The Polar OH1 sensor is very comforable to wear. All the same, an Australian paper said, it is so accurate that it can be used instead of an ECG device: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0217288 There is a Polar Sensor Logger app with MQTT which should allow to connect it to Home Assistant. We tried it once, but I …