Optimum gas sequence

[Richie]

Something that struck me as I drove the other day....

If, for whatever reason, you end up in deco with less deco gas than you/the team need and back gas is adequate and sufficient for deco; what is the optimum order to use the gas available?

Deco then back or vice versa?

Just wondering......

[neilh]

Assuming your deco gas will be richer than your backgas then I'd have thought deco first to help eliminate nitrogen or He?

[Hazel W]

yup ... rich gas first on deep stops. then back gas for remainder remembering to pad out the remaining time to allow for offgassing

[Richie]

Originally Posted by divebiatch

yup ... rich gas first on deep stops. then back gas for remainder remembering to pad out the remaining time to allow for offgassing

This was my initial thought but then I wondered if using the richer gas second would help drag the last bits of He/N from the system.

I do plan and allow for deco gas failure, just hope it doesn't happen. The boredom could be unbearable especially as I can never hit the octopus

[gibbon]

If you are talking about diving air/nitrox and deco on 50% and or 100% O2 then your perspective would likely be somewhat different to if you are on Trimix.

Unless you have major concerns about CNS then your back gas is probably the least appropriate gas you have available from a deco perspective.

Back gas is probably the most expensive as well unless you are diving air/nitrox.

If in that situation I would stay on the richest mix possible till the team had finished - making me as safe as possible, unless I needed a break on back gas due to phlem or CNS clock.

Is your main concern really the cost of O2?

[Hazel W]

using the high o2 second is helping with the "bend and Mend" school of deco ... deep stops will clear N out the system more efficiently and are usually of a shorter duration meaning more offgas in a shorter time ..the near surface stops are the mend part that clears residual bubbles .... IIRC the bubbles are physically larger at the shallow stops making them more difficult to clear but im not sure if that would have an impact on the theory ....... perhaps someone with abetter knowledge of diving phys. should answer that .

H

[Garf]

Originally Posted by Richie

Something that struck me as I drove the other day....

If, for whatever reason, you end up in deco with less deco gas than you/the team need and back gas is adequate and sufficient for deco; what is the optimum order to use the gas available?

Deco then back or vice versa?

Just wondering......

you'd have to have a pretty bad dive for the entire team to have less deco gas than it needs bearing in mind you are only using perhaps 50% of your deco gas. This means the loss of both stages in a team of two, or two stages in a team of three. If you lose any less than this then one person can deco out on the rich mix and then hand the stage over to the next diver.

I'm not what the question means to be honest. If you lose all your deco gas, then you only have your backgas available, unless you want to start breathing a travel gas as well. In this situation I would breathe whatever my lost deco gas plan told me to breathe, most likely the richest mix possible.

[gibbon]

Originally Posted by divebiatch

using the high o2 second is helping with the "bend and Mend" school of deco ... deep stops will clear N out the system more efficiently and are usually of a shorter duration meaning more offgas in a shorter time ..the near surface stops are the mend part that clears residual bubbles .... IIRC the bubbles are physically larger at the shallow stops making them more difficult to clear but im not sure if that would have an impact on the theory ....... perhaps someone with abetter knowledge of diving phys. should answer that .

H

Not quite sure where you are going with that Hazel?

Deep stops are the way to go from my perspective, however the question is nothing to do with deep stops, it is more like "should I try to get back in sync with my team by decoing more slowly to let them catch up so that our run time matches even though I have got deco gas."

If my understanding of the question is correct then I stand by my previous post.

[Janos]

Originally Posted by Garf

you'd have to have a pretty bad dive for the entire team to have less deco gas than it needs bearing in mind you are only using perhaps 50% of your deco gas. This means the loss of both stages in a team of two, or two stages in a team of three. If you lose any less than this then one person can deco out on the rich mix and then hand the stage over to the next diver.

I'm not what the question means to be honest. If you lose all your deco gas, then you only have your backgas available, unless you want to start breathing a travel gas as well. In this situation I would breathe whatever my lost deco gas plan told me to breathe, most likely the richest mix possible.

You can lose deco gas without losing all of it though.

I think the situation is that you plan for lost gas by carrying enough backgas to deco out on. However, you have a problem underwater (leaking tank o-ring, freeflow which you shut down) which means that you have less deco gas then you expected, but not zero.

So you are there with enough back-gas to deco out on, and also some rich mix as well. Do you use it first, then switch to back-gas when it runs out? Or breathe back-gas first.

My gut feel (and I haven't checked the maths) is you're better off breathing the back-gas first. The way to investigate this further is to look at the effect of losing your deco gas on different length stops. I think that for very long stops, breathing a rich mix at the start doesn't make much difference as most of the off-gassing is driven by the absolute pressure difference

Eg if your tissue tension is 10* then it makes little difference whether the gas you're breathing is 1 or 0.5. However, if your tissue tension is 2, then the difference between off-gassing while breathing 1, and off-gassing while breathing 0.5, is much more.

Janos

* - Relative units

[And]

Originally Posted by Richie

Something that struck me as I drove the other day....

If, for whatever reason, you end up in deco with less deco gas than you/the team need and back gas is adequate and sufficient for deco; what is the optimum order to use the gas available?

Deco then back or vice versa?

Just wondering......

Hi

I would use deco gas first, and worry about it running out when it runs out. You can then decide whether to share whats left within your buddy group, abbreviate the deco schedule, or extend the shallow stops on backgas, all sorts of options really.

IMHO the important thing is that you start the deco properly, and the best way to do that is to use the deco gas as soon as you can, and begin the deco schedule as planned.

Andy