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@jurelik
Thank you for all the information Gordon. As of my original post I have now found that having the watch sitting at a specific spot on my wrist yields better results. This is not ideal for the end-user as they would have to take the time to find the best spot etc. instead of it working "out-of-the-box". I'm not sure how the process works with other smart watches but as long as I know that is something to keep in mind I'm happy.
While I have you here, can I just ask another possibly dumb question - is it normal for the sensor to be affected by movement? Whenever I move my arm etc. the signal jumps up & down in response. I'm just curious because this would make it hard to make precise measurements during any exercise, no? I'm aware of the huge market of wearable fitness trackers etc. so I'm sure that problem has to be solved, I'm just not sure if the heart sensors in other devices aren't affected by movement at all?
Please look at the following video
for a demonstration of the problem. I'm obviously shaking the watch to the extreme but I just wanted to showcase that the BPM measurement is affected by movement as well. I have watched some videos on how optical heart sensors work and it doesn't seem to make sense that it would be affected by movement so any information would be much appreciated.
Also thank you for the tip about masking off the two sides of the sensor - hopefully I won't be needing to do that.