tibby

If it were me and could afford it I would wear a fully closed O2 rebreather while inside the cave area to be measured and not worry about using or contaminating the cave environment at all.
It's pretty simple to make one especially if it doesn't need to be submerged. I'm sure you can find a lot of instructions on the internet on how to make one. The most expensive part is the oxygen bottle and the button to inject it. You can adapt a paint respirator mask from Home Depot to make it easier to breathe (through nose). For a counterlung you can use a hydration bladder from an outdoors store (3-5 liters in volume) and you can buy corrugated hoses from a dive store (replacement BCD inflator hose such as Dive Rite).
Make sure to carry enough oxygen for the duration (~1-4 liters per minute depending on workload). Also the scrubber needs to be of decent size, maybe holding 2-3kg, to last several hours.
Do not attempt to dive with it though (unless you really know what you're doing of course).
There are other, more experienced people on this board that can help with this design I'm sure.
Or you can buy a mine rescue rebreather off ebay as suggested above and use it as is.
Just be aware of dangers like CO2 poisoning if the scrubber fails or there's some kind of bypass. There's also a danger of the oxygen catching on fire if not handled properly.
If you don't understand the principles and working mechanism yet try to do so before attempting this.

Tibby

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Does the voice of reason change when you add helium to the mix?...hmm