I also have an full sized upright freezer and I used a Zoomed Hygrotherm and it worked great. It never fluctuated more than 1 degree +/- 89. I will probably switch to a Helix or Herpstat in the future but I just happened to have this Zoomed lying around. I did however have 2 digital thermometers (1 on top shelf, 1 on bottom shelf) to regulate the entire thing. I have my sensor dead center of the incubator. I filled the dead air space with a bunch of gatorade bottles.