UV radiation emitted at the instant of ignition. The solar blind UV wavelength band is also easily blinded by oily contaminants. IR band are arguably the most reliable technologies available for detection of fires. Light emission from a fire forms an image of the flame at fire sensor working principle pdf particular instant.
Water on the detector’s lens will greatly reduce the accuracy of the detector, as will exposure to direct sunlight. A special frequency range is 4. This causes a peak in the total radiation emission and can be well detected. This makes the sensor in this frequency “Solar blind”, however sensitivity is reduced by sunlight. This makes infrared sensors for use in outdoor applications very unresponsive to fires. A salt film is also harmful, because salt absorbs water.
However, water vapour, fog or light rain also makes the sensor almost blind, without the user knowing. The cause is similar to what a fire fighter does if he approaches a hot fire: he protects himself by means of a water vapour screen against the enormous infrared heat radiation. The presence of water vapor, fog, or light rain will then also “protect” the monitor causing it to not see the fire. Visible light will, however be transmitted through the water vapour screen, as can easily been seen by the fact that a human can still see the flames through the water vapour screen. These cameras can be used in complete darkness and operate both inside and outside.
These detectors are sensitive to both UV and IR wavelengths, and detect flame by comparing the threshold signal of both ranges. This helps minimize false alarms. Often one sensor looks at the 4. Triple-IR flame detectors compare three specific wavelength bands within the IR spectral region and their ratio to each other.
In this case one sensor looks at the 4. 4 micrometer range while the other sensors look at reference wavelengths both above and below 4. As a result, both detection range and immunity to false alarms can be significantly increased. IR3 detectors can detect a 0. Triple IRs, like other IR detector types, are susceptible to blinding by a layer of water on the detector’s window.
Most IR detectors are designed to ignore constant background IR radiation, which is present in all environments. Instead they are designed to detect suddenly changing or increasing sources of the radiation. IR detectors become more prone to false alarms, while IR3 detectors become somewhat less sensitive but are more immune to false alarms. Furthermore, complex image processing analysis can be executed by computers, which can recognize a flame or even smoke. Unfortunately, a camera can be blinded, like a human, by heavy smoke and by fog. In this equipment the information of an UV camera mixed with visible image information. This current can be used to verify flame presence and quality.
Such detectors are used in large industrial process gas heaters and are connected to the flame control system. They usually act as both flame quality monitors and for flame failure detection. A common use in these installations is to cut off the supply of fuel if the flame fails, in order to prevent unburned fuel from accumulating. In fact, during a fire, relatively sparsely UV energy and visible light energy is emitted, as compared to the emission of Infrared radiation. It looks like a static picture, but in reality the energy fluctuates, or flickers. This flickering is caused by the fact that the aspirated oxygen and the present combustible are burning and concurrently aspirate new oxygen and new combustible material.
These little explosions cause the flickering of the flame. An Infrared detector which uses this frequency is therefore solar blind. Not all manufacturers of flame detectors use sharp filters for the 4. 3 µm radiation and thus still pick up quite an amount of sunlight. These cheap flame detectors are hardly usable for outdoor applications. 3 µm there is relatively large absorption of sunlight. Infrared flame detectors suffer from Infrared heat radiation which is not emitted by the possible fire.
One could say that the fire can be masked by other heat sources. Sometimes a moving hand is sufficient to trigger an IR flame detector. A disadvantage is that the radiation energy of a possible fire must be much bigger than the present background heat radiation. In other words, the flame detector becomes less sensitive. Every multi infrared flame detector is negatively influenced by this effect, regardless how expensive it is.