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Intelligent fire detection

The ability to detect smoke and fire as early as possible is the single most important feature of any fire prevention system and its detectors. Detection at a very early stage of a fire allows the operator to react before the fire can spread, thus reducing the risk of bodily harm and loss of valuable assets. On the other hand, false alarms are costly and can interrupt business processes. This is why fire detectors not only need to reliably detect smoke and heat, but they must just as reliably make a decision about whether what they detect is really smoke of a fire or just a disturbance, such as water vapour, dust or cigarette smoke.

To increase accuracy and detection performance, multi-sensor detectors were developed. These combine optical, thermal and/or chemical detection technologies and trigger an alarm only when the signal combination of the sensors corresponds to specific patterns of real fires. But while such multi-sensor detectors have proven to be very reliable in distinguishing smoke from dust, they cannot handle invisible disturbance values such as electromagnetic interferences (EMI).

It is estimated that between 20 and 30 percent of today’s false alarms have an undetermined cause (BRE-Study: “The causes of false fire alarms in buildings – 2014”). Research has shown that electromagnetic pollution is probably the single most important contributor to these numbers. To address this problem, Bosch has introduced its eSMOG feature to the AVENAR family of multi-sensor smoke detectors. eSMOG recognises critical EMI conditions before installation as well as during normal operations, thus avoiding false alarms due to electromagnetic pollution.

Intelligent Signal Processing (ISP)

The AVENAR family of fire detectors combines three detection technologies – optical, thermal and chemical – with different models using different combinations of those. The obvious advantage of such a combination is that a detector can detect a fire via smoke, temperature and combustion gases. This is important, as depending on its nature, a fire can exhibit a wide variety of characteristics. Some fires produce a lot of smoke without necessarily having a huge effect on the temperature while others hardly issue any smoke at all.

So, while optical detection technologies offer very good performance in detecting a TF7 test fire (slow smouldering pyrolysis wood fire) according to EN54-7, they can hardly detect a TF6 (liquid methylated spirit fire) which produces almost no smoke. Thermal detection on the other hand works perfectly with a TF6 while being hardly able to detect a fast smouldering cotton fire (TF3), as this produces a high concentration of smoke but almost no temperature increase.

Such a fire can also be detected with chemical technologies which do not perform as well with some other types of fires. Having a combination of detection technologies allows for an accurate detection of different kinds of fires. On the downside, each of the sensors brings its own set of vulnerabilities when it comes to false alarms. This is where ISP comes in.

With ISP, all sensor signals are continuously analysed and evaluated via an inbuilt microprocessor. The algorithm determines the dynamic sensor values to decide whether it is a real fire or a disturbance value. It is based on a frame of data from fire tests, tests with disturbance values and simulated data. Only if the combination of different signals matches the special characteristics of a real fire, the alarm will be triggered automatically. ISP thus reduces the chance of false alarms while increasing detection accuracy. Depending on the application site, different fire characteristics can be programmed during installation.

Another benefit of combining multiple sensors is that the detectors can also be used where disturbances like light smoke, steam or dust must be expected during the course of normal operation without producing false alarms.

AVENAR technologies

The detection performance and false alarm immunity of the AVENAR family is based on a combination of three key technologies:

• Multi-criteria detection with ISP.

• Dual-ray optical detection.

• eSMOG protection against electromagnetic pollution.

Each of these technologies has its own merits, but at the end of the day, it is the combination of all three which allows the system to find the perfect balance between detection performance and false alarm immunity in environments of any kind.

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