Dual-range smoke detectors — New level of detection accuracy.
Dual-range smoke detectors — New level of detection accuracy
Point-type optical-electronic smoke fire detectors are widely known and used to protect most objects from fire. Providing early detection of fires at the smoldering stage, fire detectors with a smoke detection channel are indeed a very effective means of protecting people's lives and health and preserving material assets. However, with all their advantages, they also have a drawback — these are false alarms under some external influences not related to fire. For example, determining the difference between steam, dust particles and smoke is a difficult, and for many detectors, an unsolvable task.
Most detectors also have difficulty detecting the low-grade, small-particle smoke produced by open wood fires, also known as open cellulose fires and designated TP1 in test fires. In the past, reliable detection of smoke from such fires was only possible with combination (multisensor) or ionization detectors, which use a radioactive element to detect invisible smoke particles in the air and generate an alarm. However, due to the specifics of operation and expensive disposal, the latter type of detector has not gained much popularity, which is not the case with multisensor models of detectors, the popularity of which has been steadily growing in recent years. In these detectors, additional detection channels are used to compensate for external interference; now, heat and gas sensors are used more often. Indeed, combined or multisensor detectors provide earlier and more reliable detection due to a comprehensive approach to the analysis of various fire factors and, in addition, are capable of detecting such a complex fire source as open burning wood (TP1), characterized by abundant emission of so-called “invisible” smoke with small particle sizes.
That is why a number of manufacturers continue to develop combined detectors and search for new technologies to solve the problem of increasing the reliability and speed of detection. However, the innovation of this type of detector in most cases is not the number of sensors, but the advanced analysis algorithms implemented in intelligent signal processing technologies (for example, Intelligent Signal Processing — ISP), allowing these detectors to achieve the highest level of fire detection.
Thanks to these technologies, the signals from all sensors are constantly pre-processed by special computing electronics, analyzed and jointly processed by the built-in microprocessor. Intelligent processing algorithms are developed in research laboratories taking into account data from test fires and all known non-fire effects. The algorithms are based on data and criteria obtained from practical analyses of fire models. An alarm is only issued if the combination of signals used in the sensor detector matches the characteristics of a specific real fire model. In addition, the parameters of the multi-sensor algorithm can be adjusted to specific operating conditions to increase reliability, reduce reaction time and the likelihood of false alarms. They also increase immunity to external interference such as dust, steam formation or temperature changes. This guarantees an exceptional ability to distinguish real fires from external influences. Speed and reliability of fire detection have been and remain the main objectives of fire alarm systems. Therefore, the advantages of combined detectors over smoke detectors are obvious here. However, they now have a serious competitor. Now there is a more cost-effective solution that not only protects the smoke detector from false alarms, but also provides high sensitivity to «invisible» smoke, comparable to the sensitivity of ionization detectors. The start of industrial production of blue LEDs served as a starting point for the creation of a new generation of detectors. 
First described by Gustav Mie in 1908, the principle of light scattering describes the elastic scattering of light by large particles (larger than the wavelength). This effect is also observed in the light from car headlights in fog and the glow around street lamps. Mie scattering is highly dependent on particle size — the larger the particle, the greater the power of the scattered light in the direction of the incident light.
In smoke optoelectronic detectorsTraditionally, IR LEDs with a wavelength of about 945 nm are used. The wavelength of the blue LED is half that and is 470 nm. Thus, using a blue LED as an emitter allows detecting smaller smoke particles. The new generation of dual-band smoke detectors uses two LEDs (dual optical principle). Based on the Mie scattering principle, the dual optical sensor technology takes advantage of the effect to determine the smoke density and particle size from the ratio of the scattered light powers from two LED sources with different wavelengths (one infrared, one blue). For an even clearer distinction between smoke particles and other particles, such as dust and steam, the microprocessor analysis algorithm uses additional parameters — smoke density and particle size in the range from 0.2 to 1 μm. This leads to earlier, more reliable detection of fires and a reduction in the likelihood of false alarms. 
Figure 1 shows data on the ratio of signals in the blue and IR ranges during studies with test sources TF1, TF2, TF3, TF4, TF5, as well as with external influences of steam, dust and aerosols (in particular, hairspray), which are the most common causes of false alarms smoke detectors. Depending on the type of fire, different values of the blue and infrared signal ratio were obtained, but in the worst case for the TF2 fire, the minimum ratio was about 2, and for non-fire particles it was approximately 1. This is due to the fact that the size of steam, dust and aerosol particles is larger than the wavelength of the blue and infrared LEDs.
These results demonstrate the possibility of identifying the type of exposure in a smoke detector with a dual optical sensor. The threshold for separating smoke and other exposures not related to fire was set at 1.4. The detector generates a Fire signal only when the signal ratio exceeds 1.4; if this value is not exceeded, the Fire signal is not generated. Dual-range detectors are capable of reliably detecting the most difficult TP1 test fire sources. These detectors became the first smoke detectors certified by VdS for the TF1 (TP1) source in addition to the other test fire sources required by EN54-7.
Another major advantage of smoke detectors with a dual optical sensor is the maximum area of the monitored room, unlike multi-sensor detectors, the maximum monitored zone of which is reduced when using a heat sensor to the standard values of heat detectors. In accordance with SP5 for rooms with a ceiling height of 3.5 m, the average area monitored by a smoke detector is 85 m2; when using a combined detector with a heat sensor — 25 m2.
The use of dual-range smoke detectors is also possible in combination with other sensors, such as heat and gas, providing an unrivaled level of reliability and speed of detection of any fire sources even in the most difficult operating conditions.
Both combined and purely smoke models of dual-range fire detectors are already now, in terms of their technical parameters, a direct alternative to ionization detectors. In many operational properties they surpass multi-sensor detectors, while remaining a cheaper technical solution. And, of course, they represent a completely new level of accuracy in detecting smoke during a fire.
T. Sulim, Head of Fire Alarm Systems at Robert Bosch
Magazine «Algorithm of Security» No. 5, 2010