Admittedly my HRM-Run is ancient (11 years) but it has been supremely reliable all those years. But lately I see abnormally high HR after more than 30 minutes or near the end of a run. The image shows HRV (Runalyze).
I replaced the strap with a new once twice, and the battery, without effect. What seems to precede the signal going noisy, is often a pause in the activity (arrow). The strap can't have dried out during a 1 minute pause (in warm and humid weather while sweating profusely). But it also happens while running, suddenly the signal goes bad. I ruled out the strap shifting or getting loose, and a bad battery.
Can these sensors get too sweaty (short-circuiting the terminals?) This is probably the one disadvantage of sensors that clip on a strap, not only the pads get soaked but the whole thing can get totally drenched.
Does anyone recognize this effect? Did you find what caused it?