I bought one online from a not particularly trustworthy shop. I have never seen it show any value other than 99%. How can I figure out whether it actually does anything and doesn't just show 99% no matter what?
I tried some simple stuff when it arrived. I believe I was able to get it to swing within the 94-99% range by holding my breath and by deliberate, deep breathing, over the span of just a minute or two. It definitely was not pegged at 99%. The changes were surprisingly fast. My wife was able to get the same response.
If it matters, I'm at about 500 feet above sea level (not high-altitude at all).
That's obviously not a confirmation that it's calibrated or trustworthy, but at least it shows a relative measure that seems real in both directions of oxygen saturdation. Surely if I have a drop due to illness I should notice, even if the number isn't exact.
2) while being an intern and sitting alone in my cube, I had dozen of test meters just sitting there and when I got bored I'd do the same thing and try to get it as low as I could by holding my breath and then snap back up. Our office is at 5000 feet so 95% is normal for healthy folks, I could get down to 85 or so and then hyperventilate back to 97% or so pretty quickly.
There is a delay of 20-30 seconds between changing my breathing to the oximeter changing its value. I suppose this is how long it takes for the blood to get around in the body.
It is hard to do so normally so I forced the air out and then pinched my nose shut with my fingers to prevent accidentally breathing in.
Actually I just tried it and yeah for me at least the full-lungs version is obviously longer but takes less discomfort for my SpO2 to start to drop (like well before contractions start), whereas on exhale I get more discomfort (around 15s before contractions) before a similar drop.