Using nitrox for diluent instead of air for shallow dives

marekm

Hi everyone,

While doing recent pool tests with my rebreather, I decided to run air diluent through the loop in SCR mode and see how it fared at 2m depth. I was a little shocked to see how quickly the PO2 dropped and how using it as a oxygen carrying gas for SCR bailout would be rather inefficient, and not much better than using the OC reg attached to the diluent tank.

After reading the great hot vs hypoxic diluent debate, I'm curious if it would be ok to use say 36% nitrox for diluent when doing dives around the 60feet/20m mark or shallower. I'm guessing that the extra moles of oxygen per litre of diluent gas would make SCR bailout more viable than just going to OC of the dil tank. Has anyone ever considered or tested this approach?

Thanks in advance.
-Marek

Dave Sutton

Yes, you are dead on with this. If you think that a SCC bailout is a possibility then more 02 in your dil is good, and if you plan to use that same onboard gas open circuit as part of your plan, it also makes sense to have it be a sensible mix too.

For nitrox range diving on my Mark-15 I use air diluent. The internal sphere and the internal 02 sphere combined equal my open circuit bailout needs. I do the same thing on the SK and homebuilts as well.

For trimix diving, I fill the internal sphere with 50/50 nitrox, use it for my suit, and feed the rig with my offboard trimix diluent (typically 21/50). The combination of the offboard gas, and bioth inboard gas supplies makes a SCC deco easy and generally also allows a OC bailout as well to be done using the internal diluent and 02 supplies open circuit for deco.

Nitrox as diluent? Sure, just be sane about the PP02 of the mix on the bottom. 1.3 is likely ideal.

Your trial at shallow depth is about as adverse as a SCC system can be used also. Deeper you would have a far longer run from, say 1.3 down to 0.5 or so. You were probably starting at 0.5 and droping to 0.3 or so? Not going to take long.


Dave

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