The temp sensor was mounted next to the ESP8266, which is fine until you turn everything on, and realise that the ESP gets warm and completely compromises the readings on the temp sensor.
I designed a second version of the board, with some physical separation between the ESP and the temp sensor, making cuts into the board to stop the heat traveling through to the sensor area.
Even with this change we still get heat transfer between the ESP and the temp sensor. Within 10 minutes of power up we have a 4°C rise in temperature readings.
At this point I have a few options
1. Re-design the board, giving even more physical separation between the ESP and the sensors
2. Add a heatsink to the ESP to try and dissipate the heat
3. Modify the software to put the ESP into sleep mode between sensor readings, to reduce heat output
I’ll try a combination of the second two, although ultimately a new board might be the better solution.