Stick-on Thermometer strip graded from 10 degrees C to 40 degrees C. Can be stuck to the outside of any fermenter to give an indication of the temperature of the liquid inside.
We find these are quite reliable but it is always a good idea to test the calibration of any thermometer from time to time. For these stick-on versions, we recommend doing this especially as it gets older or if it is exposed to excessive heat such as hot cleaning water.
