Current best practice for deco gas: 75% vs 100%

[Germie]

added benefit of being a one-step fill

@Germie EAN50 is perfectly fine as a deco gas for most normoxic profiles. Run the profiles, there is usually no benefit in using 100% alone, and using both is usually of negligible benefit

I never do a 60m dive with just 1 decogas wth 20 or more minutes of bottomtime. I know from more than 1 case about dcs where the plan showed no problems. The dive is shorter when you use a second decogas, maybe not that much, but enough to see a benefit.
A normoxic course done with 1 decogas is not a good course in my eyes, I allways do it with 2. Around 50m I prefer to take 2 decogases instead of 1 when I do bottomtimes (of course not for a bounce to 60m, then just an ean50 is enough, but that is no diving). And of course 2 decogases and a travelgas for full trimix. But if you decide to dive with only 1 decogas, then ean50 is most used, I agree. But allways remember that it is not the best gas for longer deco's.
An 100% or ean75 I have never seen as just a single decogas. Then it would be ean50.

 

[tbone1004]

@Germie courses should definitely be done with 2 gases in anticipation of full trimix, and while I would certainly prefer to have someone throw an O2 bottle on the hang line.

60m for 20min on 50/80 GF's.
Backgas, 106min deco
100%, 51min deco
50%, 37 min deco
50% and 100%, 31min deco.
While I'd rather have a safety bottle of 100%, or maybe 80% depending on the surface conditions, I'm certainly not carrying a second bottle to save 6 minutes of deco.

@ajduplessis Please explain how 50% is a less ideal choice than 100% for the dive profile above? Note the GFlo being 50 on that, it gets far more useful if you dive a lower GFlo like many still do

 

[boulderjohn]

One useful aspect of using 100% as your final deco gas is that you don't have to pay any particular attention to your depth on the final stop. During the 20 foot stop, you have to be very careful, because going a little deeper gives a high PPO2 and going a little shallower is a missed stop. When you transition to the 10 foot stop, you can now be at any depth between 20 and 10 feet with equal off-gassing. There is even benefit to staying closer to 20 feet. If you have a 75% or 80% mix, you want to stay as close to 10 feet as possible to maximize the off-gassing.

 

[Jack Hammer]

For a single deco gas, it really depends on the dive being made and the overall logistics. When looking at "the big picture", I find using either 100% or 50% to make the most sense overall. 100% is the most efficient, 50% is a good compromise to getting off back gas sooner (less needed to safely make the dive) without a huge increase in time over 100%. As a bonus, both are super simple to mix.

 

[JohnnyC]

50% and 100%. 75% is lame.

If I'm doing a dive where the delta between backgas and carrying one or two bottles is significant enough, carrying a second deco bottle, even if the deco advantage is marginal when using both gases, provides some nice redundancy in the event of a single lost deco gas.

Using the example that Tom posted, the difference between doing backgas deco because you've lost your only deco gas, and the difference even in using one deco gas, even if it's not the best mix in terms of efficiency, is pretty significant.

In that example, I would carry both 50% and 100% because even if I lost the more efficient 50%, running 100% for 51 minutes run time is a lot better than 106 minutes on whatever I've got strapped to my back. You're talking almost an extra hour of decompression as a worst case scenario. I might take both and only use the 50% as I'm not plussed by an extra few minutes, but I'd rather take both and leave the 100% uncracked, than have to float my happy a$$ there for an extra hour.

It comes down to a personal choice in this instance. Factors like surface conditions, thermal conditions, bottom topography, location, local fauna, etc. all play in to making the choice whether to hedge your bets against unnecessarily long decompression in the event of lost gas, vice having to take another cylinder. For a dive with shorter potential backgas run time, I may choose not to bring both tanks. For a dive where I know I get to park in no flow in a cave where the water is warm enough that I'm not going to get cold, I may choose not to bring both tanks. For a dive where I might end up having to do a blue water drifting deco in the ocean where the locals might be a little bitey, I may choose to take two just in case. It's all a game of comparative odds.

Now I don't know if his numbers are right, I'm not going to bother to check as the message is the same regardless. And 75% doesn't get you any real advantage anywhere.

 

[rjack321]

@Germie courses should definitely be done with 2 gases in anticipation of full trimix, and while I would certainly prefer to have someone throw an O2 bottle on the hang line.

60m for 20min on 50/80 GF's.
Backgas, 106min deco
100%, 51min deco
50%, 37 min deco
50% and 100%, 31min deco.
While I'd rather have a safety bottle of 100%, or maybe 80% depending on the surface conditions, I'm certainly not carrying a second bottle to save 6 minutes of deco.

Lose your deco bottle and you are in a world of hurt.

 

[tbone1004]

Lose your deco bottle and you are in a world of hurt.

obviously true, and the question then becomes how you give that provision. Do you alternate with your buddy and make a 37min deco a 74min deco by going 50/50? Do you put a safety bottle on the hang line and keep your fingers crossed? How bad is sorting out that 106min deco temp/environment wise?
Me? If I'm concerned about losing a deco bottle, I'll bring a second bottle of EAN50. If one bottle fails, then I still have a 37min deco vs a 51min deco. I'll still put a bottle of O2 on the hang line if it's practical to save the 6 minutes out of good measure, but I'm not going to carry the O2 on the logic of saving 6 minutes of deco or a lost bottle because a second bottle of EAN50 makes more sense

 

[rjack321]

obviously true, and the question then becomes how you give that provision. Do you alternate with your buddy and make a 37min deco a 74min deco by going 50/50? Do you put a safety bottle on the hang line and keep your fingers crossed? How bad is sorting out that 106min deco temp/environment wise?
Me? If I'm concerned about losing a deco bottle, I'll bring a second bottle of EAN50. If one bottle fails, then I still have a 37min deco vs a 51min deco. I'll still put a bottle of O2 on the hang line if it's practical to save the 6 minutes out of good measure, but I'm not going to carry the O2 on the logic of saving 6 minutes of deco or a lost bottle because a second bottle of EAN50 makes more sense

For that dive just bring 2x al40s - neither of which would be 75% lol

Two bottles of 50%? Dude why, if you are going to be up at 20ft for ~25mins you might as well be on a better deco gas.

 

[tbone1004]

For that dive just bring 2x al40s - neither of which would be 75% lol

Two bottles of 50%? Dude why, if you are going to be up at 20ft for ~25mins you might as well be on a better deco gas.

yeah EAN75 is dumb IMO, or EAN80 which is what most people call out. A pair of AL40's is obviously perfectly fine, but @Germie was talking about using EAN50 and 100% on the logic of a failed bottle but there's no logic to that. If you're worried about that, carry a buddy bottle of EAN50 for the team and don't worry about it which is a far safer call than carrying 50/100

 

[rjack321]

In my universe for a 200ft 20min 18/45 dive everyone would bring one 40 of 50% and one 40 of 100%.

If you can't handle multiple gases & switches then stick to <25-30min deco exposures until you have the experience and training to do this dive properly.