Fixed temp heat detectors are activated by the expansion of heated materials, changes in resistance, and what else?

Fixed temperature heat detectors are designed to respond to specific temperature thresholds. When the temperature in the environment exceeds a predetermined level, these detectors activate. The melting of heated material is a critical aspect because it signifies that the temperature has risen sufficiently to potentially indicate a fire or a critical hazard.

In many fixed temperature devices, a fusible element—often made of a material with a known melting point—is used. When the surrounding temperature rises because of a fire, this fusible material melts, allowing the detector to trigger an alarm or fire suppression system. This mechanism is straightforward and reliable for early detection of fires in controlled environments.

The other choices relate to scenarios that do not directly involve the primary operational principle of fixed temperature heat detectors. For example, altering the chemical composition or detecting smoke particles addresses different detection methods or aspects not pertinent to fixed temperature activation. Additionally, a secondary alarm activation does not describe the direct mechanism by which fixed temperature detectors sense heat, but rather could be an outcome after detection takes place.