Hearing about pulse oximeters more often here, I was curious about the principle, and how complicate or expensive they are. I was quite surprised how simple the measurement is! At least electrically, it is really trivial - practically just two LED's and a fotosensitive sensor. Interface to a PC (i.e. simple RS232 port), little bit of software for evaluating the data (it needs to read also the pulse to calculate the O2 level properly), and that's about all.
If you are also interested, you can find very nice explanation at
Oximetry.org. Another one can be found in
Wikipedia (together with other related info and links). In Wikipedia, they also mention "
The latest generation pulse oximeters use digital signal processing to make accurate measurements in clinical conditions that were otherwise impossible. These include situations of patient motion, low perfusion, bright ambient light, and electrical interference." - I wonder if Eric tested some of them too, and if he got more reliable results. I guess they should be quite well suited for freediving/apnea. This system allows building reflective LED/sensor combos (unlike the classical oximeters that are translucent), so you can attach them practically anywhere - hence you could choose area not too influenced by the vasocontriction. It should be also much easier to build waterproof and pressure-resistant sensors with this method (though it is really not difficult with the classical one either).
