Thermostat Switches Rise/Fall & Hysteresis
Whether using temperature, pressure,or current, the designation of ON RISE or ON FALL is very important. This is due to hysteresis.
HYSTERESIS THE DIFFERENCE BETWEEN TWO POINTS
When a thermostat switch changes states, from Open to Closed for example, the point at which it resets to Open, or the original state, will not be exactly at the original point. The difference between those two points is the hysteresis.
Example: thermostat switch closes the contact at a set point of 50°F on temperature rise. The switch does not return to open the contacts until 46°F when the temperature falls, the hysteresis is 4°F. If you want that switch to operate at a temperature of 50°F on temperature Rise it would be set at a different point than if you wanted it to operate at 46°F temperature Falling. This is why it is important to understand ON RISE and ON FALL and Hysteresis.
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