With the I.D.I.O.T completed and with waterproofing tested in shallow and fairly deep (80 ft) water, it was time to go take it for a spin in “the deep”. The (purposely) sunken boat in Hathaways ponds seemed like the perfect place to go- there’s stuff to see, and places to swim.
The logistics of a “deep” oxygen rebreather dive were not as simple as one would hope. Since the counterlung is also used for buoyancy, it is not easy to accurately predict the volume needed (without prior experience). It is also hard to descend when you are several lbs positively buoyant. It was decided to descend on a buddies O/C and then breathe the normoxic gas from O/C into the counterlung to provide a reasonable PO2 at depth. This worked more or less perfectly, giving me a rough PO2 in the 1-1.4 range.
After completing a swim around the boat, we followed the line across the great murk of the pond to shallower waters (and lower PO2s). However, due to trying to attain neutral buoyancy some gas was exhaled, causing PO2 to increase when the loop was re-inflated. So we made a stop at the “deep” (30ft) platform to take on some more good normoxic air to bring the PO2 back down to oneish. I expected the loop to get a bit rich as I vented air and played with buoyancy on ascent.
From the deep platform we continued along the string to a shallow platform, the mirror, and eventually even found a nice (underwater) chair to sit in.
With the deep testing completed, we headed back to the beach and swapped gear so my buddy could check out the rebreather in shallow water.
Things seemed to be going well until my buddy got a taste of the ol caustic cocktail when he inverted slightly. This caused him to call the dive. Neither of us can figure out where the leak came from since on my dive there was only a little bit of water in the CL after 40 minutes. His flood was at least ~1L of water, which is a significant flood. This underscores the need for backup at all times when diving the rebreather, and this is obviously a reason why people don’t run around breathing off of a single AL6 all the time.
Blowing up my rebreather 😦 + performance notes
I’ll start with the exciting part- performance. I used about 800 PSI from my AL6, which is about 1.6 CF of oxygen for a ~45 minute dive. This was supplemented by 2-4 big breaths from O/C, which would be about the same volume. the dive profile was straightforward, straight down to 40′ and then a slow ascent to the surface.
I was overweighted with 25lbs with my 7mm hooded vest and 7mm wetsuit with booties. This caused my trim to be basically vertical, and the cl volume needed was essentially the whole counterlung. This is because there is no huge cylinder strapped to my back during the dive, so the belt and counterlung create a huge moment on my body. This means I have to swim to stay in trim, which makes for a bit of a frantic dive. Reducing overweighting would help, but really what I need is to move the weight up to my back. On previous dives with a ~2mm wetsuit I held a rock far out in front of my body, which gave the weight of the rock enough leverage to counter the small amount of weight I was wearing. With 25 lbs, it seems unlikely that I will be able to balance that out without moving the weight.
Blowing up the rebreather was much less extreme than it sounds. I tried to dewater the flooded rebreather by pressurizing the counterlung- what I forgot to do was to open the vent, so I just popped the counterlung. Not ideal! But it should just be a matter of buying another drybag and cutting some holes to replace the counterlung.
It was nice to finally give the I.D.I.O.T a real test and to on a really interesting dive. I hope to return someday soon with my own diluent addition!