The Pelagian Way

Rebreatherlab

You are absolutely correct Dave.

Hence the importance of the 6 m check in pure O2 in the beginning of every dive. We had a short email conversation about this back in the days of the Rebreather list. You were wondering why I had students breathe their bail out at the 6 m check and I said it was to get their third point in a linearity check and that the O2 really had to be pure, remember?

During the dive It only takes a half and slow exhalation to get a reading that is slightly over the cylinder FG x ATA. This is due to the surrounding gas. With a bit of practise you get the same reading every time. Say Air Dil at 20 m would read 0.67 instead of 0.63. This check is not really there to weed out voltage limited cells, but rather to spot frozen cells, cable glitches, water of cell membrane, corroded Molexes etc. etc.

Another thing is that our display does NOT truncate the third decimal. If you have your cell pushed against it's voltage limited ceiling the third digit would stop wandering up and down with your inhalation / exhalation.

The reason we have to cut the thread off the cells is to avoid having to blast them head on. Now the DIL nozzles are coming from 2 o'clock and rather scrape the moist off than force it through the HP membrane.


Cheers,

Andy

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dave t

Wow good memory Andy Agreed as many checks as you can do is always good I always push my po2 up above setpoint when I reach bottom, I dont care if it wont go to say 1.6 as long as it goes above my chosen setpoint on all three cells I am happy.....for the moment.

Very nice product you have there I wish you every success

best

Dave

kieranu

I'll put my neck on the chopping block, but others are probably thinking the same thing - isn't this potentially suicidal if you're diving below around 10m unless you're really confident on how much oxygen has been injected into the loop?

Rebreatherlab

Thanks Dave. I will be in Sharm between Sep 28 - Oct 5 and at DEMA plus another 10 days after the show to run diver level and Instructor X-over. If you happen to be around, find me and you can have a closer look and a test dive, (time and logistics permitting).

Cheers,

Andy

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Dave Sutton

Uhhh..... no, why would it be?

If you've done any reasonable cell validation routine (6M check) and have sufficient common sense to not to push the 02 manual add button for 20 seconds after the cell stops increasing the PP02 reading (bad cell scenario) you'll be just dandy. And if you are using a sensible diluent, you're probably *already* slightly above setpoint when you bottom out anywayso you don't need to do a thing to make this check.

On all rigs I dive, I try to bottom out somewhere at about 1.6-ish, with a desired setpoint of 1.4-ish. Only after "some time period" do I expect to either (A) have the solenoid fire, or (B) manually add 02 (depending on the rig design), as loop PP02 decays to setpoint from initial bottoming-out PP02.


Dave

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dave t

Exactly as Dave said when you descend you should be on low setpoint and with a descent dil no o2 will be added by the solonoid, it takes a very short push on the manual dil button to bring the po2 above setpoint if the dil hasnt done it already. I am suggesting bringing it up to 1.4 if my setpoint is 1.3, no more.
I would have thought that manual control is a simple skill that should be taught in all classes. Maybe not!!

best

Dave