (which is why it is a subject for a course which takes hours rather than a post which takes 10 minutes to digest (unless it is a Chasey one of course )

Plan your dive. Look at your deco - where are your stops going to be, what gas are you best to be on? What gas reserves are you carrying, how far do they have to get you? How does this change if you chnage yoru back gas, your decompression gas, your set point etc etc etc. Just wanted to make the point that standard gases are fantastic and take a lot of the hard work out of this - but they don't stop us from thinking

When dives get deeper and we start looking at third deco gases this gets even more interesting and the choices get more wide as well. So much to learn - and so much that we still don;t understand no mater how much we try. Go do your course - and keep asking questions

[big_si]

Originally Posted by Clare Gledhill

You have got hold of some of the variables there - especially in the short recreational diving that we are all doing here but it's not quite as simple as that (which is why it is a subject for a course which takes hours rather than a post which takes 10 minutes to digest (unless it is a Chasey one of course )

Oh.... so you mean I'm not qualified now? I want my money back then! LOL

Originally Posted by Clare Gledhill

Plan your dive. Look at your deco - where are your stops going to be, what gas are you best to be on? What gas reserves are you carrying, how far do they have to get you? How does this change if you chnage yoru back gas, your decompression gas, your set point etc etc etc. Just wanted to make the point that standard gases are fantastic and take a lot of the hard work out of this - but they don't stop us from thinking

At present I do plan the dive on a basic level. Due to the small amounts of deco that I'm getting into my current dives have been more to familiarise myself with carrying a stage and getting my buoyancy spot on at all stages as preparation for my Adv. Nitrox + Deco. Little steps and all that/ From the skills workshop I did with Mark Powell the other week all these pieces of practice have certainly paid off!

I cut various tables for the depth and time (with other plans incase of change) and create back up tables based solely on back gas (or more commonly just air regardless of the mix I'm using to build in some extra safety padding)

At present I'm surfacing with about 1/2 of my back gas and deco gas so with plenty of redundancy. One of my next steps will be to more accurately plan my gas usage and extend my bottom times and I'm guessing that the Mark will help me with that much more than I could hope to do on my own!

Originally Posted by Clare Gledhill

When dives get deeper and we start looking at third deco gases this gets even more interesting and the choices get more wide as well.

Sounds interesting! Will cross that bridge if / when I get to it! I'm in no hurry at the moment. There will still be plenty for me to see tomorrow, next week, next month, next year! Plan is to just carry on building my knowledge and experience at a steady level.

Thanks loads for your help

S

[Tony Jay]

Originally Posted by big_si

Deco Gas Run time
50% 72mins
80% 67mins
100%

This works for me.

o2 as
1. I can use a smaller cylinder (3litre rather than 7)
2. cheaper (my dive shop does top offs and you pay per litre) -
3. available for theraputic use (it has been used in anger when a bent casualty exhausetd the boat's supply)
4. I'm climbing the ladder on 100% if I want to
5. easy to mix and analyse (no need to wait overnight)
6. Can use for homebrew nitrox
7. can top off 80 bar to get a 50 % for teaching advanced nitrox course (so it is multifunctional cylinder !)
8. I can do my 3m stop anywhere bewteen 3 to 6m and it does not lengthen my deco time (and some would say it should shorten it)

there are disadvantages, but you can guess them.

For me diving is about compromises

Tony