Mechanical thermostats are an inexpensive tool to set your temperature of your heater using a capillary and bulb that is inserted in a thermowell within your heater. The limitations of this device are the amperage load it can handle (often around 15 Amps for a 3 phase, and 25 amps for single phase). Applications that require more precise measurements use digital controllers that have relays, fuses, contactors and are available in standard NEMA 1 terminal housing, NEMA 4 moisture-proof and NEMA 7 explosion proof housing for more volatile environments.