I have tested two of these and both give incorrect readings for both temperature and humidity. I am using up to six weather stations to compare results. Rain, wind and pressure measurements from the Pimoroni Weather HAT seems ok.
I suspect that the BME280 on the Pimoroni Weather HAT is too close the the Raspberry Pi board or alternatively my enclosure also raises the temperature.
My solution is to forget about the BME280 readings on the Pimoroni Weather HAT and use an independent device that support better thermal isolation. I am using a M5StickC coupled to a Grove BME280 and this provides more accurate and reliable temperature and pressure readings.
I only use the Pimoroni Weather HAT for rain and wind measurements and a node-red flow on the Raspberry Pi provides an integrated report via MQTT.
I have the temperature reading calibrated with a function from: bme280-python/bme280/init.py
Also changed the bme280 to be forced mode as recommended.
My issue is that the rain gauge (reed switch) seems to be triggered randomly with absolutely zero rain. I’ve added ferrite beads (no help) and have switched back the the large USB wall wart for power which has seemed to help a little. I was getting a false reading every couple of hours now it is about one every 8 hours.
Not sure how to get the right setup to reduce the false readings. Seems like it has to do with how the Weather HAT is powered. Any help would be great.