[brok3nmachine]

The cheapest 02 sensors that I know of are around $50 each, and have a shelf life of 1-2 years. Average price for a decent sensor is closer to $90. And digital solid state O2 sensors go for $400+.

*Edit*: google dive gear express O2 sensors to see these.

I use them for technical scuba diving. In particular with a rebreather, which is capable of mixing oxygen with other gases such as nitrogen and helium. And as the gas is recirculated CO2 is scrubbed away.

CO2 sensors are also available for rebreathers but are not always reliable (same for O2 sensors, so a minimum of 3 are used). The human body is actually quite sensitive to CO2, and it determines our breathing rate. So there are ways to know if you're likely being exposed to elevated CO2 levels by being aware of your breathing rate, paranoia, etc.

[amoshebb]

Yes, these are the galvanic ones I was rounding up to $100. I was curious if some other technology or sensing technique I wasn't aware of existed that was less expensive, more reliable, or lasted longer.

[joseda-hg]

Completely off-topic, but do you happen to use text-to-speech for comments?

The usage of 02 and O2 interchangeably piqued my interest; it seems like a way for such an error to occur.