How Do Heat Detectors Work?

Heat detectors have evolved considerably in the last 20 years. Research & Development and the emergence of new technologies have improved these detection systems and made them more efficient and sophisticated.

Heat Detectors

The two main ways of detecting fires due to the presence of heat are: rate-of-rise heat detectors and fixed temperature heat detectors.

When the ambient temperature increases over a period of time that is equal to or more than the rate of change of the heat detector’s pre-set level, the alarm is activated. This is the principle on which the rate-of-rise heat detectors work.

Fixed temperature detectors are activated when the ambient temperature increases significantly when compared to the pre-determined level of the heat detector.

Heat detection technologies have evolved over the years and can be categorized into 4 types:

Electro-Mechanical Heat Detectors

These heat detectors work when a mechanical movement completes an electrical circuit. There are 4 kinds of electromechanical fixed temperature heat detectors.

The first type of electromechanical heat detector has a thermostat made of a bimetallic strip. One end of the strip is fixed while the other is end is free to move as the temperature changes. The bimetallic strip also forms one part of the electric circuit. An alarm is sounded when the temperature increases and the metal strip moves and completes the electrical circuit.

The second type of fixed temperature detector is a fusible link that is made of a eutectic alloy, which is basically a mixture of two or more metals. The melting point of the alloy is lower than that of the individual metal. When the surrounding temperature increases to the temperature of the eutectic alloy, it changes to a liquid from its solid state. This causes a spring that is held under pressure to be released and complete an electric circuit, which sounds the alarm.

The third kind of heat detectors are found in very old fire protection systems. It consists of a heat sensitive wire that is stretched between two points. One end of the wire is fixed and the other is wrapped around a pulley wheel. There is a weight at the end of the wire to maintain the tension of the wire. When the ambient temperature rises, the wire expands causing the weight to drop down. The system is calibrated in such a way that at a pre-set fixed temperature, the weight falls to a point where it creates an electric circuit and activates the alarm.

The fourth kind of heat detector in this category is a distributed heat detector, also called the line-type heat detector. This contains a pair of twisted insulated electrical conductors that are enclosed in a protective covering. When the heat rises, the insulator melts to a molten state and the twisted conductors complete an electrical circuit and activate the alarm.

Opto-Mechanical Heat Detectors

These are modern versions of the electromechanical line-type detector. Opto-mechanical heat detectors have optic cables that are separated by an insulator that are sensitive to heat, which is protected by an outer covering.  A focused light beam is passed through the optic cable. When the temperature rises, the insulator changes to a molten state which causes the focused light beam to be distorted. This change in the light signal is monitored by the detector that triggers the alarm.

Electro-Pneumatic Heat Detectors

The electro-pneumatic rate-of-rise heat detector was patented in 1941. These detectors contain a vented chamber that have a diaphragm which moves due to the change in pressure according to the change in the ambient temperature. If the ambient temperature rises faster than the predetermined rate, the diaphragm moves to complete an electric circuit which sounds the alarm. The main advantage of electro-pneumatic heat detectors is that they can operate at a range of temperatures.

Electronic Heat Detectors

This device has a thermistor that acts as the element sensitive to heat. Basically, a thermistor is a resistor whose resistance changes according to the change in temperature. These heat detectors can operate both as a rate-of-rise heat detector or a fixed-temperature heat detector depending on their construction.

Heat detectors are quite reliable when it comes to detection of fires; however, they are not commonly recommended as “life safety” devices. Heat detectors are commonly used to detect fires when there is a chance of deceptive triggers from one of the by-products of combustion i.e. smoke, heat and electromagnetic radiation.

Cygnus range of heat detection devices have indeed changed the face of fire security. The top-of-the-range product line has been carefully designed to protect valuable assets such as the worksite and the people on the site from the potential hazards of fire.