oxygen sensor

That's a good question, I don't know how they are calibrated. You could contact a company like this:

https://www.pulmolab.com/respiratory/oximeters/SPO5500.html

I would suspect they are not user calibrated.

Alledgedly, they are colour-sensitive. The colour of blood changes with oxygen and carbon dioxide levels, and it is the colour that is used to trigger an alarm.

Is there a biotech/biomed reader that wishes to chime-in, please?

instrumentman36:

Oxygenated blood absorbs more infrared light and allows more red light to pass through. Deoxygenated blood absorbs more red light and allows more infrared light to pass through.

Pulse oximeters uses a light emitter with red and infrared LEDs that shines through a reasonably translucent site with good blood flow. Opposite the emitter is a photodetector that receives the light that passes through the measuring site.

These units are not user calibrated. Rather they come with calibration curves stored in chip memory.

Here is a site with good background info:

http://www.oximetry.org/pulseox/principles.htm

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Just my $0.02...