Jon said:
So, I could just pick up a used rebreather and run it without the drive bottle to get the same effect- assuming it has an 02 guage on it to monitor what I am breathing?
Jon
Absolutely not!!! The loop would eventually go hypoxic because there is nothing to replenish the oxygen content. It is a closed system, the oxygen has to come in from somewhere. With the above DIY model the air comes in through a valve. With a closed-circuit rebreather pure oxygen is added from an external supply with the bulk of the breathing gas being air or a helium based gas (the diluent). This is usually done by an electronically controlled solenoid valve which will inject oxygen when it is needed (as in the Inspiration, Mk15, Megalodon, Prism, etc) or there is a tiny constant flow of oxygen getting bled into the loop through a precision orifice to replenish it (the Jetsam KISS).
This kind of rebreather is very expensive, even buying used. A used Inspiration doesn't sell for much less than a new one. They also don't come up for sale that often. You'd be looking in the region of $4000(US) at least. It would also be very difficult to get the O2% down as low as 8%, the units usually have a failsafe in their control systems to prevent this ever happening. It would be possible with the Jetsam KISS as this is manually controlled and has no electronics.
Oxygen rebreathers work differently. They ONLY have oxygen (no diluent) in the loop. As oxygen is consumed then the loop volume decreases and a valve is triggered to inject more oxygen. There is also a manual pushbutton over-ride to add oxygen. These are usually ex-military technology or used in firefighting or working in contaminated environments. Oxygen rebreathers can be picked up very cheaply on e-bay now and again (the Biopak 45 often goes for about $150). It would need a bit of work, you would have to add an oxygen monitor (diving rebreathers usually use 3 not just 1) and you would need to disable the automatic oxygen valve.
In practice you would inflate the rebreather with air by blowing in it. Then you would breathe it down to the required %O2 and use the button to put a bit of oxygen in. It all seems a lot of hassle though, and you still need oxygen to drive it. The DIY model doesn't.
Jeff Bozanic's Mastering Rebreathers is an excellent read for anyone interested in rebreather diving.
Cheers,
Stuart