The dual stage electronic thermostat utilizes microprocessor technology to provide a very user-friendly programmable thermostat that offers unparalleled flexibility of use. The dual stage controller will control a chiller and heater to keep your tank at a steady set-point. Designed for chillers with a probe well built into the heat exchanger. If you wish to place the probe in the sump, be sure to order a Temperature Controller Probe Well. The probe is not waterproof.