The thermostat probe needs to be directly against the UTH, OUTSIDE the tank (preferably wedged between the UTH and the glass). Otherwise, your probe is measuring ambient temps, not the temp of the actual hot spot. You need a thermometer with a probe, also, to attach to the glass inside the tank over the UTH (this gives you your true surface temp). Walmart carries an Acurite indoor/outdoor digital thermometer that works well for this.