Wrist heart rate monitor WHR inaccurate

That's an interesting observation as I have also done all the OHR activities on a copy of the cardio app. For everything else I've used a chest strap. Perhaps you're on to something... definitely worth to test it out. I've done 7 activities with the OHR and one of them looks like it could be somewhat plausible(still a bit high hr), the rest all display the same behavior that hr jumps at the start. Here's an example where the hr graph could be quite accurate if not for the initial 40 bpm jump. It's 40-50 bpm too high for the rest of the activity. 

Deleting and reinstalling the Cardio app didn’t fix the issue. So I created a new app similar to the Cardio. The app did not exhibit any abnormal high HR but the HR response was very sluggish. I redid the new app by copying and modifying the run app (I was hoping for a better HR response time). It seems to work reasonably well. The very high HR is gone and I think I can use this app for the lower intensity indoor training without the need for a strap.

it seems like the built in apps or activity types have their own OHR filter and it may not be working properly for the Cardio, Yoga and possibly other apps.

I'm unable to duplicate what you and Ommike are experiencing.  I was casually walking around the house maintaining a HR around 68 bpm.  I started both the "cardio" and "yoga" apps twice, continued walking for a few minutes and my HR stayed stable.  That being said, when the "cardio" app was mentioned, I remembered reading a thread about this same issue about different HR readings with different apps.  I believe it was in the VA3 forum, but after doing a search on the subject I can't find the thread and I don't recall the outcome of the issue.

Thanks for the effort. I am set for the time being and will wait for the next one or two firmware updates to try the Cardio app again. If it is bad, I may consider replacing the watch. BTW, I called support yesterday and was offered to have the watch replaced, which I was reluctant to do at this stage (support suggested a replacement just because they had no other option at this stage).

I have noticed the exact same thing. My HR when sitting down is around 45-55 which is correct, but as soon as I get up and take a few steps it shoots to about 75-90, sometimes as high as 100. I know this is very far from reality both by knowing my HR and by switching back and forth to Strap HR (if I'm after an activity and before I've showered) and seeing my Strap HR at 61 (while walking around) and the 945's at 85.

My 645M was much more accurate in this regard and never gave me reason to doubt and check it. With the 945 it's so far from plausible that it leaves no room for doubt.

I wore both (albeit on different wrists) for 4 hours. The 645M's graph (left) is rather flat, and the 945 has loads of peaks at 80-90.

I'm hoping this is rather software than an inferior sensor?

Mine just showed a 140bpm HR whilst sitting still, took a second reading with a Kardia which showed 65-70bpm! Power cycled the watch and it did the same for a bit before returning to normal :(

talsela said:

It doesn't seem to me like one of the hot topics that needs to be resolved in this watch, or am I wrong?

I'm seeing no difference with WHR from my 935 to the 945, but those two are nearly identical in weight and size.  Your 645 is 42.5 x 42.5 x13.5 and 42 grams. The 945 is 47 x 47 x 13.7 and 50 grams. Is it possible the difference in HR you're seeing is because of the size difference and more movement on your wrist with the 945?

The size issue crossed my mind but since I wear my watches very tight, I don't believe this is the issue.

Went downstairs today (5 stories) - HR reached 104bpm and took a long time until it settled back down. Usually 105-110 is what I reach walking upstairs with an HR strap.

Just walked upstairs and WHR raised from 65 to 72. Continued to monitor after arriving at home to check for delays, but 72 was indeed the maximum.

From where I am, WHR (non activity) is almost random...