Not sure i get your question... just keep the unit on the cool side... and run the probe to the heat pad... u may have to move the probe around and find out where your getting the hottest reading... Usually if you have the probe on the ouside of the heat pad youll get a couple degree difference compared to the middle of the pad... just play with it and keep moving it around... I use duct tape to hold the probe in place against the bottom of the tank... Just make sure you only use a very small piece and you get all the sides down as good as possible... Ive evn had to change out some pieces of duct tape bc the piss on the paper and it gets on the tape.