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Will the Series 4, or any Series for that matter, measure the blood oxygen levels? I had read that the circuitry is there, but can't find any info on it's current availability. Any ideas about this?
 

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Apple watch Pulse Oximeter function?

Will the Series 4, or any Series for that matter, measure the blood oxygen levels? I had read that the circuitry is there, but can't find any info on it's current availability. Any ideas about this?
I have no inside Apple information, but I do have some thoughts
on the topic. First, for those not familiar let's have a little
explanation.

How could this work? If you look at fresh oxygenated blood
you see a bright red color. The red color is strongly reflected.
Blood without so much oxygen does not appear as bright, though
still red enough to get your attention. In the infrared the blood
carrying oxygen absorbs more light than the non. The different
absorbing rates at the different colors can be used to calculate
and look up the fraction of blood carrying oxygen. To put a
number on it, for red light the non-oxygen carrying blood absorbs
about 10 times more light than the oxygenated blood. I think
of it like this. When blood comes out of the lungs it is bright.

For about $20 you can buy a small device, an oximeter, that
clips on the end of a finger and makes this measurement.
The blood pressure in a finger tip gets a little boost every time the
heart beats and for a moment gets a little peak in the amount of
blood there. That absorbs a little more light, and the sensor can
see the difference. The typical oximeter also reports the pulse rate.
Hence these are called "pulse oximeters".

As should be obvious from looking, both oxygen conditions of blood
absorb green light pretty strongly. For green light they absorb
almost the same. So green light can't see a difference.

It appears Apple put both green and infrared LEDs in the Apple watch.
I guess green light was chosen to get a stronger signal on the light
reflected for finding the pulse rate. That leaves the infrared light
alone to detect the difference oxygenated blood and non. However,
for near infrared light where oxygenated blood absorbs more, the
ratio is about two to one. That provides not a lot of signal to work
with. Further into the infrared there is likely more signal, but then
silicon photo detectors stop working. Maybe there is an infrared
color where everything works without driving up the cost. I don't
know, but they have not done it yet.

What we do know is that almost everyone else chose red light and
infrared light for their pulse oximeters. So, living dangerously, I am
going to predict that unless Apple changes the colors of their optical
system they will not turn on an oximeter function on the Apple watch.

There may be some Apple watch "apps" from developers that claim
to provide an oximeter. Now that you have read the memo, you
might want to see evidence to support that.


Thanks,
rationaltime
 
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