Each spa is equipped with a thermistor that is used as temperature sensor. The thermistor is a semiconductor that changes resistance with temperature changes (as the temperature goes up, the resistance goes down). A comparator circuit then compares the voltage to the set point voltage. If the voltage differs the heating circuit is sent a signal to energize the heater system or terminate the system if voltage is equal to the set point. The set point is the spas default temperature (102°F) unless reset at the control panel's up or down temperature set buttons.