Thermometer reading hot even though I have it dialled to low?

[Slither Seeker]

a rheostat is quite different from a T-stat. with a rheostat the energy flow is constant despite fluctuations in relative humidity and thus temperature. the dryer the substrate gets the hotter it will be on a rheostat. a t-stat is better at minimizing these swings, but in incubators I've still seen humidity swings mess with even t-stats. it's also possible that the rheostat failed... anything can fail, but I'd check the humidity right over hot spot.

I've taken to sealing up my enclosures and heavily insulating them, and eliminating heat lamps and by doing so keeping the humidity more constant to reduce big swings in temp and humidity. if you want to get really into it, when water changes states from a liquid to a gas, it literally absorbs heat/energy, so a moist hot spot will literally remain cooler with a constant voltage than a dry hot spot. screen tops make for tedium.