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

dteubner

Post the argument not the names of people who might be able to explain something which makes no physiological sense because they are experts

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Dave T

Hanlon's Razor - Never attribute to malice that which can be adequately explained by stupidity.

Janos

But gas in the body is a isn't unsaturated on the way up. It's supersaturated, but fortunately the body can tolerate a degree of supersaturation. This is a ratio of tissues pressure to ambient pressure, and the tolerable ratio is alos dependent on depth.

Have a look at erik baker's paper on M-values (there's a link on www.rebreather.ca amongst others) which goes into it in more detail and explains it far better than I can.

Janos

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Jon Adams

[quote=Mark Chase;176708] You are joking aren't you mark

you attach a jump or travel line from the shot, to the lazy or deco station last pair up disconnect if all the tags are gone and you all float off, then if the lazy is full up you can bag off and you all move in the same direction with the tide!! that is really basic ffs

what charlie t said is correct,

you won't catch me hanging on for dear life next month old pal

jon

Janos

Nine times out of ten the above makes perfect sense, but the example Mark gave where he was decoing at 8m was on a dive out of Dover, where you can't drift, no matter how much you (and I!) would like to.

Janos

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Mike

Surely, when we are decompressing, our tissues ARE supersaturated. They contain a higher partial pressure of dissolved gas than they can hold at the depth/pressure we have ascended to.

The can of coke analogy surely fits - take a bottle of coke into the chamber with you and pour yourself a glass at depth - it will appear flat (coke is pressurised to about 3 ata, pretty close to the 18m of a Table 6). The liquid is able to hold more dissolved gas due to the ambient pressure (and I'm assuming that there is no CO2 in the chamber!). As you reduce the ambient pressure gas will dissolve out of the liquid - if you reduce pressure slowly enough you could decompress coke without it bubbling. Isn't that all we are trying to do on deco?

Extending the analogy, consider the pressure delta between dissolved gas pressure and ambient. If that delta is kept very low, the CO2 diffuses out without bubbling, however it takes a long time to decompress the coke. If the delta gets too high, the coke bubbles. However, there is an optimum pressure delta at which you get the maximum rate of gas dissolving out of the liquid without bubbling.

In order to maintain the pressure delta at this optimum, you need to vary ambient pressure relative to how much gas is left in solution. Seems to me that this is very much like bringing a diver up from 6m to 4m.

Mike

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dteubner

Fair point.

It doesn't, however, change the fact that the determinant of the amount of inert gas which leaves the venous blood (and hence the body) is the difference between the partial pressures of the gas(es) in the venous blood (PvIg) and the alveoli (PAIg).

The PvIg and PAIg are completely independent of the ambient pressure unless there is inspired inert gas.

If there is inspired inert gas then, for a given fraction of inspired inert gas, the PAIg will be smaller with decreasing depth - i.e. off gassing will actually be faster at shallower depth.

No-one has proposed a reason why off gassing should be faster at 6m vs 4m. If you are not breathing 100% O2 (and we never are really for reasons discussed in the thread) then the reverse is clearly true.

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Dave T

Hanlon's Razor - Never attribute to malice that which can be adequately explained by stupidity.

UWSojourner

It’s difficult to determine if this discussion is trying to get at what is happening physiologically or practically (i.e. how your dive watch actually performs the calculations).

The latter is quite clear. Buhlmann algorithms and VPM both use the tissue half time concept. Tissues are loading according to the following equation:

TC1 = TC0 + (InspiredPressure – TC0) * HTFactor.

The important thing to determine is what changes each symbol.

TC1 and TC0 are the dissolved gas pressures of the tissue compartment at time 0 and time 1. The value TC0 only depends on what happened in the past, not your current stop depth. So whether you are stopped at 6m or 3m, TC0 does not change (it is assumed your tissues have TC0 pressure at the start of the process and you’re starting at 6m or 3m based on the original question).

HTFactor is dependent on the half-time of the compartment and the elapsed time from time 0 to time 1. Again, it is not dependent on depth.

InspiredPressure is the pressure of the inspired inert gas. It is directly dependent on depth, but only if an inert gas is in the breathing mix. Once you are breathing O2, InspiredPressure is 0 at all depths. In this case (breathing O2), TC1 is not dependent on depth because TC0, HTFactor, and InspiredPresssure are not depth dependent, and your Buhlmann/GF and VPM dive computers will not reflect any differences in speed of offgassing based on depth (i.e. TC1 will only be dependent on elapsed time from time 0 to time 1, the half-time of the compartment, and TC0).

If the question is physiological, then all the complex issues surrounding how a body responds to decompression come into play and I'll simply wait for Dr. Mitchell to stop by at some point and provide a case study illustrating why Dr. is not in front of my name.

dteubner

Typing at the same time.

The ambient pressure is only about bubbling and bubble formation. Unless significant bubbles are present then gas is eliminated by diffusion accross the alveolar membrane. The rate of diffusion is dertermined by the difference in inert gas partial pressures between the blood and alveoli. This partial pressure difference is either independent of ambient pressure (on 100% O2) or larger at shallower depths with a constant fraction of inspired inert gas.

Your can of coke analogy should be looked at like this. The fastest way to get the CO2 out of the coke is to explosively decompress it (by opening it). If you don't want bubbles to form then you put it in the chamber and open it at pressure. CO2 will still leave the coke by simple diffusion accross the air/coke interface. The rate of this diffusion will not be affected by chamber ambient pressure as long as the pressure is high enough to prevent bubbles forming. So once you are deep enough to stop bubbles the coke will still go flat and the rate at which it does will be independant of chamber depth.

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Dave T

Hanlon's Razor - Never attribute to malice that which can be adequately explained by stupidity.

Mike

It should be noted that David Teubner also has 'Dr' in front of his name, and does some of that unbending of diver stuff too.

Whilst trying to find my old physics texts to look up Raoult's law, I've had a bit more of a think, and I suspect Dave is right, and I've been thinking about vapour pressure as well, rather than just partial pressure of a dissolved gas. And forgetting that dissolved gas does not change volume with pressure

So I now think;
Rate of inert gas dissolving out solution is independant of pressure if O2 is the only inspired gas.
Bubble formation in blood only helps move the gas around in the blood, it doesn't help gas dissolve out of tissues any faster
High PPO2 has a beneficial effect in adition to the 'lack of inert' effect.
CNS clock is too conservative at <~1.6 PPO2
Dave Teubner remembered more from uni physics than I did.

Therefore, I should deco at 6m all the time :-)

Mike

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Mark Chase

[quote=Jon Adams;176841]

We are not diving out of Dover next month so we will be using a lazy

And no i am not joking about Dover. a lazy shot will drift straight across the path of the Sea-Cat just as easy as a SMB will and you will be just as dead

We do fixed shot line ascents near the lanes in Dover and use john lines. We do have problems with masks being ripped off and regs free flowing and you usually come up with 10lb of shrimp attached to you as well

Its the only way to get on some of the wrecks. On or near springs its hard work.

If we started bagging off or using a lazy shot in the busiest shipping lane in the world we would be banned from diving there faster than you can say French Port Authorities.

Thanks to a disgruntled ex skipper stirring things up we are already banned from diving inside the lanes and have lost access to several excellent wrecks as a result.

When doing a shot line ascent 100% for deco is not advisable.

ATB

Mark

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