Don't let you be fooled: smartwatches provide pulseoximeter readings, not ECG. I've developed ECG systems for HRV (e.g.: https://www.vital-monitor.com/ ) for quite some time. When doing research and comparing a ECG with the said-to-be "ECG accurate" data it turend out that virtually all systems lie in creative ways.
@ bt protocol: Garmin, Zypher, Polar are all "opened up" aka the documentation of the prorietary parts are to be found on the net - or you can buy then and sign an NDA.
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