What is a Heat Detector

There are two methods for detecting fire from the presence of heat;

1. Fixed temperature heat detectors operate when the ambient temperature increases sufficiently to predetermined level where the heat detector will operate; or
2. A rate-of-rise heat detector operates when the ambient temperature increases over time equal to or greater than the rate of change the detector was manufactured to operate.

Over the years there has been continuous development in the technology to detect heat. These technologies can be broken down into four main types.

Electromechanical

Electromechanical - As the name suggests electromechanical heat detectors operate due to a combination of mechanical movement creating an electrical circuit. # There are four fundamental types of electromechanical fixed-temperature heat detector;

1. Not used in modern fire safety systems, the thermostat comprises a bi-metal strip, with one end fixed into position and the other end free to move depending to the change in its temperature. The bi-metal strip also forms one part of an electrical circuit. When the temperature increases the bi-metal strip completes an electrical circuit to actuate an alarm.
2. The second, and most common type of fixed temperature heat detector is a fusible link comprising a eutectic alloy. This type of heat detector was the most common heat detector found between the years 1970 and 2000. An eutectic alloy is mixture of two or more metals whose melting point at a lower temperature than the individual metal. When the ambient temperature increases to the eutectic temperature, the alloy changes state from a solid to a liquid, like solder. This enables a spring held under pressure to release and make an electrical circuit to actuate an alarm.
3. The third type may be found in very old fire systems and includes a length of heat sensitive wire strung between two points. This type of heat detector is a very old method of heat detection and is no longer used. The detector comprises a length of heat sensitive wire with one point fixed into position, and the second point draped over a pulley wheel. At the end of the wire is a weight to maintain the tension on the wire. As the ambient temperature increases the wire expands and the weight drops down. This system is calibrated so that at a predetermined fixed temperature the weight drops to a point where it can create an electrical circuit and actuate an alarm.
4. The fourth type of electromechanical fixed-temperature heat detector is a distributed heat detector (also known as a line-type heat detector) that comprises a twisted pair of electrical conductors separated from each other by a heat sensitive insulator, and enclosed in an protective sheath. When exposed to heat the insulator changes physical state from a hard solid to a molten state enabling the twisted conductors to create an electrical circuit to actuate an alarm.

linear heat detector

Optomechanical

Optomechanical - This type of heat detector is a modern variation of the electromechanical line-type heat detector and could be classified as an optomechanical distributed heat. Optomechanical heat detectors contain one or more fibre optic cables separated by a heat sensitive insulator and protected by an outer sheath. A focused light signal is passed through the fibre optic cable. When exposed to heat, the heat sensitive insulator changes state from a solid to a molten state which has the effect of degradation or discontinuation of the focused light signal. The signal change is monitored by a device which then would actuate an alarm. 

Electropneumatic

Electropneumatic - The principal for an electropneumatic rate-of-rise heat detector was first patented in 1941 and has been subject to ongoing product development since that time. Electropneumatic heat detectors comprise a controlled vented chamber containing a diaphragm that moves due to a pressure differential according to the rate of change of the ambient temperature. When the ambient temperature changes faster than the calibrated rate which the vent has been designed to release, the diaphragm moves sufficiently to create an electrical circuit to indicate an alarm. The main benefit of electro pneumatic heat detectors is that they operate at a range of temperatures because they respond to the rate of change in temperature, not at a fixed temperature only. 

Electronic (Thermistor)

Electronic (Thermistor) - The most common form of heat detector used throughout Australia is electronic device with a thermistor acting as the heat sensitive element. A thermistor is a type of resistor whose resistance changes significantly according to temperature. These detectors can operate as a fixed-temperature and a rate-of-rise device or both depending on their intended design. These detectors may also include features to reduce the possibility of deceptive phenomena causing false alarms.

There are five classifications of heat detector established for use, they are classified according to their fixed temperature operation or rate-of-rise (if fitted).

The aforementioned classifications have been by the ISO standard adopted and modified for Australia as (Australian Standard AS 7240.5) and covers point type heat detectors with a wider range of classes as listed below.

While heat detectors are a very reliable form of fire detection, they are not normally recommended as a life safety device. Heat detectors are often selected to detect the presence of fire where there is the potential for deceptive phenomena from one of the other by-products of combustion. The next part in our series on the Principles of Fire Safety Series will focus on Smoke, Flame and Gas Detectors.