Transdermal monitoring is old technology, mainly used in slerp medicine. There's decades worth of research available. It's not terribly accurate in numerical values but it will track a change in baseline well enough. As an example we use both transdermal monitoring and arterial samples on our patients. The transdermal might be off upwards of 0,8 mmHg when we determine starting values. It will track a change in pCO2 at around +/- 0,1 though. Ie actual CO2 is 5,3, transdermal says 6,0. It rises to 6,5 and transdermal says 7,3. Just an example but is a fair approximation of my experiences. It should be noted that the equipment I use is ancient though, newer stuff is likely more accurate but either way I'd argue that it's really only the change from baseline that is important.
When it comes to response times ours are in the region of 2-3 minutes. Modern units claim 30 seconds or less. Personally I do not think instantaneous CO2 symptoms are likely in a rebreather setting unless we're talking bad gas so even the slower response is likely ok and if the fast response is true that's definitely enough.
The idea of transdermal monitoring has been a pet for me the last few years and I've posted on rbw a few times and even emailed Dr Mitchell once (no response though). Now that I actually dive a ccr it's a higher priority, I've actually pondered doing a combined CO2-hit/monitoring project. No technical know-how and no entrepreuneurial spirit means it'd be for the sake of curiosity more than anything else.
As an aside, Giana. Did you try placing the probe on the sternum? Reasonable blood flow, thin skin and little movement.
