These heat detectors are widely used because they have a unique advantage over detectors of the same class, which is the compensation of the temperature gradient. The fixed heat detectors have to have the whole unit heated to the set temperature to sound the alarm and are therefore slow.
The sharp rise detectors are based on the principle that an increase in the ambient temperature per unit of time above a set value triggers the active element of the detector, and which can give false alarms due to sharp inoffensive increases in the surrounding temperature.
The secret of the Fenwal detector lies in the idea of an exterior alloy sheathe with a high expansion coefficient that adapts to the changes in the ambient temperature. The internal core is constituted by an alloy with a low expansion coefficient. A slow fire will expand the internal core and the external sheathe equally, which will trigger the alarm at the detector’s set-point.
A sharp temperature rise of 40ºF a minute will cause a different expansion in the outer sheathe and the internal core, which will not be sufficient to trigger the alarm.
By eliminating the temperature transitories, virtually all of the false alarms in the temperature gradient detectors are eliminated. If there is a sharp rise in the temperature, the rapid expansion of the outer sheathe will close the interior core and therefore close the alarm contact too.