So I just bought an H10 a few days ago. I had the same questions and couldn't find good answers. Here is my experience...
Hardware:
- iPhone X
- Series 3 Nike Apple Watch
- Polar H10
- Precore Treadmill (will be relevant)
When I got the H10 I downloaded the Polar Beat app to pair the sensor with the phone. I then opened bluetooth on the watch and could see the H10 under "health devices" so I paired there as well. I also installed the Polar Beat Apple Watch app because I wanted to check that out. My first exercise with the H10 I did through the Polar Beat app. Heart rate readings were consistent and accurate (more so than the Apple Watch during hard running...not a hardware issue, this is just anatomy/physiology in action). The nice thing about the Polar sensor is that my heart rate also appeared on the treadmill (I think this is the ANT+ capability...again, relevant in a minute). What was frustrating is the Polar Beat app doesn't, apparently, use the Apple workout API; they try to go their own and it was buggy in documenting my run. It failed to capture distance, steps, etc as part of the workout (though the watch captured them in general).
My next workout I used the Apple workout app. I wanted to see what happened...I was not expecting the H10 to work. But alas, I was surprised. Where I would normally get erratic heart rates and/or greyed out numbers on the watch, I got consistent and seemingly accurate readings. This, coupled with the fact that the exact same readings were appearing on the treadmill, lead me to believe that when there is an external heart rate monitor paired with the watch and active, that the Apple Watch defers to that for readings (which is awesome).
My next test will be to turn off my phone and see if the watch captures the workout and uses the H10 with no phone present; my confidence level is high because of the former results and that I can see the H10 paired directly with the watch.
