- Describe, with the aid of a sketch, the construction and operation of a thermistor type rate of rise heat detector.
(10)
Thermistor Type Rate-of-Rise Heat Detector
Construction (5 marks):
- Housing – protective outer casing with ventilation holes to admit hot air.
- Sensing element – two thermistors (temperature-sensitive resistors):
- Exposed thermistor – directly exposed to the ambient air through vents, responds quickly to temperature changes.
- Reference thermistor – enclosed within the body, insulated, responds slowly, and tracks ambient background temperature.
- Bridge/Comparator Circuit – electronic circuit that compares resistance changes between the exposed and reference thermistors.
- Output relay/contacts – when triggered, closes the circuit to raise an alarm.
Operation (4 marks):
- When the rate of rise of temperature exceeds a preset limit (e.g. 8–12 °C per minute), the exposed thermistor’s resistance falls rapidly, while the reference thermistor lags behind.
- The resulting imbalance in the bridge circuit produces a signal.
- Once the signal exceeds a set threshold, the detector activates the alarm circuit.
- If the temperature rises slowly (normal warming), both thermistors change resistance at a similar rate → no alarm, preventing false activation.