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The Model 302 operates on the principles of a rate compensation detector. The detector consists of a high expansion aluminum tube which encases two insulation struts with opposing open contact points

The high expansion sensing shell and the expansion struts have a different coefficient of expansion. A slow rate of temperature rise allows the heat to penetrate the inner expansion struts. Therefore, the tubular shell and the struts expand slowly until the total device has been heated to its rated temperature level of 135°F./194°F. At this point, the silver contact points close which initiates the alarm. When subjected to a rapid rate temperature rise there is not as much time for heat to penetrate the inner strut. The rapid lengthening of the shell allows the struts to come together at a lower level which compensates for thermal lag inherent in conventional fixed temperature detectors. When the surrounding air temperature goes below the rated temperature level, the shell contracts which forces contacts to open, thus automatically resetting the detector.