BPM very different on 2 fit bit devices Sense 2 & Charge 4

[webbo1066]

Good afternoon

Just a little confused as to why 2 fitbit devices can give very different BPM readings ?

I have been using the Charge 4 for couple years now and recently bought the Sense 2.

Just as a comparison I have for last couple weeks ware both at same times, but on different wrists.

Sense 2 on left wrist with non-dominant setting & charge 4 on right dominant wrist.

This morning for approx 2.5 hours out for 6mile walk. Walk contained following, starting with Large downhill, then mild up/down, large uphill, small uphill. 3 short breaks in-between 4 hill sessions.  

Why do I get very different BPM readings? Now not sure what I can trust ?

Charge 4 BPM readingSense 2 BPM reading

[LZeeW]

@webbo1066   The Charge 4 and Sense 2 use different optical heart rate monitors (OHRM).  It's possible that the two OHRMs are made by two different companies.  Even if they are made by the same company, each emitter/sensor model has its own performance accuracy.  If you want to know which is more accurate, then you should compare the heart rate trace with a device that measures heart rate using ECG.