Heart Rate accuracy

Yes! I get this too. I've had my V4 since July, and it's generally been fine (apart from recording high stress and rapid breathing during yoga which is clearly wrong). But I tried doing runs with just the wrist optical monitor and my HR seemed too high a couple of times, so I went back to using the chest strap I used with my old forerunner.

Now a couple of weeks ago, I went for an easy run and my HR was low during the first km and I looked at it during the 2nd k and it was quite high (my max is probably in the 160s and I'm aiming to do easy runs averaging 120, which usually end up as 123 or 124) about 140, and it just kept going up to 180 which is higher than I've ever seen recorded. Then, at the 2km mark (I checked on the graph on Garmin Connect), it dropped back to 120. I know this wasn't me, because a high HR like that *hurts* and I felt fine. 2 more runs, all normal. Next run: low HR, sudden surge, drop back down at very close to 2k (bit annoying for the average HR, but no big deal over 10 miles). Another run: fine. This morning, gentle start, a little peak around 1 km at 140 and the sudden drop down.

What's going on? It's connecting to the strap and today I ran with the old Forerunner, also connected to the strap, although I couldn't read that at all without my glasses. I'm convinced it's the watch and not the strap. I walk around quite a lot during the day, but my HR rarely reaches 100 even so, and the V4 duly records this -- most of the time. Every day lately, I've had peaks of 120 or 130, running pace, when I can't recall having high stress, certainly nothing that's lasted for five or ten minutes as the watch seems to record. This seems to be the same as your problem. It may be a predictive software thing (HR has increased by 10% over last minute, and watch decides to keep doing that for a few minutes).

It's becoming a huge annoyance, because I like having all the data the V4 records, and I'm think of sending it back as faulty in that I don't trust the HR measuring and it's not fit for purpose. I don't want to send it back, but I may have to.

Yeah quite frustrating, I have raised a queries via support and I was told to get it to the service center to have them look at it. I have yet to done so. 

I started using the 1st version of vivoactive HR and I have never ever have any issue with the heart rate.

I have even once seen my vivoactive 4  move fast reaching abive 200 while sitting. I quickly restart it and it stabilise at 70+. However the value of > 200 was not recorded at all at connect. 

Now I just restart it whenever I saw such weird reading..

Thanks. I've tried restarting it. Let's see what it does from now on. The manual says that restart may erase data or settings, so I wasn't keen to do so, but it seems fine.

There is someting fundamentally flawed with the HR measurements... While in a activity mode, like "walking" it will correctly record your heartrate, but if you take a walk without any activity mode activated, the HR measurements will be atleast 15-25 bpm lower overall, on the same route!!

This device feels wonky...

There is also another way to try but it does not helps after a while same thing happens again.  Do refer to attached screen capture on the solution given by support 

The watch is using different settings for the heart rate measurement algorithms depending on what it thinks you are doing. If you indicate that you are bicycling or walking or running or something else, it optimizes the settings to match that activity. If I select "cardio" as activity, and do weigh lifting instead, my heart rate readings will be too high. I think this makes sense. It is up to us users to help the watch by telling it what we are doing. 

But when the watch is trying to guess/measure your heartrate, it is just receiving some light sensor input. It needs an algorithm to process that into a heart rate. Measuring on the wrist is not a very accurate science. When you move your hands, there will be light leakage. The algorithm will probably have several possible answers, and need to rely on heavy filtering to report a correct value. If you heart rate has been steadily 120 for 5 minutes, and the most recent measurment says 175, it is most likely wrong. The watch can choose to discard that reading and keep reporting 120, which is more likely still correct. And if the sensor is reporting that the HR is now probably either 80 or 160, and the watch knows you are doing cardio, it will go with 160. 

The algorithm that gives you raw HR data, works better if the settings are optimized for the activity type you are doing. If you are doing a steady state run, heavy filtering can be used to get a stable and correct heart rate. If you are running intervals (or lifting heavy weights), your heart rate will change very quickly. That means less filtering, less stable and accurate value, but higher sensitivity. This kind of software processing is what has made wrist heart rate measurement possible.

If you just got the raw data from the light sensors, it would probably not make much sense to you.

OK thanks. That makes sense, as it seemed to be making calculations based on the rise during the first k (which is about 50 bpm, from 70ish to 120ish) and assuming the same rise will happen in the 2nd k (which it doesn't; I slow jog to start, but after about 800m, I'm usually at the pace I want to continue at, and my HR should only rise when I fatigue). Also, I use a chest strap because I know that wrist based monitoring is as you say, and this happens after it's told me that the strap has been detected (which I assume means it's using strap data).

Right, I think I understand now.

The chest straps are significantly more accurate. Mine sometimes picks up my running cadence instead of my heart rate, typically the first few minutes of my runs. After that it is spot on. External straps are always used instead of wrist heart rate in activities if there is one connected.