Did you know that most bulb type thermometers and even digital thermometer probes will measure only the ambient temperature?
The yeast activity can easily increase the temperature by a few degrees. Plus if you have immersed your fermentor in a chilled ice there is going to be significant difference between the temperature of the air and that of the vessel. LCD temperature strips although simple and crude sticks on the vessel and measures only the vessel temperature.
They are stickers that will last many years and just need a torch light/room light to operate.