Choosing a smoke detector.
Choosing a smoke detector
Smoke detector
It is no secret that detecting a fire at its earliest stages helps to eliminate it as quickly as possible, minimize the risk of fire spreading, and make it possible to achieve a quicker end to the fire. The best and so far the only assistant in this matter is a smoke detector. These devices detect a signal at the earliest stage of ignition, since smoke is the only indicator of a future fire at the stage before the appearance of fire. When choosing one or another device, it is necessary to analyze the information on the classification and purpose, allocated, in particular, for smoke detectors. So, these devices are divided into ionization and photoelectric. The first devices of this kind functioned with the help of ionization chambers, which could detect small particles of combustion products. Ionization detectors remain the most popular among customers of security equipment to this day. With the development of production technology in this area, a photoelectric smoke detector appeared.. This type has a number of advantages over its older brother, since it is virtually immune to changes in the external environment, which reduces the likelihood of false alarms, in addition, it responds more effectively to large smoke. If you are the owner of such a device to ensure fire safety, of course, you should take care of the cleaning process of the device. Cleaning is necessary to maintain sensitivity and prevent false alarms of both types of detectors, in fact, the fire safety of the facility depends on it. To clean the photoelectric detector, it is enough to disassemble the device, rinse, wipe and blow with a stream of air. The ionization detector cannot be washed in this way. Any ionization sources are most effectively cleaned by immersing them in a cleaning medium. However, as a result, the cleaning agent itself may be radioactively contaminated and special conditions will be required for its disposal.
Fire smoke detector
We are all more accustomed to such a name as a fire alarm, however, this device is based on a fire smoke detector. The operating principle of such a device is to detect fires accompanied by the appearance of smoke by registering optical radiation scattered by smoke particles. As radiation sources in fire smoke detectors, as a rule, LEDs are used, which have a radiation divergence from 20 to 90o. Inside the device body there is an emitter with the first focusing lens installed in front of it. The optical axes are combined, there is a photodetector, a reflector made in the form of a hollow truncated cone, the inner surface of which is made mirror, and the base plane of the conical reflector is an elliptical surface, its geometric axis is combined with the optical axis of the photodetector and is located at an angle j to the optical axis of the emitter. The operation of the fire smoke sensor is carried out according to the following mechanism: in the absence of smoke, the light flux of the emitter is focused by the lens into the area in front of the conical reflector, the plane of the elliptical section of which makes an angle U with the optical axis of the emitter, which excludes direct radiation from reaching the photodetector and thereby prevents false alarms. When smoke appears, radiation scattered forward, mainly by particles of «black» smoke, enters the conical reflector and is focused by the lens onto the photodetector. At the same time, radiation scattered in all directions, mainly by particles of «white» smoke, is reflected from the spherical and conical reflectors and is focused by the lens onto the photodetector. The appearance of the output signal of the photodetector indicates the presence of smoke in the monitored volume. The sensor is located inside any device aimed at indicating the state of the environment, for example, an aspiration fire alarm — the efficiency of its operation depends on the quality of the sensor.
Smoke sensor circuit
In order to understand in detail how the smoke sensor functions, the circuit shown in the figure has special symbols. The device, according to the figure, consists of a VD1 LED and a VD2 photodiode, operating in the infrared spectrum. Light from the AL301 type LED passes through the gap between the tubes and illuminates the FD206 type photodiode VD2. The photodiode opens, and a positive voltage appears on its cathode, which is fed to the inverting input of the operational amplifier DA1. The reference voltage is fed to the second input of the amplifier from the output of the variable resistor R4. This resistor sets the sensitivity. In the absence of smoke or dust in the air, the voltage on the cathode of the VD2 photodiode exceeds the voltage taken from the slider of the variable resistor R4, so the voltage at the output of the DA1 operational amplifier is close to zero. In this case, the LED VD3 does not light, and the element DD1.1. is locked.