The sensor is the most important element of a fire alarm. Appliances of this kind can be seen on the ceiling of every office and shop. They are small white boxes. It is the fire alarm sensors that react to the onset of a fire, that is, to smoke and fire. When choosing such a device, it is necessary to correctly take into account the possible nature and path of propagation of the fire.
Content
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general information
- Classification system
- Change parameter and localization
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Types by sensor type
- Heat and smoke
- Flame and gas detectors
- Combined and manual
general information
The use of the term "sensor" is not very correct. After all, this is only a part of the element called "detector" that reacts to light, smoke, heat. There are different types of products that differ in the principle of operation, price, installation features, fire detection method, and also the purpose, which is determined by the type of room.
Classification system
Since there are many detectors for detecting a fire, the system of their classification is quite wide.
The detector, in addition to the sensor, consists of many elements. The design of the device necessarily includes a system for transmitting an alarm signal. That's why by the type of signal transmission, the detectors are:
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Single-mode. - Dual-mode.
- Multi-mode.
- Analog.
Single-mode - generate a fire signal and transmit it if the impact of an external factor (for example, temperature) is sufficient. In our time, detectors of this type are not used.
Dual-mode - they can transmit two signals at once - that there is a fire and that there is no fire. Initially, it may seem that the last function is superfluous. However, the “no fire” signal indicates that the device is fully operational and well monitors the situation.
Multi-mode - have the function of a signal, which indicates that the device is partially out of order. This enables the technician to quickly troubleshoot problems. In addition, a detector of this type can generate signals indicating that the sensor is dirty and needs cleaning.
Analog - not only react to a fire, but also determine its scale.
Change parameter and localization
By the type of changes in the parameter that is monitored, the sensors are divided into maximum, differential and combined.
The maximum ones, they are also threshold ones, are triggered at the moment when the threshold value specified in the instructions for the temperature or smoke concentration is reached.
Differential differ in that they react not to the value of the parameter, but to the dynamics of the change in values.
Combined include the two previous types.
By localization, detectors are:
- Point - familiar from childhood. They have one sensor, which is assembled together with the detector. They are used in everyday life.
- Multipoint - several sensors are subject to them at once.
- Linear - are divided into singles and paired. Trace the space along the line.
Types by sensor type
It is worth dwelling in more detail on the types of detectors by the type of sensor. This type of classification is fundamental. According to him, devices are divided into heat, smoke, gas, manual, flame detectors and combined ones.
Heat and smoke
Thermal ones were invented by the very first. The essence of their work is in the use of materials with a low melting threshold, as a result of which they are destroyed at high temperatures. However, such devices are disposable.
Nowadays, there are several types of heat detectors, they are united by the fact that the sensors are triggered when the temperature rises sharply. They can be installed in almost any room. The response threshold of heat-sensitive elements is 70C.
It is advisable to install this type of fire detector only if the main symptom of a fire is a rapidly increasing temperature.
Advantages: cheap - from 50 rubles, service is not difficult.
Minuses: useless in rooms with a high ceiling; useless when a fire occurs against the background of burning substances that do not generate heat.
Smoke alarm fire detectors are in most cases the most effective. They are installed in closed rooms at a ceiling height of up to 12 meters. It is also allowed to install devices on walls, at a distance of no more than 30 centimeters from the ceiling.
The smoke sensor can be radioisotope and optoelectronic. The latter is the most common. It consists of a smoke chamber, inside it a photodetector is placed on one level, and an LED on the other. When smoke enters the chamber, the light beams are refracted and shine on the receiver, an alarm is triggered.
Pros: fire detection at the initial stage, low price - about 200 rubles.
Among the cons: a false signal is triggered due to dustiness, low reliability, does not work on black smoke from burning rubber.
Flame and gas detectors
Flame detectors react not only to open flames, but also to slowly smoldering objects. They are installed in industrial (most often oil and gas and chemical) and industrial premises, where most of the liquids do not emit smoke when ignited.
Disadvantages include high cost and possible false alarms.
Gas devices are triggered when gas (carbon dioxide and carbon monoxide) appears in the air from a flammable product or object.
The principle of operation of a gas detector is based on the transformation of an electrochemical process. Recommended for apartment installations due to the danger of carbon monoxide (colorless and odorless) for humans.
Combined and manual
Combined ones can immediately consist of a heat, smoke and flame sensor. They record many signs of a fire at once. The most common combination is a smoke and heat detector. Today there are 3-4-channel options, however, due to the high price, they are rarely used.
The manual type of sensor starts signaling an alarm when it is started by a person, since the device itself does not react to signs of fire.
To turn on the alarm, you need to press the lever, glass or other device. To return the device to a standby state, you need a key, the loss of which can cause some difficulties.
The installation site of this type of alarm is the walls at the points of entry and exit from a building or room. The distance between them should not be more than 50 meters. The devices are used in shopping centers, hospitals, institutes and other places of mass stay of people.
How to choose the right device for a fire alarm depends not only on personal preference, but also on the type of room. And also it is worth taking into account the existing requirements for the device and carrying out installation work only with the help of devices that are mandatory in the fire alarm system.
