Off gassing at 4 vs 6m (from the complacency thread)

CharlieT

I remember Rich Stevenson being a strong beliver in 50% and 100% deco gases when I did my Intermediate and Advanced Open Circuit courses with him.

He might hold different views today, I don't know.

Are you telling me he was talking cobblers?

Mark Chase

100% for deco is definitly the best gas but for the purposes of diving its a PITA

First off you might not be albe to do your deco at 6m or above. If your on a shot line or deco station dive you might be at 6.5m or 7m. On a shot line ascent out of Dover once i did 90% of my deco at 8m due to the other 10diver being above me on the line.

Then there is the fills issue.

I go to the dive shop and ask for an 02 fill in my 10ltr deco bottle and they say no because they are running short or Yes but they only have 110bar in the banks OK if 110bar is enough but not if its not.


80% rocks

If your using 80 then 50s a bit to close so I used 36%

ATB

Mark

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Mike

Henrys law assumes at STP - pressure is constant. As ambient pressure drops, so does the amount of gas that will stay dissolved in the liquid. Think CO2 in a can of coke.

Mike

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Janos

Hum. What I was trying to say is that divers off-gas through bubble formation when ascending, as well as through simple diffusion. Remember that all ascents produce bubbles. It's the size that varies.

Conside a diver who rockets to the surface after a big long dive. Gas will rush out of his tissues and form bubbles and will cause him severe problems. However he will have less dissolved gas in his blood!

Now consider two divers, Alf and Betty. Being good buddies they have stuck together throughout the dive and done identical profiles. They arrive at 6m together, and prepare to do an hour long stop on pure O2. [1]

Alf spends the whole of his stop at 6m, but during her stop Betty ascends slightly and finishes her stop at 3m.

During Betty's ascent to 3m, gas will come out of solution and form tiny bubbles. If her ascent is sufficiently slow, then these bubbles will be small and be filtered out by her lungs. Alf will produce no such bubbles.

Because Betty is offgassing through bubbles and diffusion, she will have less inerts in her blood than Alf.

Suppose there was a third diver in the team, Charlie. Let's say that Charlie ignores the 6m stop and does all of his time at 3m. He will produce bigger bubbles than Betty, and so off-gas quicker, but it may well be that these bubbles are so big that they are not filtered out properly by the lungs or they cause damage. So Charlie off-gasses quicker, but runs a bigger risk of being bent.

Well. That's the theory at least. I've no idea how practical all this is. My own plan is to clear the 6m stop, then ascend slowly (ie over five or ten mins) to 4m or 5m if conditions allow.

Janos

[1] - Let's ignore CNS issues for the time being.

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Janos

If he believes that a diver offgases breathing 100% at 6m offgases more efficiently than one breathing 100% at 5m (or 4m or 3m) then yes I think he's wrong.

I've seen no evidence to suggest this is the case and no theories to explain
why it might be the case, but if you or he have anything to show me why this might be then I'll happily look at it with an open mind.

Janos

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CharlieT

[quote=Mark Chase;176686]100% for deco is definitly the best gas but for the purposes of diving its a PITA

First off you might not be albe to do your deco at 6m or above. If your on a shot line or deco station dive you might be at 6.5m or 7m. On a shot line ascent out of Dover once i did 90% of my deco at 8m due to the other 10diver being above me on the line.

Then there is the fills issue.

I go to the dive shop and ask for an 02 fill in my 10ltr deco bottle and they say no because they are running short or Yes but they only have 110bar in the banks OK if 110bar is enough but not if its not.


80% rocks

If your using 80 then 50s a bit to close so I used 36%

ATB

But I was only referring to the qualities of 100% O2 not the practicalities of using it.

If there are too many divers above you on a shotline then deploy your DSMB but remain CLOSE to the other divers taking up the space at 5-6m. This is a safe and accepted practice even on 100m + dives as was practiced by several extremely well known and experienced expedition divers during our Alderney trip last year.

I find out what depths the one or two deco bars will be at (on a big dive) and plan accordingly. If the plan goes awry then deploy that DSMB and be at the depth you've planned and want to be at.

Deco stations are often virtually unusuable in UK sea conditions with the bar(s) bouncing violently up and down. In these conditions they are uncomfortable to use and I blob off but remain close to the station.

I understand what you're saying about getting 100% O2 fills at dive shops but how often do you need to do this if you're a rebreather diver? Once a year, maybe twice?

You cannot have a rigid approach to deco gases as it very much depends on the depth of the dive. That's why I've ended up with a collection of ali deco cylinders from 5.6L to 11L each containing various different gases.

No, you don't always jump in with the perfect combo but I'm damned if I'm emptying them all the time and putting up with all that hassle at the dive shop

CharlieT

Then email or ring him with your views and you'll learn something new during the process

I use Martin Robson of Eau2 Technical Training nowadays and that's exactly what I did about several queries I had when I met up with him last week.

If you feel you're more of an expert than the likes of Rich Stevenson or Martin Robson then I wish you the very best, M8.

Janos

I don't claim to be an expert, just a coward!

If someone says that you can offgas more efficiently by [Your Choice Here] then unless they can come up with convincing evidence that shows it works I'll just keep doing the extra time on the stops.

However I am genuinely interested if you or they have evidence that shows that offgassing on pure O2 at 6m is better than offgassing on pure O2 at 5m. I did research this substantially a while back but didn't find anything. Do you have anything you can post here?

Janos

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Mark Chase

[quote]
Then disappear off in the 2 knot current straight across the path of the cross channel ferry and the 4-500 container ships in that area each day

We don't deco on the shot for a giggle. Its horrible hanging on like a flag most of the time. We do it because we have to. Lazy shots don't work and deco stations just get dragged to the surface by the current. Buoys just get dragged under. We have a anchor line connected to the boat.



Only works in a flatish sea, we tend to use a slightly deeper bar to aloow for surface movement.



We use them every dive in our gas diving group. We havent had any issues we couldent manage so far. Uzing a lazy shot bagging off is an option.


Sorry i thaught we were discussing the practice of 50% and 100% for OC deco.

I used 21/35 36% and 80% for all my OC deco diving between 40m and 100m


On a CCR i use 10/55 18/45 50% and 80% Thats it, I never change the gas list for bailout I use ratio deco as a back up so its not advisable to mess arround with the list.

ATB

Mark

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I took my unit to the dive shop and demanded they bolt on every thing that would fit.

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dteubner

What the CO2 in coke shows you is what happens when you explosively decompress a supersaturated solution - hopefully not relevant to CCR divers!

All the ambient pressure does is determine the point at which bubbles have a chance to form. When the partial pressure of a dissolved gas exceeds the ambient pressure then there is the potential for gas to come out of the solution as bubbles.

If you have an unsaturated solution of a particular gas in liquid in contact with gas which does not contain that patricular gas then the ambient pressure has absolutely nothing to do with the rate at which the particular gas will leave solution.

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