Devices that allow to warn about the beginning of a fire at its early stage are called fire detectors. The fire sensor (sensor) is part of the fire detector and is the first element to respond to external influences. It is 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 the fire detector. The complex combination of different types of fire alarm detectors allows you to create an effective emergency fire extinguishing system in different conditions of its use.
Types 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, and the release of carbon dioxide and carbon monoxide. Each of these factors must respond to the named device.
According to the principle of determining the start of a fire, fire alarm sensors are divided into several main groups:
- thermal;
- smoke detection;
- flame;
- gas.
A warning about the onset of a disaster can be issued in a 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 the physical quantity into an electrical signal, and a fire detector.
Thermal 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 low-melting metals to create junction points.
The electric circuit, which includes such a sensor, is broken under the action of high temperature in the area of its installation. This is fixed by the control circuit and is 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 practically not used today.
Another technology uses the thermal dependence of the resistance of certain metals on ambient temperature. With increasing temperature in the controlled zone, the resistance may increase or decrease. A thermal fire detector is included in one of the shoulders of the bridge circuit. A device measuring current is included in the diagonal of such a bridge.
In the absence of heating, current does not flow through the device - the bridge is balanced. And in the process of heating due to changes in the resistance of the fire heat sensor, the balance is disturbed. Current flows through the meter. Exceeding its permissible (threshold) value, the control circuit perceives it as a fire and issues a warning signal.
Another principle implemented in the fire sensor circuit of this type is the occurrence of an electromotive force (EMF) in the joint of two dissimilar metals (chromel-alumel) with an increase in temperature in the sensor location zone. The magnitude of the EMF depends on the temperature and its rate of increase. Combining several sensors in 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 sensors is advisable in small enclosed spaces.
Smoke detectors
In domestic and administrative buildings, smoke detectors are used to determine the occurrence of a fire at its initial stage. As a sensitive element they can be used smoke fire sensors, the work of which is carried out using different principles for determining the optical density of air. The most widely used devices are ionization and optical type.
The main element of the sensor of the first type is an ionization chamber in which air particles acquire a volumetric electric charge under the influence of a corona discharge. When a constant voltage is applied to the electrodes, the movement of charged particles occurs - an electric current.
Smokey air is sucked into the chamber through 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 strength in the chamber, which will be determined by the detector as a fire.
When determining a fire by the optoelectronic method, a smoke chamber is used, in which an LED and a photodetector of the same wavelength range are mounted at opposite heights at different heights. In the absence of smoke at the installation site, current in the circuit does not flow. When smoke particles enter an open chamber, the LED beam is refracted. The amount of light reflected from particles and incident on the photodetector depends on the degree of smoke in the room at the installation site of the smoke detector. The start of a fire alarm depends on the electronic circuit setting.
Flame sensors
Devices of this group are used where combustion products do not emit enough smoke in open industrial areas. The electromagnetic radiation accompanying the combustion process depends on the temperature of the flame and its intensity. A sensitive element (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 using stove heating (fireplaces) and with gas stoves. Substances released during combustion or decay in the gas analyzer are subjected to electrochemical conversion and comparison of the received signal with an acceptable value. If the concentration of carbon monoxide or carbon dioxide is exceeded above the permissible level, an audible alarm sounds “Alarm”.
Combined sensors
Sensors of this group are multi-channel combined devices. One device is capable of recording various signs of fire. The most common combination of smoke and heat sensors. A fire warning signal is issued at the command of any of them.
Before starting operation after installing the device, a sequential test check of each channel is performed by pressing the corresponding buttons on the product body. IP fire detectors are self-contained detectors. For their operation, a battery is required, the capacity of which is sufficient for the normal functioning of the device during the year.
Conclusion
After reading the material presented in the article, the reader should understand that the white boxes with buttons and bulbs mounted on the ceilings in production rooms and in crowded places are designed to automatically warn of a fire. Fire alarm sensors are installed by the technical fire protection service without fail. To install the device in the premises of the homeowner requires only his desire.