[bennelson]

The temperature controller is ON/OFF only.

You can program it for either a heating or a cooling mode. That is, it completes a circuit anytime the temperature is above or below your set-point, depending on how you program it. It's very simple, and it accurate to 1 degree (f).

I wasn't using any kind of water circulator, and it all worked fine. If I did want to use a water circulator, I wonder how a "power-head" from a tropical fish aquarium would work?

I made a batch of yogurt using this setup, and it turned out great. I put the milk and yogurt starter in mason jars and put those in the water. it turned out much better than the same yogurt made in the oven with the "hot-box" technique.