| If I was you i would get myself a digital thermometer with probe. You can calibrate the thermometer by placing it in a slurry (ice and water) place the slurry in the fridge -not freezer. In a short while the slurry should be a constant 0-degrees. Place the probe of the thermometer in the container, leave for about half an hour. The temp should read 0- degrees. If the temp is up or down which should only be a fraction or so mark variation on your thermometer. of course the accuracy of your thermometer is more important when incubating, but hopefully this will give you a good indication of whether your thermometer is working. | 
