April 17th, 2012 by Jay Harding
Essentiall Fenzy PO-68 Mod
Okay, if you read my hypercapnia post you'll know that I almost died on a PO-68 chest mount pure O2 rebreather a few years ago. Below is text from a post on that "other rebreather site" detailing my forensics on the unit and my mod to fix the problem. Unit dives safely now.
Well, I think I've finally figured out definitively what happened with my CO2 hit. I chalked up the breakthrough to wet sorb - which I still believe - but now I think I know why the sorb was wet.
I changed the hoses and DSV on the PO-68 to a set of double hose regulator hoses (1 inch cuffs on one end and 1.5 inch on the other) and a borrowed KISS BOV. This swap was what allowed me find the problem. With the new softer hoses, I was able to see that my negative and positive pressure checks were holding, but not absolutely. When I drew a hard negative on the loop, the softer hoses allowed me to see that there was some leakage, but only a little bit right after the loop was sealed, then the leakage stopped. I noticed this because the hoses would relax just slightly and then stop. So, I tried a positive pressure check: same thing. When I sealed the loop, the hoses would shorten just a bit, then stop. It first I thought is was just the the suppleness of the hoses fooling me. So I tried both again paying close attention to the counterlung (removed from the shell). Sure enough, the bag would slightly inflate after being crushed with hard negative and slightly deflate ater being overfilled during a hard positive. So, I started my forensics again, checking all the seals, etc.
Here are my conclusions:
1. The seal in the counterlung to scrubber fitting on the inhale side of the loop was failing - but only when the counterlung was either overfilled by a positive pressure situation or crushed right down by a negative pressure situation.
2. The seal failure only lasted about a half second until the overfill or crushed state was relieved. Once the counterlung tension was slackened, the seal worked again.
3. When the counterlung was forced out of it's normal working shape by either overfilling or vacuum crushing, the torque on the fitting caused the seal to fail until the torque was relieved by slight leakage (which restored the counterlung's normal working shape).
So, why was the seal failing when the fitting was under torque? It's a barrel seal with an o-ring. WRONG!! When the o-ring was picked out of the fitting (it's pushed into a groove on the female side of the fitting) I found that it wasn't an o-ring at all. It's a ring seal with a channel shaped cross-section. It's not solid. So, when the fitting gets torqued, the seal gets distorted and fails until the torque is relieved.
SO! Under the normal working cycle of the PO-68 (which has no manual add valve and only has an ADV) the counterlung gets crushed every couple of minutes as your body metabolizes O2. The loop runs on minimum volume by default and every couple of minutes the bag bottoms out and the ADV fires replacing loop volume and adding gas. ONLY . . . everytime the bag crushes, the seal on the inhale side of the loop fails. I believe that everytime this seal fails a teaspoon or two of water trickles through that fitting and right into the scrubber. The longer the dive, the more potential for breakthrough. OR in my case, it was at the beginning of the next dive on the same fill (the next day).
I've since done a series of tests on this loop (out of the shell) and sure enough, as you overfill the loop with a positive pressure check, the fitting bubbles. But, only for about a half second, then the seal integrity restores. Torque the fitting with your fingers and it leaks again. It's truly a fluke that I figured this out. If I had kept the original hoses (which are mil spec and heavy duty and stiff) I probably wouldn't have noticed the volume change or figured it out that the seal was failing.
Needless to say that I am on a quest for perfectly dimensioned solid o-rings to replace those hollow cross-section seals. I have to wonder why Fenzy used them instead of o-rings in the first place. At any rate, I consider replacement of those seals a necessary mod. Other PO-68 users please take note. Dive Safe, Jay
NOTE: Found o-rings with a good tight fit. Unit now breathes and dives flawlessly. No drips, no wet sorb, no breakthrough.