Mirror: G K Bhat
We discussed electromechanical and opto-mechanical system in our previous topic. However these systems are discussed in view of academic interest. The system nowadays generally used is electronic (Thermistor) system. Some places Electro-pneumatic system is also used. Therefore in this topic we will discuss both these systems.
Electro- pneumatic – The principal for an electro- pneumatic rate-of-rise heat detector was first patented in 1941 and has been subject to ongoing product development since that time. Electro- pneumatic 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): is a system which are commonly used system nowadays
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).
Type A (white dot): Normal temperature duty, incorporating both fixed-temperature (58ºC – 88ºC) and a rate-of-rise actuation
Type B (blue dot): Normal temperature duty, incorporating fixed temperature (58ºC – 88ºC) actuation only.
Type C (green dot): High temperature duty, incorporating both fixed-temperature (88ºC – 132ºC) and a rate-of-rise actuation.
Type D (red dot): High temperature duty, incorporating fixed temperature (88ºC – 132ºC) actuation only.
Type E (yellow dot): Special purpose fixed temperature. This type of heat detector is selected when the purpose of protection cannot be satisfied by types A to D
In the near future, these classification Types will be replaced by the ISO standard (AS 7240.5) heat detectors with a wider range of classes.
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.
How Heat Alarms Works?
A heat alarm is designed to detect heat instead of smoke, the alarm contains a thermistor which is set to respond to temperatures above 58°C.
Step 1 – Heat Enters the Sensor Chamber
When a fire breaks out hot air from the fire will rise and enter the sensor chamber.
Step 2 – The Alarm Sounds
When temperature inside the chamber reaches 58°C a signal is sent to the integrated circuit which causes the alarm to sound alerting the occupants to the fire.
Will be continued.
G K Bhat
Mirror: G K Bhat