Often the perceived operating temperature of a thermostat in the actual application will differ from the published temperature range because of thermal lag. This is the result of the thermal mass of the device and the thermal resistance of the insulation which slows down the reaction of the bimetal disc. The bimetal disc always operates at roughly the same temperature, however the temperature inside the thermostat and outside may be significantly different and time must be allowed for the heat to penetrate and leave the housing. The faster the temperature rises and the larger the total temperature difference from the beginning of the heating cycle to the specified operating temperature the more lag will be experienced.
