I put the unit on the cool side(indoor temp reading) and the probe on top of the uth and under the substrate to measure the warm side(outdoor temp reading). I also suggest switching the reostat for a thermostat so it can keep the temps steady. With the rheostat, the top goes up and down all the time! You can get the repitemp 500R for $25! Some on here don't like it because it's cheap but I never had a problem with them!