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There are two basic types of thermostats, snap-action and slow make break. Snap-action opens contacts by a bi-metal disk snapping when the opening temperature is reached. The snap-action quickly opens and separates the contacts. The contacts will not reset until the bi-metal disk cools to approximately 20°F degrees below the opening temperature; this is called differential. With a snap-action thermostat the closest control you can expect is +/-10°F. The advantages are long life, ability to handle resistive, inductive and DC loads, high amperage and act as a high limit switch. The snap-action thermostats can offer manual rest features.
Slow make break (SMB) switches operate by slowly pulling the contacts apart a short distance at the opening point and then resetting when the differential is reached, usually 2 to 5 degrees F. SMB switches offer close temperature control, but because the thermostat opens and closes often it does not have the life of a snap-action thermostat. These thermostats are small and sometimes have an electrically live case and must be insulated, but can be molded into silicone or butyl heaters. They handle only AC resistive loads.
Fuses are used to shut down a heater when a critical temperature is reached so the thermal system cannot operate and possibly cause damage or personal harm. Fuses are small and can be built into heater assemblies or molded in place, if the fuse opening point is higher than the heater molding temperature. Most fuses have electrically live cases and must be insulated.
Resistance temperature detectors are used in conjunction with an RTD controller to control the temperature of a thermal system. The RTD changes resistance with the change in temperature, which is fed to a controller which adjusts power to the thermal system. Birk can build an RTD into a heater or assembly. RTD’s come in various resistances and shapes and sizes as small as .050″ diameter. The resistance used is determined by the algorithm curve required by the controlling instrument. RTD’s come in materials to meet the application temperature requirements.
Thermistors act similar to an RTD but have a greater resistance change over the same temperature range. Thermistors also come in various resistances and temperature capabilities. The change in resistance is fed back to the controller which adjusts the power to the thermal system. Thermistors come in many sizes as small as .020 inches diameter and can be built into a heater or thermal system.
Thermocouples are made of two dissimilar metals which are welded together, when the temperature changes, a mil-volt signal is generated which is fed to an instrument which amplifies the signal and uses that signal to adjust the power to the thermal system. Thermocouples are available in various wire sizes down to 30 gauge and can be built or molded into a heater.