Here is a quick fix for issues I used to have, and have seen others reporting, with Polar H10 heart rate monitors. It relates to random drops in heart rate during an activity, as seen towards the end of this long steady run;
When the strap was new it worked perfectly for a while, but then I’d start to see these dropouts in every run after a warm up. In an effort to resolve it I had tried replacing the battery, but would end up having to replace the strap, only for the same problem to return within 3 to 6 months. Others have said that this is what they do, claiming that the strap is a consumable item that needs regularly replacing, but that seems wasteful and doesn’t sit well with me.
Fortunately there is a simple solution; O-rings. Before attaching the HR monitor to the strap, pop an o-ring on each of the connector posts – this creates a tight seal that prevents sweat from getting on to the connectors, and stops the dropouts.
It seems that the small lip at the top of those posts, that should be creating the seal, wears down over a short period of time, and although adding the o-rings is an extra step that needs to be done every time you reconnect the monitor to the strap, it’s a lot cheaper and less wasteful than replacing the strap every 6 months.
Here is an affiliate link to the O-rings I use (19mm Outside Diameter, 15mm Inner Diameter, 2mm Cross-Section Diameter).
One other tip is to make sure you’re using the correct battery – a CR2032 will fit and appear to work, but the battery enclosure will not be tightly closed due to the slightly thicker battery, and that can cause loose connections (and moisture ingress). The battery needs to be a CR2025.