I have been having a lot of success with my new Garmin chest strap type HRM until today. I have mostly been using it to monitor my HR during long slow runs - and force myself to slow down during these. But I have also used it during a hard 10K "race" very recently and a couple of hard 5K parkruns.
But today - although Autumn has supposedly arrived here and it has cooled down to the low 20's C around 7AM, after around 20 minutes of hardish running at parkrun ( I went out TOO fast!!), my running singlet was absolutely sodden with sweat - and the HRM started to give out ridiculous numbers -- and continued that way for some time. Put me right off my run really - I did feel like today wasn't my day anyway , but a HRM constantly telling me that I was about to die didn't help either.
I think I will have to put this run down today as a 5K Tempo run with 1K tempo intervals. It wasn't meant to be one of those - but that is what it turned out to be.