In a fixed temperature heat detector, which component is used as the sensing element?

In a fixed temperature heat detector, which component is used as the sensing element?

• A. Bimetallic strip
• B. Fusible alloy
• C. Thermistor
• D. Ionization chamber

Answer: (B)

In a fixed-temperature heat detector, the sensing element is a fusible alloy because it provides a definite, preset temperature at which the detector triggers. When the environment reaches that set temperature, the alloy melts, releasing a mechanism that activates the alarm. This gives a reliable, fixed-point response without needing complex electronics. A bimetallic strip bends with heat and is more characteristic of rate-of-rise or other temperature-sensing designs, where the response depends on how quickly the temperature increases rather than hitting a precise fixed temperature. A thermistor is an electronic temperature sensor that varies its resistance with temperature and would require additional circuitry to produce a fixed trigger, not a simple mechanical release. An ionization chamber is used in smoke detectors, not heat detectors.

In a fixed-temperature heat detector, the sensing element is a fusible alloy because it provides a definite, preset temperature at which the detector triggers. When the environment reaches that set temperature, the alloy melts, releasing a mechanism that activates the alarm. This gives a reliable, fixed-point response without needing complex electronics.

A bimetallic strip bends with heat and is more characteristic of rate-of-rise or other temperature-sensing designs, where the response depends on how quickly the temperature increases rather than hitting a precise fixed temperature. A thermistor is an electronic temperature sensor that varies its resistance with temperature and would require additional circuitry to produce a fixed trigger, not a simple mechanical release. An ionization chamber is used in smoke detectors, not heat detectors.