Decco mix

[Mark Chase]

Originally Posted by ardhill

Question, not an argument

For those of you who feel that O2 is the best 'single' gas - why?

I understand the concept of having no nitrogen. But sometimes a bottle of 50% gets you out of the water quicker with the same algorithm (well it does on the settings I use - VPM-B +3).

So, do you believe it is the best simply because of the absence of nitrogen and does time to surface not count?

Or is there anything else?

Thanks.

In terms of "best" deco for a purley physoligical point of view 100% is the best gas. No interts = maximum inert gradient you could end up getting out of the water with less nitrogen in your body than when you got in.

In terms of fastest deco in the 40ishm zone using a single gas then 50% has the edge by a masive 8mins on my deco plan for 45mins at 45m on air. However getting on the deco gas at 21m extends the range of the back gas and gives some small advantage in deep bailout.

02 deco = 68mins

50% deco = 61mins

50 & 100% = 46mins

32% and 80% deco = 48mins and a bottom bailout option.

ATB

Mark

[nigelH]

Originally Posted by ardhill

For those of you who feel that O2 is the best 'single' gas - why?

Well gettiing out of the water a few minutes earlier is not a priority.
Getting onto a deco mix earlier is not a priority either.
I am an advocate of doing more time than the tables or computer ask for and adding various types of conservatism in my head.

Having lots of options for the day it all goes wrong is a priority.

Now on the rebreather I can pull more 'get home with bust gear' stunts with oxygen and bottom gas on whips then I can with anything else.

Back on the boat I have already stuffed it into the buddy, who missed some stops, of the guy who missed a lot of stops who was on the boat's O2. Since that day our local helio was out rescuing somebody else we waited for the next one up the coast to come and take them away so that day it paid for itself in one.

[Garf]

I've been asked what decompression gas I use a lot, and often it is phrased as "what is the best gas to use". Well, this is not a simple question, and the answer often leads to a realisation that there is more to consider than just assuming one gas is "ideal".

We'll look at 50%, 70%, 80% and 100%, and we'll use the example of a 30 minute dive to 45 metres on air, and use gradient factors of 30/85 to do the maths.

When selecting a decompression gas there are a number of factors that we have to consider

Decompression time
for some people, this is the be all and end all; "How fast can I get out the water?". Thinking rationally, we are looking for a large pressure gradient between our backgas and our decompression gas. Logically then, 100% would always be the ideal choice as it contains no inerts. Life's rarely so simple though. Using our example above, pure Oxygen would get us out of the water in 68 minutes, but 70% gets us out of the water quicker in only 64 minutes. Bizarre huh. Well, not really when you realise that the decompression model you are using is another factor to consider. If you use tables and elect to use a model which forces deep stops, then you are going to be doing deeper stops on your backgas and racking up decompression, so Oxygen does you no favours. If, however you can get on your gas earlier, you will shorten the stops at 6 metres. Looking at it from another perspective, if you dive a computer that is aware of the gases you have, such as a VR3, then it will minimise the time you have at depth to get you onto your Oxygen as soon as possible. So, when asking the question "how quickly will it get me out of the water", the answer is is is a factor of both the amount of inert gases in the decompression mix you choose, and also the decompression model you are following, and only by taking both into consideration can you move forwards towards a choice. However, when looking at gas gradients alone, 100% wins as the gas which will get rid of the nitrogen the fastest with no other variables.

Bailout
For many people, this is another important factor. How quickly can you get onto the gas. Obviously, the richer the mix, the further up the water column you have to go before you can get on it. This gives us a couple of things to consider. Firstly, as we have seen above, a richer mix is not always ideal if we plan to introduce deep stops into the equation. Secondly, we may or may not want to consider having the decompression gas as a bailout option. It may be psychologically reassuring to know we can swim up to 21 metres in a hurry to take a breath of our 50% decompression gas. It may be more worrying to think we'd have to bolt to 6 metres and actually stop before we hit the surface if we are carrying only 100%. If we are considering bailout as the option, then 50% wins among the most frequently used decompression gases as it will allow us to get on it the deepest, and 100% is the worst.

In Water Skills
The difference between a one metre difference on 50% at 6 metres is the difference between a PPo2 of 0.8 and a PPo2 of 0.85. However, the difference in the same depth change when breathing Oxygen is the difference between a PPo2 of 1.6 at 6 metres and a PPo2 of 1.7 at 7 metres. Obviously, if we are in any doubt of being able to keep a stop precisely at 6 metres, then Oxygen is a poor choice of decompression gas. Now there are those that will say if we cannot hold a stop at 6 metres then we shouldn't deco diving, but I've seen plenty of people who move between 1.5 and 1.7 continually throughout a 6 metre stop so the point is a valid one. I've listed this under a heading of Inwater skills, but obviously environment needs to be considered as well. contrary to popular belief, us DIR types don't restirct ourselves to inland diving, and one of the questions I ask myself when planning a deco dive where I am going to use Oxygen is "are the conditions going to allow me to comfortably remain solidly at 6 metres" and if the answer to that question is no, then I may well select a different gas than Oxygen or even consider whether I want to do the dive at all.

Available Gas
This is a complicated one. There is a balance between the amount of gas you need to carry in your back cylinders, and the amount of decompression gas you need to carry. Let's say we are using 100% as a decompression gas. The chances are we will only need a small cylinder to carry it in - we can only breathe is at a depth of 6 metres so a small amount of gas will last a relatively long time. This is why the GUE mob use ALi40's for O2 bottles, as we don't need anything bigger. This all seems very nice until we remember that becuase we cannot get onto the 100% until 6metres, we are breathing our backgas all the way up through the water column, which means we need a very large amount of backgas - or will in fact be able to to do less bottom time. It is often the case with deco dives where you carry only 100% that the backgas usuallyu becomes the limiting factor rather than the decompression gas. Look at the other extreme, 50%. Now, depending on the decompression model we choose, we will be able to get on this gas at 21 metres, and may well have decompression stops to do on the way up to 6 metres. This means we are going to use far more decompression gas, and indeed far less backgas. In dives such as this, the decompression gas tends to be the limiting factor when planning the dive, certainly for the fairly standard Ali 7s, which is why you see us GUE types carrying AL80's to use with 50%, as the numbers just work out with twin 12s and an Ali80 of 50%. Assuming most people dive Ali7s as a decompression bottle, then all of a sudden 70% and 80% becomes the strongest gases - there tends to be a good balance between backgas and decompression gas with these gases so that neither becomes an outstanding factor and limits the dive.

CNS
The CNS clock needs to be considered when planning the dive. These days, is easy to do this with stunning programs like vplanner, but it's important to note that the richer the mix you choose, the faster the CNS clock is going to run and the more likely the final CNS count of going to be the limiting factor of the dive. Obviously, with 100% you are going to be basically doing all of your decompression at around 1.6 bar ppo2 which means we need to keep an eye on CNS, especially if we are planning repetitive diving. From the CNS perspective, 50% is the winner.

Team
without getting into a big debate on this one, I personally find it madness that people in the same team will dive different gases on the same dive. I guess it's everyone's personal choice, but to the new deco diver, I would say think about the ramifications of choosing a different gas than your team mate or buddy. You will potentially be doing your decompression at different depths, you will have to switch gases at different depths, you may have to leave the bottom at different times, and you will almost certainly surface at different times. whist certainly possible, I would hope its unarguable that it makes the dive planning and "what we will do if.." procedures more complicated to work out. There is no winning gas here, but what gases the team are using is certainly a factor.

Training and Familiarity
when I do a decompression dive, I know what depths I have to change gas at. Because it's the same ddepths every time I dive. There is arguably a value in picking a decompression gas, or a couple of them, and then sticking to them, rather than choosing a specific mix, such as 76%, on each unique dive. There is value in everyone knowing what depths we all have to change at, and you become familiar with just doing a double take at specific depths in case we have something to do at those depths.

Exposure
A personal admission here, becuase I am guilty of this one. For nearly 18 months I carried 50% on every decompression dive, without ever thinking about it. 50 metres depth, 50% decompression. 30 metres depth, 50% decompression gas. Until one day, Andy Kerslake asked me in his own delightful way "what the **** are you doing that for, you ****". Whilst 50% is a great gas when you have been diving to 45 metres, it's bloody useful when you have been diving to 30-35 metres, becuase you are now getting shallow enough before you get on it to actually achieve anything, and there is no really significant benefit over diving the backgas all the way up. At worst, diving to 30 metres, many of your tissues will still actually be ongassing whilst you are happily switching to 50% at 21 metres. Doh!. However, in the same depth range, 30-35 metres, carrying a bottle of Oxygen gives amazing results in terms of how fast you get out of the water. So, no winning gas here, but obviously the "best" gas can change depending on the type of dive we have been doing.

Conclusions
So, how do we wrap this all up. From an inerts perspective 100% wins. From a bilout perspective 50% wins. From a gas management perspetive, 70 or 80% usually wins. Either of the gases can be ideal depending on the type of dive we are doing, and we also have to consider the rest of our team, the conditions of the dive, the size of cylinders we have at our disposal and our own in water skills. At the end of the day, there really is no right or wrong answer, but there is value in picking a couple of gases and sticking with them, as you can then start to learn what is actually happeing to you during decompression, and which combinations of variables will result in your getting out of the water safe and feeling great which, at the end of the day, is the only thing that really matters

Dive Safe

G

[Mark Chase]

Excelent post M8

ATB

Mark

[ardhill]

So Garf, what about 40% then

Nice post!

[broady]

A great green-grollie to Garf incoming !

Graham

.

[big_si]

Originally Posted by Garf

.... lots of wise words and generally what Si said.....


Dive Safe

G

Good post Garf

LOL

[RogerViking]

and your view on 40%

another great post garf

[Twinset & Wing]

Garf I would green you but I appear to be to generous to you so I will have to bank it till I have spread it around a tad....excelent post

[japandiver]

Excellent post .... and just one other pragmatic comment on deco gas choices - logistics of gas blending.

Using 50 and 100 is an excellent choice if you are getting shop-blended gas and/or have access to a booster pump / reasonably sized O2 bank.

If you are not / can not / don't wish to boost O2 then using 32 and 80 may be a pragmatic choice. 32 acts as a good travel / initial deco gas and while 80 is not as efficient as 100 it is lot more efficient than 50. Filling multiple 40's w/ 80% can realistically be done with one or two J's and you will typically find that a used 80% can be easily topped-off to give a 32% stage for the next dive.

So if you are currently diving / filling in the boonies (like me) then using 32 & 80 are the gases of choice for a 60 - 70m gas dive and 50 gets used on more moderate trips.

Hugh