No rheostats just change the amount of power to a light or in reptiles case heaters so they do not get as hot. They work great if they are in a constant temperature room. Thermostats turn on a off a heat unit based on a measured temperature. It will measure the temp where its probe is placed and either turn a heater on or off according.
Proportional thermostats do the same but rapidly very very quickly and 'flutter' the on off cycles so the heating devise is never on for long enough to go beyond the set point of the thermostats effectively they will bring a heater to a given temp and hold it exactly there. On off style will average the highs and lows to deliver the average temp that the user set on the unit.
I'd suggest that if you are in a variable room and using a light thin bottoms enclosure a proportional unit is the way to go I believe the base herpstat is the cheapest of them.
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