How to measure Heart Rate Variability

Apple allows you to inspect all data using the Health app itself. In addition to the graphs it prepares, you can tap the graph and then Show All Data

As you know, there's no export function on iOS itself, but there's no reason any third party app can't request access to that data (which you also can see and approve from the Share Data tab of that app.

I would check out apps like:

• https://itunes.apple.com/us/app/heart-rate-variability-logger/id683984776?mt=8

If the watches are different, and the phones are different, then the only thing that appears to remain the same is you.

You don't mention, but have you ruled out the possibility of there being anything about you? For instance, I remember reading that tattoos could prevent the heart monitor obtaining a clear reading, and there was some talk about people with very black skin also having difficulties. Example article here http://www.bbc.co.uk/news/technology-32537030

The other possibility is whether there is something with the tightness with which you wear it?