I've built a homemade incubator, I have tried two different water heater thermostats, neither was very accurate or steady.
Can anyone recommend a good thermostat?
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They aren't. You can try a wafer style thermostat. Their accuracy is also debatable, but a little better than the hot water stat. Other than that, your choices are limited without stepping up to a digital control. I think you can buy a digital control for the Little Giant as a replacement. That works with a potentiometer (variable resistance dial to control temperature), I believe it controls a 40w element in the Little Giant and similar for other bators. Those are a little more accurate yet. GQF makes one, kinda pricey around $100, which are good up to 400w.
I've got two homemade bators that use wafer thermostats. They are much better than a water heater thermostat. No, they are not digital precise but they are good. Around $20 for the wafer and switch assembly
my friend just built his with an ebay electronic digital thermostat... one from taiwan or korea... don't member... but it took forever to get here, but so far it's spot on according to a water wriggler thermostat...
let me find one, hold on...
it was like 30 bucks shipped... cheaper than my ronco that's for sure...