Deco dive with divers on different back gas

[Boarderguy]

That’s 18/45 on a rebreather? It’s a phenomenal waste of good helium for open circuit; 45m/150ft is a 25/25 dive on OC, or just use 25% and put up with the narcosis.

Narcosis is bad and scary, how dare you suggest such a thing!... 150 on air is fine. 170 on air is fine. 190 the air gets a bit thick and the narcosis is much more in your face (for me).

YMMV

 

[boulderjohn]

No, GFs are a personal preference based on risk appetite.

Not when you are diving as a team that needs to stay together.

 

[LFMarm]

Not when you are diving as a team that needs to stay together.

Agree. Going back to the initial statement "there is no harm to the diver who is staying a little longer." is not true if that diver has a very low GF Low which will cause unnecessary slow tissue loading. This doesn't happen if they have same GFs. But again they may have different GFs given that they are even diving different gases.

 

[Matan]

That’s 18/45 on a rebreather? It’s a phenomenal waste of good helium for open circuit; 45m/150ft is a 25/25 dive on OC, or just use 25% and put up with the narcosis.

putting up with narcosis is either liking it (in which case there are cheaper ways than diving haha) or being cheap on helium which makes no sense - you take enough to avoid narcosis. for me 25/25 I'll be a 60's hippy at 45m.


Matan.

 

[boulderjohn]

or being cheap on helium which makes no sense

One of the factors that led me to retire from teaching technical diving was the fact that for quite a while I was unable to get helium at any price, and I was the only source of helium for my students.

 

[boulderjohn]

But again they may have different GFs given that they are even diving different gases.

It only takes a minute or so to change your GFs. If you are diving together as a team, you want to match everything as closely as possible. In the specific case provided in this thread, diving the different gases is not a deal breaker, but if you choose to do that, then you need to match everything else as much as possible. Otherwise, you are just two solo divers.

 

[boulderjohn]

Here's a little anecdote about the idea of applying specific dogma to all situations.

When I was a UTD diver and the organization was still very new, we were all absolutely required to use EANx 32 for dives shallower than 100 feet. Never, ever were we do use air.

At one point, UTD released a whole new set of videos of divers doing skills in an exemplary manner. They had gathered some of their instructors together to make those videos. One of those instructors was ours. He told us that they got all their gas for those videos donated by a local dive shop, a shop that did not have the ability to make nitrox. All the skills in those videos were being preformed by divers breathing air.

I just completed a week of NDL diving on a LOB whose nitrox making ability was down. We all did the dives on air. We all lived. It can be done.

 

[LFMarm]

Here's a little anecdote about the idea of applying specific dogma to all situations.

When I was a UTD diver and the organization was still very new, we were all absolutely required to use EANx 32 for dives shallower than 100 feet. Never, ever were we do use air.

At one point, UTD released a whole new set of videos of divers doing skills in an exemplary manner. They had gathered some of their instructors together to make those videos. One of those instructors was ours. He told us that they got all their gas for those videos donated by a local dive shop, a shop that did not have the ability to make nitrox. All the skills in those videos were being preformed by divers breathing air.

I just completed a week of NDL diving on a LOB whose nitrox making ability was down. We all did the dives on air. We all lived. It can be done.

This is more or less the same way as I do it. If EAN is available, I will always use it for shallow diving; if air is the only option, I would do that still staying shallow. Air diving is way better than no diving and if you respect MOD for density and narcosis is not dangerous, just not optimal. Not diving is also far from optimal

 

[rjack321]

So we go from a pretty simple, barely-into-deco dive to full on hypoxic with all sorts of other issues. That is just an absurdity.

The "slippery slope" argument is a fallacy, and you are showing why. The fact that something makes sense in one situation does not mean it will make sense in another. That is why we have to use our minds and not follow dogma that might make sense in one case and not in another. Once you have become part of the dogma, though, it can be hard to see that. It took me a long time to clear that junk from my brain.

Shrugs so what? The EAN32 diver isn't even on a deco dive at all with 30mins at 100ft. And if you tweak the GFs enough the air diver can get away with 30mins and a stop which basically mirrors a safety stop. Or they could just admit that this isn't a great idea even if it could work and do a 20 min NDL dive to the same depth. Saying "it's fine" or there are acceptable workarounds is a poor & sloppy attitude to have for deco diving.

In this case you can avoid all this super easy:

1. Dive solo,
2. Do it as a (marginally shorter) NDL dive, or
3. Skip the dive cause you're unprepared or miscommunicated the plan back at the fill station.

Those are the options that make sense, not having some contrived OOA plan along with mismatched deco plans.

 

[rjack321]

That’s 18/45 on a rebreather? It’s a phenomenal waste of good helium for open circuit; 45m/150ft is a 25/25 dive on OC, or just use 25% and put up with the narcosis.

People have definitely toxed at 150ft on 25/25. Not something I'd make a habit of