Self-replying to note with the T-Rex 3 at least, the HR sensor is extremely sensitive to pressure, in that it easily reads 10bpm high if the watch is tight enough not to slip easily on the wrist. I like a watch this size and weight a little loose so that's OK, but the Band 7 seems to read most accurately when it's tighter. Either way, I doubt I'd rely on these for more than general trend and variation, a finger clip pulse ox is probably a lot more reliable.