Probe MODEL 302-AW 135 Heat Detector

All models operate on the principles of a rate compensation detec
tor. The detector consists of a high expansion aluminum tube which
encases two insulation struts with opposing open constant points (see
cut-away view). The high expansion sensing shell and the expan
sion 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 temperatures
goes below the rated temperature level, the shell contracts which
forces contacts to open, thus automatically resetting the detector.