Inerts Offgassing and Sharing Deco Gases

[Naomi S]

(220-50)x15/(1.3bar x 71min) = 26lpm

Thanks well tha's not as bad a s expected

[Morag]

I didn't even work mine out for swanage i was blowing somewhat with the effort it took to keep warm!

Could someone who is not supposed to be working do the sums pls

71 mins
Chugged a full 15l (say 220 bar) down to 50 bar
How deep is it under the pier? 3m?

OK.... 170 bar of a 15l is 2550l of gas.

Lets say 5m depth because the maths is a bit easier - or you can work it out from the average depth.

that is 1.5 ata

2550 divided by 71 gives you the amount you used per minute = 36 lpm. divide this by the 1.5 = 24.

Your SAC was, on those figures, around 24.

[nigelH]

I think I worked out that mine went over 100 at various points on Tech1

I'm impressed.

I sat on the floor with a pony of air and an Apeks TX50 and deliberately maxed out my breathing for 60 seconds and managed a sustained 100L/min (well 100.6 by my digital blending gauge before and after readings).
My head was spinning at the end of it.

At depth the viscosity of gases rises so it would be harder so you have better lungs than I have unless you had some nice low viscosity helium in there.

[MonkeyPony]

Another thing that came up this weekend.

If you are diving a 21/35 mix to 48m for 30mins and have 35mins of deco to do. You end up with 18mins of deco at 6m and then an ascent. With 8 mins of deco left to do, your buddy signals that he is running low on deco gas (50%).

I'd finish my few minutes dec, hand off my stage and a let him complete his deco. A few extra minutes can only make it safer. If there was still a lot of deco left then it would be a yellow blob for a drop tank.

I would debrief with my buddy back on the boat and ensure that gas planning is more realistic for subsequent dives.

[Juz]

I forgot one thing, before I handed off my stage, I'd pull out my wet notes and have them write down their credit card details - after all, this is going to cost them!

Juz

[MattS]

I have found that not worrying about my SAC (and hence cutting my buddies dive short) so much on a dive makes it go down too. Which sounds like an obvious thing to say but i think that your SAC is what it is and shouldn't be a competition.

To a point yes. The breathing cycle is pretty interesting physiological and psychologically. It sits on the border of voluntary and involuntary.

We usually live on the surface though, where there is 0.21bar of O2 floating about in our lungs. As soon as ambient pressure increases, when we dive underwater, so does the ppO2 of the air in our lungs, double at 10m, treble at 20m etc. Our cardiovascular systems have evolved to balance increased CO2 production with increased O2 respiration. What happens underwater is we have an increase in CO2 production, because we are exercising, but there is a great deal more Oxygen floating around the lungs and absolutely no reason to breathe faster to get more of it.

Your breathing rate underwater needs to be sufficient to excrete through the lungs the CO2 you are producing...and no more. The really interesting bit is that an imbalance of CO2 causes psychological stress. You can reduce your anxiety level simply by paying a little attention to your breathing. There is no debate about this. It is an absolute fact which the psychologists and physiologists agree happens, how and why it happens.

I dont really buy into this whole go diving alot and it'll go down business.

So don't. It does not stop it being a fact. The more you dive the less threatening being underwater is and that causes your breathing rate to drop. The more you dive the more efficient you become as your brain learns how to do things by expending the least amount of effort.

I would have thought it'd vary massively from person to person and even from dive to dive depending on a whole bunch of things like your size/ fitness levels/ how tired you are/ stress/ exertion levels on dive etc.

Bigger people are always going to breathe more because they have bigger muscles and organs producing more CO2 for a similar effort. Apart from that the variation is down to anxiety and exercise levels. Reduce both and you can significantly reduce SAC with no adverse effects.

Out of interest does anyone have a consistent SAC most of the time?

Yep, I can fairly accurately estimate what my SAC has turned out to be for any given dive. I dived with an Air Integrated computer for a number of years, which sort of helped to associate particular levels of exercise with the resulting SAC. Drift dives in warm water it drops to about 8Lpm which is about as low as it will drop. Most of the time for UK deco dives it is around 12Lpm. In the Winter it is about 15Lpm. I was wearing an AI computer when I had to pull a diver up from 36m and the high levels of stress and exercise during the initial ascent saw my SAC rise to between 75Lpm and 100Lpm. The situation was brought under control at about 20m and my SAC subsided to a more leisurely 15Lpm. What I take from that experience is that if you do not attempt to control your breathing in the face of a high stress situation, you have little hope of getting out on a 1/3 reserve.

[Air Guzzler]

Ive not read the other 10 pages yet but should this situation not have arisen
Due to the fact that you should have planned all your gas consumptions prior to entering teh water

just MHO

Another thing that came up this weekend.

If you are diving a 21/35 mix to 48m for 30mins and have 35mins of deco to do. You end up with 18mins of deco at 6m and then an ascent. With 8 mins of deco left to do, your buddy signals that he is running low on deco gas (50%).

Do you?

A. Give him your Deco Reg and switch to backgas and increase the stops by 1.5x or 2x, and then when he has finished his time, you take the deco reg back, he does * amount of time extra on his backgas, you finish your extension on deco gas and ascend.

or

B. As you only have 12 or 16 mins at 6m, you continue to breathe your deco gas and he switches to BG, and remain there until your normal stops would be completed and ascend

B is based on the assumption that the nitrogen makes up 46% of the BG you are breathing and the He will both ongas and offgas more quickly and as at 6m your deco method is primarily based on pressure gradient, rather than gas gradient, it won't make much difference for the timescales involved.

Again, please try not make this an agency thing, just interested in people's thoughts about the deco theory.

Regards

[GLOC]

Then you should have read the other 10 pages then as that was covered!

[Diving Dude]

A better option if you think a yellow bag is going to be required, ie very long times in water and there might be an occasion when the 'team' runs out of gas, is to use support divers.

l'm sorry Gareth, but get real, this only works in the "l'm a disciple" world of the KP nut guys in florida, in the real world the guys doing half reasonable run times in the 70 m + range don't get the comfort of support divers.

If you disagree, l'll happily accept you and the chimps as support divers for Chasey and me on our next 70m+ gig.

EDIT

You'll have to pay for yoursleves though

EDIT 2
And bring the donuts

[GLOC]

Like Jack Ingle uses on his trips or Joe Cornish?

I agree that support divers are not always easy to come by, but I still think they are a better option than a yellow bag especially for OC where you don't have the luxury of the very long supply of gas which you do on a CCR.

Having a discussion last night with Garf over Ada's little ascent and what we do brought it home, would we do an 85m dive without support divers? Not sure but probably pushing the limits if anything goes wrong.

Regards