USN decompression 30fsw/9m with 100% O2

[Wibble]

If I could dislike a post on this forum, this would be the one.

Aww, bless.

Now explain why you don’t agree with what was said. Which bit is incorrect?

The thing I dislike is how inflexible 100% is and how difficult it is to get a top-up without a booster — most dive shops don’t have a booster.

I hate how 100% jacks up the CNS especially if that’s your only deco gas. I hate how you are stuck at exactly 6m/20' — caught between your ceiling and your PPO2=1.6. I hate that it can’t be used except at 6m, so you’re on an inefficient backgas at 9m/30'.

And I hate that you can’t give it a simple air top to get the pressure up a bit.

80% allows you to get on it at 9m/30'. It doesn’t need air breaks. It doesn’t excessively increase the CNS and pulmonary toxicity. It’s more flexible and less rigid at 6m and 9m (MOD is 10m). It’s just as efficient as 100% for deco as you’re on it at 9m. It’s fine to do an air top so you get more gas volume. And it’s cheaper.

And deeper dives requiring two deco gasses, use 50% and 80%. All the same benefits and just as fast as 100% — in fact faster if air breaks are taken.

My throat and chest is like I’ve been smoking after using pure oxygen. Literally wheezing in bed at night.

But please fill yer boots in attacking it. Ooooh, it’s like the DIR wars of yore.

 

[Degenerate]

Aww, bless.

Now explain why you don’t agree with what was said. Which bit is incorrect?

Honestly, for the dives we do here (which are on CCR, but for the purpose of this lets pretend they are on OC).
The total runtimes between using 50% and 80% are very similar, and in some profiles even identical.
With that in mind, I can get on the 50% at 21m, and in an emergency I would not hesitate to jump on it earlier during an ascent if the situation called for it.
The 80% mix I'm not supposed to (following "guidelines") get on before the 10m mark which gives me less margins if **** hits the fan at depth.
Same is true here, I can absolutely get on it deeper and grab a few breaths on the way up without risking imminent death, but for the dives we're doing you'll end up with quite a bit of time between your first stop (21-27m) and the stop where hopping on your 80% bottle would be a good idea which basically renders it quite a problematic gas compared to 50%.
You would have to seriously push the limits of your body (80% at 20m+ is a pretty bad idea, I'm sure you know).

Personally I think 80% is a poor deco gas choice compared to a bottle of 50% and one of 100%.
I am sure there are specific profiles/dives where an 80% deco bottle could be a decent idea, but generally speaking (and especially for OC diving) being able to get on your deco gas earlier is a massive win if something happens, as your gas tends to go away fast on OC at depth.

Short version; if you are limited to one deco bottle then 50% is an ideal deco gas, you can get on it earlier and the runtimes compared to 80% will be near identical.
You are basically increasing your margin for error and increasing your safety by quite a lot without increasing your dive time.

Super short version: If 80% was a good deco gas it would be a lot more common.


Edit: You edited your post A LOT after I posted this, and it's pretty hilarious because pretty much every point you used to advocate using 80% over 100% can be used to advocating using 50% over 80%.
But please fill yer boots in attacking it.

 

[rjack321]

Aww, bless.

Now explain why you don’t agree with what was said. Which bit is incorrect?

The thing I dislike is how inflexible 100% is and how difficult it is to get a top-up without a booster — most dive shops don’t have a booster.

I hate how 100% jacks up the CNS especially if that’s your only deco gas. I hate how you are stuck at exactly 6m/20' — caught between your ceiling and your PPO2=1.6. I hate that it can’t be used except at 6m, so you’re on an inefficient backgas at 9m/30'.

And I hate that you can’t give it a simple air top to get the pressure up a bit.

80% allows you to get on it at 9m/30'. It doesn’t need air breaks. It doesn’t excessively increase the CNS and pulmonary toxicity. It’s more flexible and less rigid at 6m and 9m (MOD is 10m). It’s just as efficient as 100% for deco as you’re on it at 9m. It’s fine to do an air top so you get more gas volume. And it’s cheaper.

And deeper dives requiring two deco gasses, use 50% and 80%. All the same benefits and just as fast as 100% — in fact faster if air breaks are taken.

My throat and chest is like I’ve been smoking after using pure oxygen. Literally wheezing in bed at night.

But please fill yer boots in attacking it. Ooooh, it’s like the DIR wars of yore.

Huh? What kinds of profiles are you running (OC or CCR BO?) where 50% and 80% are even remotely a good idea?

Sounds like a whole bunch of bottles and needless switching for a dive the rest of us would do on a single al80 of 50%. But I dunno maybe enlighten me.

 

[PfcAJ]

Hot take: CNS% is a baloney measurement.

 

[Degenerate]

Hot take: CNS% is a baloney measurement.

What?!
You're saying that exceeding 100% CNS won't kill you instantly?
No way!

 

[rjack321]

What?!
You're saying that exceeding 100% CNS won't kill you instantly?
No way!

*twitches as I read munching on my bratwurst*

 

[boulderjohn]

Hot take: CNS% is a baloney measurement.

I have tried to find the basis for the published limits on CNS, and I have not found it. (I have to admit that I have not tried all that hard.) I strongly suspect that is was something of a guess, somewhere in the range between "somewhat educated" and "wild."

Now think what that does to me as a technical instructor. The course materials require students to make sure that students do all the CNS and OTU calculations for their dives, and they learn, of course, that it takes some pretty big dives, dives that we won't be doing in training, to exceed those limits. But they know that people do indeed do dives that exceed those limits, sometimes by a wide margin.

So when they ask me how that happens, what do I say? I answer honestly but very carefully. I certainly don't tell them that they should go ahead and ignore those limits with my blessing. I have developed something of a spiel that walks the tightrope.

What I am really saying in this post is that it is time for someone in the scuba agency to do some kind of investigation and report on the origin and accuracy of those numbers. I am not expecting it, though, because I am sure they are walking the same tightrope I am. If they were to write anything that sounded like a blessing to ignore those limits and someone were to do that and die, ....

 

[Wibble]

Huh? What kinds of profiles are you running (OC or CCR BO?) where 50% and 80% are even remotely a good idea?

Sounds like a whole bunch of bottles and needless switching for a dive the rest of us would do on a single al80 of 50%. But I dunno maybe enlighten me.

OC would use a standardish backgas (32%, 28%, 25/25, 21/35 or 18/45). For shallow dives or limited dive time would use a single 80% deco gas and for all dives deeper than 40m would always use two deco gasses of 50% & 80%. If <50m/170', they would be ali7s, if deeper then two ali80s again with 50% & 80%. Runtimes of 2ish hours. If doing a long shallow dive <40m would probably take two ali7s but maybe only use the 80%.

The point of two deco gasses is redundancy as much as efficiency. Should one fail the dive could be completed on the remaining cylinder under the principle of one failure only.

CCR is 60% for the deco gas — obviously taking the bottom bailout too.

Will be investigating using an ali40 of 50% and an ali80 of 80% for deeper & longer self-sufficient CCR dives, e.g. in the 70m range or long dive times. From messing around with three ali80s on OC, three stage cylinders is a massive embuggerance and isn’t fun. Won’t be any easier on CCR.

 

[Wibble]

You would have to seriously push the limits of your body (80% at 20m+ is a pretty bad idea, I'm sure you know).

Personally I think 80% is a poor deco gas choice compared to a bottle of 50% and one of 100%.


Edit: You edited your post A LOT after I posted this, and it's pretty hilarious because pretty much every point you used to advocate using 80% over 100% can be used to advocating using 50% over 80%.
But please fill yer boots in attacking it.

Just to be absolutely clear, the 80% would only be for OC dives and would normally be in conjunction with the 50% deco gas.

The min gas planning calculations would be to have sufficient backgas to get to the second gas switch, e.g. 9m for 80% in the case of a failure in the 50% cylinder. Easy to calculate on MultiDeco.

 

[rjack321]

OC would use a standardish backgas (32%, 28%, 25/25, 21/35 or 18/45). For shallow dives or limited dive time would use a single 80% deco gas and for all dives deeper than 40m would always use two deco gasses of 50% & 80%. If <50m/170', they would be ali7s, if deeper then two ali80s again with 50% & 80%. Runtimes of 2ish hours. If doing a long shallow dive <40m would probably take two ali7s but maybe only use the 80%.

The point of two deco gasses is redundancy as much as efficiency. Should one fail the dive could be completed on the remaining cylinder under the principle of one failure only.

Shrugs. Do what you want. 50% and 80% are not anything magic here that an 80 of 50% and a 40 of 100% couldnt do as well or better.

Of course with 2 hour runtimes you are looking at carrying a stage if you want to have a reasonable min gas to actually get you up to the 50%. And with ~70-90mins of deco you really can't cover a lost deco gas alone or even sharing with a buddy either - especially on the 7s.