CCR in O2 mode as a "Deco Check"....

[Padowan]

OK, I have been thinking. I run my unit as an O2 CCR on deco starting from about 7m briefly to get the buzzer sounding, then at 6m to keep the PPO2 nice and high (and hence maximise the inert gradient). After a reasonable exposure dive (particularly with Helium in the dil) it is apparent that I am still offgassing as the loop PPO2 drops - (before anyone who's not thought about this properly says, "But the PPO2 will drop because you're metabolising O2" - this is not the case, if the unit is running as an O2 breather, there should always be 100% O2 in the loop, and any drop is caused by the addition of inerts from your body, not the metabolisation of O2)

So my question is this: Could you use the monitoring of the rate of PPO2 drop whilst running a CCR in O2 mode as an indication that you are "clean"? For example if you're at 6m, and you flush the loop and get 1.6bar, you know you've got 100% in the loop, if after 5 minutes that's only dropped down to say 1.58 bar, could you assume that you are no longer off gassing as a rate that gives a hightened risk of DCS. (obviously I only picked the numbers 1.58bar and 5 minutes arbitrarily, more investigation would be required...)

In the case that your body is still loaded, but because of some external factor is not offgassing efficiently (for example like el_presidente frezing his nads off after a suit flood) this check would tell you that you might as well get out early, blow the stops, get on O2 and call the chopper as the stops are not working efficiently anyway, and even if you do them, you'll probably still be loaded. Would save possibly complicating a bend with hypothermia...

Any thoughts on this as a possible additional "cleanliness check"?

[DecoWarrior]

we have thought about this and noticed the effect that you are talking about

especially on long dives, it does seem to be a way of measuring your off gassing , but i would be very reluctant to rely oon that soley as an indication of offgassing being completed, if the shit had hit the fan and i had to get out the water, then i would probably risk it but on a dive where there was no rush to get out i would almost be tempted to sit it out if you havent any major probs and youve got loads of gas, i dont think its worth the risk for what may effectivley be a few mins

after all sitting in the water for a few mins aint going to bend you or kill you is it!

[Padowan]

I'm not really talking about using it as a replacement for a schedule. People have been known to do 'perfect' deco profiles and still come out bent, perhaps doing this check after the scheduled stops, would provide extra info as to whether the schedule has been effective?

[Andy Phillips]

Do you know mate, I am not certain whether or not you think too much or have some really good thoughts.

I shall go away and think about this, you've gone and piqued my curiosity, damn.

Andrew

[Padowan]

Originally Posted by Andy Phillips

Do you know mate, I am not certain whether or not you think too much or have some really good thoughts.

I think the first one is true for sure. But hopefully there's some truth in the second one....

[ahar]

Originally Posted by Padowan

(for example like el_presidente frezing his nads off after a suit flood)

Interesting - in the case that el_presidente posted about, I suspect this would have been of help if he had an idea of what the ppO2 drop should have been - then as he was at his 6m stop 'freezing his nads off' he could have seen that the actual rate of inert gas entering the loop was less than expected.

In fact, if you are making this complicated....I assume that you know the volume of gas in the loop, and so will be able to work out the volume of inert gas exhaled in a certain time period. Given a particular dive to a particular depth for a specified amount of time, I suspect it would be possible to work out very rough figures for the amount of inert gas you would expect to exhale when decoing at 6m. This could be an interesting back up for tables or a computer - if the rate of exhaled inert gas is less than expected you could extend the stop just in case. I would require a large amount of experimental data to work out however, and it would be a very individual thing. The limiting fact as always would be the accurancy of the ppO2 read out

[Decodiver]

Originally Posted by Padowan

(before anyone who's not thought about this properly says, "But the PPO2 will drop because you're metabolising O2" - this is not the case, if the unit is running as an O2 breather, there should always be 100% O2 in the loop, and any drop is caused by the addition of inerts from your body, not the metabolisation of O2)

Hi Padowan

This is correct if you have ramped up your setpoint to 1.6, but is not symptomatic of O2 RBs per se.

If you use the unit as an O2 RB, after say a complete electronics failure, where you are flying by the seat of your lungs and running minimum loop volume and manually adding O2, then the drop will be as a result of O2 metabolisation.

However I do see your reasoning, my point of caution is that it has to be said that one of the earlier fatalities on the unit is believed to have been from running the unit at high PPO2 on deco resulting in an ox tox.

Cheers

Dave Cooper

[DecoWarrior]

i know people who have considered risking getting out of the water as we have spoke about thhis many times in the pub, i see what you mean about an indication of your cleaness,

the point i was trying to make was that if you were in a position where getting out of the water was a necessity you could use that as a guide with a hope of damage limitation

i dont know of any one who has actually tried it though, although we have talked about it on many occaisions

[Andy Phillips]

As I think about this I find myself wondering how accurate a test this would be.

Sure, you breath out the off gassed inert gas, but this is no measure of how much is still dissolved in your system, all you are really proving is that the gas that has been removed, is removed. So called "dirty" deco is caused by gas sill being disolved, or rather contained somewhere within the body.

Doppler is really the only answer, you need to check for bubbles.

Just a thought or two, still pondering this one.

Andrew

[Padowan]

Originally Posted by Decodiver

...
This is correct if you have ramped up your setpoint to 1.6, but is not symptomatic of O2 RBs per se.

If you use the unit as an O2 RB, after say a complete electronics failure, where you are flying by the seat of your lungs and running minimum loop volume and manually adding O2, then the drop will be as a result of O2 metabolisation....

I disagree. Taking the example of an O2 rebreather diver, where there is no inert loading (above atmospheric air) on the diver, the loop is flushed to contain pure O2, 100%, at 6m the PPO2 will be 1.6bar, whatever the loop volume. As the diver metabolises some of the O2 in the loop, the volume of O2 reduces, but it will still contain 100% O2, only less of it, so the PPO2 will still be 1.6bar so long as there is gas in the loop, no matter how much he metabolises... If we add in the element of a diver who's got an inert loading from doing a proper dive, his body will be offgassing the inert loading into the loop as he ascends, and will continue to offgas these inerts into the loop at the 6m stop, so the loop now starts to contain more inerts (plus less O2, from metabolism, but as illustrated previously this alone will not effect the PPO2 of a 100% O2 loop) and the addition of the inerts mean the %ages of the gasses change, effecting the PP of the gasses.

It's a moot point, but the metabolism of O2 in a loop will only affect the PPO2 if there were either inerts already in the loop (eg a poor O2 flush) or if the body is putting inerts into the loop (offgassing)

Not starting a fight - I'm a relative RB newbie - but I think I've got a good point, that can take a while to get the head around...