I don’t understand how the watch will be able to determine this data
It will use the same technology or very nearly so as the hospital pulse oximeters that are clipped to your finger - infrared light beamed into your skin. The oximeter measures how much light is absorbed by the oxygenated blood. This isn't that different from radar or sonar as far as methodology is concerned. You can by standalone oximeters that are very accurate, fairly inexpensively.
It could be just a matter of a software update, or Apple may be using different/improved sensors for this data. Either way, I'd be surprised and impressed if Apple made this available on earlier Watches.
Turning off Bluetooth and wifi makes a huge difference. I typically use Airplane Mode as a wireless toggle switch.
Turning on Airplane Mode on the Watch also turns it on in the phone, which I don't care for. I'd like a software option that lets me toggle turn BT and WiFi off, or Airplane Mode on, then automatically turns them on and off respectively, at 0500, as happens on the phone. Otherwise I'm partway through the day before getting that "D'ho!" moment.
I think there's something with the Sleep Tracking coming in watchOS 7 that turns off the display during sleep (very sketchy on the details). That would save some battery life as well. I tried toggling Theater Mode at night. Did I mention "D'oh!" moments? Noticeable improvement in battery life though.
You know that's a great idea to turn everything off except sleep tracking while the user is asleep. Everything can "wake up" if the use does also.
As a walker and 'fixie' cyclist in my mid-70s (ditched my car a decade ago) I welcome any additional instant and to hand information on my personal health.
That being said, I hope Apple have fixed the "glass face detaching" issue I have experienced twice in the past 9 months on a 42mm Watch 3 due probably to sweating, but not any knocks, rain or water ingress. The first time Apple replaced the Watch, but this time I may try an Apple store in Paris for a quick fix since I'm unsure whether there will be a further 1-year guarantee on their replacement product.
I had that happen once with my Series 0 and got a replacement from Apple out of warranty, but haven't had it happen on my S1 or current S3, and I shower every day with my Watch — so I don't think it's necessarily a moisture thing, maybe just bad luck in your case.
While I don't have much need to upgrade my Watch (or iPhone) this year as I don't use as much as in the before times, this feature would likely get me to upgrade this year.
Comments
You know that's a great idea to turn everything off except sleep tracking while the user is asleep. Everything can "wake up" if the use does also.