The "pulse ox" measurement on 255 is too low.

I have used my O2 sensor only once. My doctor made some measurements with professional equipment, and got very low readings that did not make any sense. I ran a quick check with my watch, getting a normal value (98 or 99?). That convinced the doctor to find another equipment to measure again. This time it got the same readings as my watch, so we concluded that the first equipment was broken.

Oh. Well in my experience, comparing it to a medical grade pulse oximeter, the garmin always has lower measurements, and almost never reaches 100%. So I don't really trust it.

So, probably, the trend (going up or down) is right, but the number is too low/wrong.

I agree, and that is why I usually have it disabled.

Update: I've noticed that the watch tends to give wrong results when it's placed near to the hand and tends to give better ones when it's further up the arm.

For example, while keeping the watch near the hand I got a oxygen level of 89%, while the professional oximeter gave me a 100% reading. Placing the watch farther from the hand gave me a 99% reading.

After having observed this I conclude that the "all day"/"during sleep" measurements are often useless, because you will not pay attention to the position of the watch while the measurements will be taken, so the results will be unreliable; even the trend cannot be trusted: just think that if the watch moves on the arm, your reading might drastically change.

I have an Epix Pro 2.  I’ve been having lung issues (of all the things I didn’t see coming) so I have 2 fingertip oximeters, Apple Watch Ultra and a Wellue Finger ring.  I’ve mostly been wearing the AW and it was the one I caught the problem with.  Anyways, I’ve done a lot of looking at O2 and here is how things look to me.

Wellue Ring - This is a cool little device that wakes you up if you O2 drops and it records every 4 seconds.  I’ve mostly resolved the drops so it records a pretty consistent 95/96% with the occasional mild drop.  I believe it’s fairly accurate.

Apple Watch - Not enough reads, maybe up to 10 a night, but it seems acceptably accurate, comparable to the Wellue.

Pulse Oximeters - Both are cheapies, but they run a bit lower than the AW.  I have a new one incoming that isn’t a cheapie.

Last night the Apple Watch read 94 or 95 all night.  The Wellue in the same ballpark, maybe a point higher, with one drop to 92%, for just a minute.  So basically the same, as the AW doesn’t read often enough to catch a short drop.

Now the Garmin.  It recorded long drops, was all over the place, basically, crazy stuff.  It had me in the 80’s for long periods of time.  The graph looked like a drunk trying to walk a straight line.

Now, this is a Garmin forum, so someone may conclude everything else was wrong, but the conclusion I’ve reached is that the sensor, at least when sleeping, is PURE GARBAGE.  In fact, it’s so bad it’s not even a joke.

Maybe, it’s better on manual reads?

Yeah. The problem is that the accuracy of the Garmin sensor does seem to depend heavily on where the watch is positioned on your arm.

So manual readings are not, alone, more accurate. But if you do a manual reading you have the possibility to check where the watch is and to position it better in order for it to be able to give a better and more consistent reading.