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A thermistor type rate-of-rise heat detector primarily comprises the following components:

1. Thermistors:
   • These are the core sensing elements, usually a pair of them.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk
   • Thermistors are semiconductor devices whose resistance changes significantly with temperature.   1. What Is A Thermistor And How Does It Work? – Omega Engineering www.omega.com
   • One thermistor is exposed to the ambient air, while the other is sealed within the detector housing, acting as a reference.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk
2. Electronic Circuitry:
   • This circuitry processes the signals from the thermistors, comparing their resistance values to determine the rate of temperature change in the surrounding environment.
   • It also includes a fixed temperature threshold setting to trigger an alarm if a certain temperature is reached, even if the rate of rise is slow.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk
3. Housing:
   • The detector components are enclosed in a protective housing, typically made of plastic or metal.
   • The housing has vents or openings to allow ambient air to reach the exposed thermistor.
4. Base (Optional):
   • Some detectors come with a separate base that provides electrical connections and mounting points.   1. Rate of rise Thermal Detector – Honeywell Building Automation buildings.honeywell.com
   • The detector head then plugs into the base.   1. System Sensor 5451 Rate-Of-Rise Heat Detector – Fire Alarm Max firealarmmax.com

Operation of a Thermistor Type Rate-of-Rise Heat Detector:

1. Normal Conditions:
   • In the absence of a fire, both the exposed and sealed thermistors experience similar temperatures, and their resistance values remain relatively balanced.   1. Thermal Detector – Demco www.demco.com.my
   • The electronic circuitry monitors this balance and keeps the detector in a standby state.
2. Fire Condition – Rapid Temperature Rise:
   • When a fire occurs, the temperature in the surrounding area rises rapidly.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk
   • The exposed thermistor experiences this increase in temperature faster than the sealed thermistor.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk
   • The resistance of the exposed thermistor decreases significantly compared to the sealed one, creating an imbalance.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk
   • The electronic circuitry detects this rapid change in resistance and triggers an alarm, signaling the presence of a fire.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk
3. Fire Condition – Slow Temperature Rise:
   • If the temperature rise is gradual, both thermistors may eventually reach a high temperature.
   • Even if the rate of rise is not fast enough to trigger the alarm initially, the detector will still activate once the exposed thermistor reaches its preset fixed temperature threshold.   1. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited www.citysecuritysystems.co.uk

Advantages:

• Early Detection: Responds quickly to rapidly rising temperatures, providing early warning of a fire.   1. Rate-of-rise heat detector IQ8Quad – Honeywell Building Automation buildings.honeywell.com
• Adaptable: Can also detect slower temperature increases, ensuring detection even if the initial rate of rise is not significant.   1. Difference Between Rate of Rise and Fixed Temp of Heat – Ravel Fire ravelfire.com
• Reliable: Thermistors are generally reliable and have a long lifespan.

Disadvantages:

• Sensitivity: Can be sensitive to environmental factors like drafts or sudden temperature changes, potentially leading to false alarms.
• Limited Area Coverage: Typically monitors a relatively small area around the detector.

Applications:

• Commonly used in areas where rapid temperature increases are likely, such as engine rooms, boiler rooms, and machinery spaces.   1. Exploring the Varied Landscape of Heat Detectors in Building Fire Alarm Systems firesystems.net
• Also suitable for areas where a combination of rate-of-rise and fixed temperature detection is desired.

In conclusion, thermistor type rate-of-rise heat detectors are valuable fire safety devices that provide early warning of fires by monitoring the rate of temperature change in their surroundings. Their ability to detect both rapid and slow temperature increases makes them suitable for a variety of applications where fire detection is critical.  

1. Exploring the Varied Landscape of Heat Detectors in Building Fire Alarm Systems

2. Apollo CR (90oC) Rate of Rise Heat Detector – City Security Systems Limited