CMF Gas matching - 4 parts [reposted with working photo links].

Freef

Thanks, Mathias, green sent.

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David.

Currently owner of two differently sized ankles.

Dsix36

Man, I wish this post was here back when I was diving my Dolphin. I was on the verge of killing myself, with all the messed up calculations and spreadsheets that I did, just for more bottom time. Maybe with this info somebody won't have to learn the hard way, like I did.

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THE MORE THAT I LEARN, THE MORE THAT I STILL NEED TO LEARN!!!!!!

Freef

What happened? Did you post an incident report?

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David.

Currently owner of two differently sized ankles.

Dsix36

I wasn't a member of Rebreather World back then, I just read the posts Thus I did not file any incident posts. Plus, I did not want to look stupid, but maybe this will help someone else!!!!

I was experimenting with different gas mixtures and flow rates in an attempt to maximize my bottom time. I never liked the fact that I was depth limited by my mix - If what I was actually breathing was not as rich as what the tank held. I made several spreadsheets (to do the math for me) and began to input the mix, flow rate, and usage to determine a new EAN (this was suppossed to be what was in the loop).

My biggest mistake was in the amount of gas used. I was calculating by volume rather than what was actually metabolized. I found it very hard to estimate these figures using a SCR that vented.

My loop was always much higher than expected, but my dives were planned with a bottom depth that would yield a max PO2 of 1.5 in the event of calculation errors (full tank flow and FO2)

On the dive in question, I was diving a max depth of 72 ft. to a wreck that I was very familiar with. I was using 60% ( which would actually breath as 44%, 1.5Lpm usage) and my MOD was 72ft. Max PO2 @ 72ft was 1.4 I thought that all was fine. When I hit the sand at 72ft my PO2 was 1.9 I began to ascend, at 65' (still above the wreck) my PO2 was 1.7. I did about 15 minutes there before aborting and ascending.

PO2 of 1.9 scared the crap out of me. All of my training tells me 1.6 is the absolute maximum. I was even nervous about 1.7

Did I mention this: When I got home and rinsed and disinfected my gear, I had connected the input hose to the wrong orifice, It was still on the 32% and not the 60%. This doubled the flow. Remember, distraction will kill me too.


The problem with my calculations was this: I was figuring usage as "volume of gas used" (1.5Lpm) and it should have been "volume of O2 used" (.23Lpm). This makes a big difference in the actual loop mixture and the resulting PO2 at depth.

That was my last attempt at experimenting and decided I needed and OXY and Uwatec to be sure. I still never felt safe and decided to bite the bullet and buy a CCR. I spent almost 2 years researching and studying Rebreather before picking an instructor and buying a unit. I am still waiting for technical training though.

If I never see a PO2 over 1.2 again, that is fine by me!!!!!!!!!!!!!!!!!

Even at the expense of proving my braindeath, I hope that this post may prevent another from doing the same thing

__________________
THE MORE THAT I LEARN, THE MORE THAT I STILL NEED TO LEARN!!!!!!

Freef

That would about do it!

It's easy from the comfort of my chair, but you may have been able to see the difference in flow rates by the howl of gas into the loop from the 32% jet compared with the 60% jet gentle hiss. I had a dive once where I got out straight after getting in because the 60% flow seemed high [it wasn't] and I swapped the hose to the 50% jet. Using 40% in the tank it still let me do the 20m depth we were aiming for anyway.

The other thing to remember about the mixing described in the article is that a loop flush at a depth beyond the MOD of the tank gas will cause a spike the fO2 of the loop, and hence the pO2.

__________________
David.

Currently owner of two differently sized ankles.