I quickly learned this product is unreliable due to lack of a temperature probe and temperature display. I found my temperatures varying by 10-15 degrees just days apart even though the dial was taped in place. You're better off getting a thermostat; I got the Jumpstart thermostat intended for sprouting plants.
This ZooMed Rheostat allows me to dial in precisely the temperature emmitted from the undertank heat strip. Too warm is bad, and summer ambient temps affect heat requirements. So nice to be able to turn elements down or up as needed for exact requirements. Adjust and monitor until you get it just right.
I use these when and if one of my main proportional thermostat fails. They are not a permanent solution to heating amd environmental control but work in a pinch when you need a inexpensive backup fix.