Quote:
| 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