its measuring the UTH.... the bulb is for me to view, and also because sometimes the cool side temps get too cool so i turn it on for a little to regulate. the probe for the thermostat is under the tank, on the glass where the UTH is, as is the probe for the thermometer.
i feel like something has to be wrong though when it won't turn on my lamp. makes me wonder if its not regulating the UTH, which is also plugged into it.