Application and principle of various types of smoke sensors
Smoke alarm sensors are now installed in vehicles such as trains, airplanes, etc. During the operation of trains, if the smoke sensor detects the presence of smoke, it will immediately send an alarm signal to the cab. At the same time, the in-vehicle control system will automatically reduce the speed of the train or even stop it.
Smoke sensors are used to detect whether there is smoke and the concentration of smoke in the environment, such as detecting thick smoke during a fire. When the smoke probe encounters smoke or certain specific gases, the internal resistance of the smoke probe changes, generating an analog value, thereby controlling it. The smoke sensor uses the principle that the resistance of the smoke sensitive element changes due to the concentration of smoke (mainly combustible particles) to send an analog signal corresponding to the smoke concentration to the host.
Types of smoke sensors
Smoke sensors mainly include ion smoke sensors, photoelectric smoke sensors, and gas-sensitive smoke sensors.
Ion smoke sensor
The smoke alarm uses ion smoke sensors inside. Ion smoke sensors are advanced, stable and reliable sensors that are widely used in various fire alarm systems.
It has a radioactive source of americium 241 in the inner and outer ionization chambers. The positive and negative ions generated by ionization move to the positive and negative electrodes respectively under the action of the electric field. Under normal circumstances, the current and voltage of the inner and outer ionization chambers are stable. Once smoke escapes from the outer ionization chamber, the normal movement of charged particles is disturbed, the current and voltage will change, and the balance between the inner and outer ionization chambers will be destroyed. Then the wireless transmitter sends a wireless alarm signal to notify the remote receiving host and transmit the alarm information.
Application and principle analysis of various types of smoke sensors
This type of detector will produce charged particles (ions) formed by ionization of uncharged particles, so it is called an ion type smoke detector.
The air between the electrodes is modulated by DC voltage, and ions are formed by the radiation source, and current is generated. After the smoke particles attach to the ions, the current signal weakens, and this signal change is proportional to the amount of smoke particles. Ion detectors are suitable for the detection of open fires.
Photoelectric smoke sensor
There is an optical maze in the photoelectric smoke alarm, which is equipped with an infrared pair of tubes. When there is no smoke, the infrared receiving tube cannot receive the infrared light emitted by the infrared transmitting tube. When smoke enters the optical maze, the receiving tube receives the infrared light through refraction and reflection. The intelligent alarm circuit determines whether it exceeds the threshold value, and if it exceeds, it will issue an alarm.
Photoelectric smoke detectors can be divided into dimming type and scattered light type:
- Dimming photoelectric smoke detector
The detection room of the detector is equipped with a light-emitting device and a light-receiving device. Under normal circumstances, the light-receiving device receives a certain amount of light emitted by the light-emitting device; when there is smoke, the emitted light of the light-emitting device is blocked by the smoke, which reduces the amount of light received by the light-receiving device, reduces the photocurrent, and the detector issues an alarm signal.
- Scattering light photoelectric smoke detector
The detection room of the detector is also equipped with a light-emitting device and a light-receiving device. Under normal circumstances, the light-receiving device cannot receive the light emitted by the light-emitting device, and thus does not generate photocurrent. When a fire occurs, when smoke enters the detection chamber, the light emitted by the light-emitting device is diffused due to the action of smoke particles. This diffuse light is received by the light-receiving device, causing the impedance of the light-receiving device to change and generate photocurrent, thereby realizing the function of converting the smoke signal into an electrical signal. The detector receives the signal and then determines whether an alarm signal needs to be issued.
The internal structure (detection chamber) of this type of sensor is equipped with an optical sensor (transmitting light source and photoelectric receiver). Smoke entering the detection chamber will block the emission of light and cause scattering. The photoelectric receiver will receive the signal that changes due to the scattering of light, and then produce a change in the current signal.
Gas-sensitive smoke sensor
A gas sensor is a sensor that detects specific gases. It mainly includes semiconductor gas sensors, contact combustion gas sensors and electrochemical gas sensors, among which semiconductor gas sensors are the most commonly used. Its main applications include: detection of carbon monoxide gas, detection of gas gas, detection of coal gas, detection of Freon (R11, R12), detection of ethanol in exhaled breath, detection of human oral halitosis, etc.
It converts the type of gas and its concentration-related information into electrical signals. According to the strength of these electrical signals, information related to the presence of the gas to be tested in the environment can be obtained, so that detection, monitoring and alarm can be carried out; it can also form an automatic detection, control and alarm system with a computer through an interface circuit.
Among them, there are several types of gas sensors:
- Combustible gas gas sensor, including various alkanes and organic vapor (VOC) gases, currently widely used in range hoods, leak alarms and air purifiers;
- Carbon monoxide gas sensor, carbon monoxide gas sensor can be used for industrial production, environmental protection, automobiles, homes and other carbon monoxide leaks and incomplete combustion detection alarms;
- Oxygen sensor, oxygen sensor is widely used, and there is a great demand in environmental protection, medical, metallurgy, transportation and other fields;
- Toxic gas sensor, mainly used to detect environmental pollution gases such as smoke, exhaust gas and waste gas.
Typical models of gas-sensitive smoke sensors include MQ-2 gas sensor. This sensor is commonly used in gas leakage devices in homes and factories, and is suitable for detecting liquefied gas, butane, propane, methane, alcohol, hydrogen, smoke, etc.
Comparison between ion smoke sensors and photosensitive smoke sensors
Ion smoke alarms are more sensitive to tiny smoke particles and can respond evenly to various types of smoke; while forward photoelectric smoke alarms are more sensitive to slightly larger smoke particles and respond less to gray smoke and black smoke. When a raging fire occurs, there are more tiny smoke particles in the air, while when it is smoldering, there are more slightly larger smoke particles in the air.
If a large number of tiny smoke particles are produced after the fire occurs, the ion smoke alarm will alarm before the photoelectric smoke alarm. The time interval between these two smoke alarms is not large, but this type of fire spreads very quickly. It is recommended to install an ion smoke alarm in such places. After another type of smoldering fire occurs, a large number of slightly larger smoke particles are produced. The photoelectric smoke alarm will alarm before the ion smoke alarm. It is recommended to install a photoelectric smoke alarm in such places.
Comparison between gas-sensitive smoke sensors and ion-type smoke sensors
Fire smoke is a mixture of gas, liquid, and solid particle groups, with physical properties such as volume, mass, temperature, and charge. Ion-type smoke detectors are devices that sense the microcurrent changes of smoke particles through voltage changes caused by an ionization chamber equivalent to a smoke-sensitive resistor. When smoke particles enter the ionization chamber, the ionization state of the air in the ionization chamber is changed, which is macroscopically manifested as an increase in the equivalent resistance of the ionization chamber, causing the voltage at both ends of the ionization chamber to increase, thereby determining the smoke condition in the air.
Gas-sensitive sensors detect the components of certain combustible gases in the air, so in terms of fire detection, gas-sensitive sensors are not as good as ion-type sensors. Detect the content of combustible gases in the air. Effectively detect trace leaks of various combustible gases such as coal gas, liquefied petroleum gas, natural gas, carbon monoxide, etc. Applicable to industrial enterprises such as petroleum, chemical, coal, electricity, metallurgy, electronics, and places where combustible gases are produced and stored, such as gas plants, liquefied petroleum gas stations, and hydrogen stations.
Application of smoke sensors in fire prevention network systems
Networked smoke alarms use photoelectric or ion smoke sensors, which are stable and reliable, and have far better performance than gas-sensitive resistor fire alarms. Networked smoke alarms can be used to promptly alarm smoke emitted by various early fires. The product is compact and can organically combine wireless transmission and fire smoke sensors. Networked smoke alarms are mainly suitable for public places such as hotels, warehouses, guesthouses, restaurants, hostels, factories, oil field well teams, and mobile cabins. When the smoke in the air reaches a certain concentration, an alarm signal is immediately issued to effectively prevent fires and avoid loss of life and property.
Networked smoke alarms can be used alone. When in use, first fix its mounting base on the ceiling of the room to be monitored, connect the battery to the alarm, and then screw it into the mounting base. In the working state, once the smoke concentration and duration of the prevention space are detected to reach the alarm value, the buzzer will sound an alarm immediately. When the networked smoke alarm is matched with a wireless anti-theft alarm system, the installation and use are the same as when used alone.
When the networked smoke alarm sounds, it will also send a wireless signal to the wireless anti-theft alarm host. The alarm host will respond to the alarm regardless of whether it is in the armed or disarmed state. This way of use has a wider alarm range, and the fire alarm can be grasped in time when no one is at home, so as to respond in the first time.
Application of smoke sensors in hotel fire automatic alarm system
Hotels are places for domestic and foreign tourists to stay, eat, and hold various meetings and banquets. Modern hotels are generally multifunctional and comprehensive, integrating catering, accommodation, entertainment, and shopping, which can easily cause fires. The automatic fire alarm system plays an important role in early detection and early extinguishing of fires, and maximizing the reduction of casualties, economic losses and adverse social impacts caused by fires.
For hotel buildings, in order to accurately predict the location of the fire and minimize the false alarm rate of the detector, smoke detectors and temperature detectors are selected to form a regional fire detector network. The bedroom, study, dining room and reception room in the guest room use detectors with buzzer bases. Detectors in any area of the guest room send fire alarm signals to the control room and trigger the buzzer to sound, reminding guests who are sleeping or working to evacuate quickly.
A smoke fire detector is a fire detector that uses a small smoke sensor to respond to smoke aerosols (solid or liquid particles) generated by combustion and (or) pyrolysis suspended in the atmosphere around it. Generally, there are ion type and photoelectric smoke fire detectors.
Taking the photoelectric smoke detector as an example, the photoelectric smoke detector has a smoke detection chamber with a light source and a photosensitive element. The light of the light source generally cannot shine on the photosensitive element, but when smoke enters, the light is scattered in the smoke, so that part of the light shines on the photosensitive element. The thicker the smoke, the more light is scattered on the photosensitive element. The photosensitive element then converts the light signal into an electrical signal for output to the fire controller.
Key Applications of Car Smoke Sensors in Automobile Fire Prevention
In recent years, automobile fire accidents have occurred from time to time, causing huge losses to the lives and property of the country and the people. The lessons are profound. At present, automobile fire accidents have become the focus of media opinion and have attracted wide attention from all walks of life.
In particular, urban buses and long-distance buses use air-conditioning systems to put people in a relatively closed environment, which brings a lot of inconvenience to fire handling and personnel escape. It is particularly important to control the occurrence of fires and early warning. Therefore, to do a good job in fire prevention, it is necessary to use high-tech fire prevention products in the automotive field to detect the disaster early and control and eliminate hidden dangers in the bud.
For the monitoring of fire smoke, smoke sensors and temperature sensors are usually used. Among them, smoke sensors are mainly ion type, photoelectric type and gas sensitive type.
In recent years, with the development of gas sensing technology, the detection technology combining gas sensors and traditional fire detectors has been widely used in the field of automobile fire smoke detection.
When a fire occurs, the main products of gas combustion are CO and CO2. CO is a unique sign of an extremely early fire. In general, the content of CO in the air is extremely low. However, during a fire, almost every substance will produce incompletely burned CO, especially in the smoldering stage.
From the incubation of a fire to the intense combustion, CO undergoes a regular change process from nothing to something, from small to large, and then gradually decreases. Moreover, CO has a lower density than air and is easier to float earlier to achieve early warning. Therefore, CO is suitable for early fire detection, which is very important for capturing fire information at an earlier time.
