Devices that warn of the onset of a fire at an early stage are called fire detectors. The fire sensor (sensor) is part of the fire detector and is the element that first reacts to external influences. It is also sensitive to sudden changes in the state of the environment in which it is located.
It is the type of sensor that determines the properties and basic characteristics of a fire detector. Comprehensive combination of fire alarm detectors of different types allows you to create an effective emergency fire extinguishing system in different conditions of its use.
Varieties of devices
A fire in a room is accompanied by the appearance of smoke, an increase in temperature in the surrounding area, the appearance of an open flame, the release of carbon dioxide and carbon monoxide. The named device must respond to each of these factors.
According to the principle of determining the beginning of a fire, fire detectorsalarms are divided into several main groups:
- thermal;
- smoke definitions;
- flames;
- gas.
Distress warning can be issued in sound form (siren), light indication, electrical signal. This is provided for in fire alarm systems. The fire detector circuit of any group consists of a sensitive element (sensor), an electronic circuit that converts a physical quantity into an electrical signal, and a fire detector.
Heat sensors
There are several varieties of sensors of this type, but all of them are designed to signal the excess of the permissible temperature in the controlled area. The very first sensors used the property of fusible metals to create junction points.
The electrical circuit, which includes such a sensor, breaks under the influence of high temperature in the area of its installation. This is fixed by the control circuit and perceived as a fire. A chain of devices connected in series allows you to control a large area. Fire detectors of this type are disposable and are hardly used today.
Another technology uses the thermal dependence of the resistance of certain metals on the ambient temperature. As the temperature rises in the controlled area, the resistance may increase or decrease. The thermal fire sensor is included in one of the arms of the bridge circuit. A device that measures current is included in the diagonal of such a bridge.
Bno heating current does not flow through the device - the bridge is balanced. And in the process of heating, due to a change in the resistance of the fire thermal sensor, the balance is disturbed. Current begins to flow through the meter. Exceeding its allowable (threshold) value, the control circuit perceives it as a fire and issues a warning signal.
Another principle implemented in the fire detector circuit of this type is the occurrence of an electromotive force (EMF) in the junction of two dissimilar metals (chromel-alumel) with an increase in temperature in the sensor location area. The magnitude of the EMF depends on the magnitude of the temperature and on the rate of its increase. Combining several sensors into a group allows you to determine the beginning of an increase in the indicator with high accuracy. The response threshold can be set for any temperature value that determines the start of a fire. The use of thermal fire detectors is advisable in small enclosed spaces.
Smoke detection sensors
In domestic and administrative buildings, smoke detectors are used to determine the occurrence of a fire at its initial stage. As a sensitive element, smoke fire sensors can be used in them, the operation of which is carried out using different principles for determining the optical density of air. The most widely used devices are ionization and optical types.
The main element of the first type of sensor is an ionization chamber, in which air particles under the action of a corona discharge acquirebulk electric charge. When a constant voltage is applied to the electrodes, the movement of charged particles occurs - an electric current.
Smoky air is sucked into the chamber by means of a small-sized electric pump through a cylindrical tube. Smoke particles entering the device attach to the ions and make them neutral. The magnitude of the electric current decreases. The degree of reduction depends on the amount of smoke present in the controlled room. The threshold electronic device allows you to set the value of the current in the chamber, which will be determined by the detector as a fire.
When detecting a fire by the optoelectronic method, a smoke chamber is used, in which an LED and a photodetector of the same wavelength range are fixed opposite each other at different heights. If there is no smoke at the installation site, no current flows in the circuit. When smoke particles enter the open chamber, the LED beam is refracted. The amount of light reflected from the particles and hit the photodetector depends on the degree of smoke in the room where the smoke detector is installed. The start of the fire alarm depends on the setting of the electronic circuit.
Flame detectors
Devices of this group are used where combustion products do not emit enough smoke, in open industrial areas. The electromagnetic radiation that accompanies the combustion process depends on the temperature of the flame and its intensity. Sensitiveelement (sensor) responds to radiation intensity in one of the ranges - infrared, visible or ultraviolet.
Gas sensors
Devices of this group are recommended to be installed in apartments with stove heating (fireplaces) and gas stoves. Substances released during combustion or smoldering are subjected to electrochemical transformation in the gas analyzer and the received signal is compared with an acceptable value. When the concentration of carbon monoxide or carbon dioxide exceeds the allowable level, the "Alarm" siren sounds.
Combined sensors
The sensors of this group are multi-channel combined devices. One device is able to capture various signs of a fire. The most widely used combination of smoke and heat sensors. A fire warning signal is issued at the command of any of them.
Before operation, after installing the device, each channel is tested in turn by pressing the corresponding buttons on the product case. IP fire detectors are autonomous detectors. They require a battery with enough capacity to operate the device normally for a year.
Conclusion
After reading the material presented in the article, the reader should understand that white boxes with buttons and light bulbs mounted on ceilings in industrial premises and in crowded places are intended forautomatic fire warning. Fire alarm sensors are installed by the technical fire protection service without fail. All that is required to install the device in the owner's premises is the owner's desire.
