New additions to Bosch’s Fire Detector 420 Series provide faster fire detection than was previously possible, along with reduced rate of false alarms.
Although no two fires are the same, they all have certain obvious characteristics in common, including the generation of heat, smoke and combustion gases. Detecting any one of these can give early warning of a fire, but can also lead to false alarms due to other particulate material being mistaken for smoke. That is why multisensor detectors were developed by companies such as Bosch Security Systems with products such as its FAPOTC 420 optical, thermal, chemical multisensor introduced IN 2001. Where Bosch’s Fire Detector 420 Series differentiates itself, however, is in the algorithm in the company’s intelligent signal processing (ISP) technology, which enables the 420 Series to achieve a high level of intelligent fire detection.
ISP enhances multisensor performance
In general, the more sensors a fire detector has, the earlier it can detect a fire and the fewer false alarms are generated. Bosch’s ISP technology, through which all sensor signals are pre-processed continually by dedicated internal evaluation electronics, analysed and linked with each other via a built-in microprocessor.
The sensor signals are processed by an algorithm developed using data from fire tests and tests with known disturbance values. The algorithm itself is based on rules derived from the experience of 5000 fire patterns. An alarm is triggered automatically only if the signal combination of the sensors corresponds to the specific pattern for a real fire.
In addition, the multisensor algorithm parameters are adapted to application type to further optimise early fire detection and false-alarm immunity. They also enhance immunity from ambient influences such as dust, humidity and temperature variations. This ensures best-in-class differentiation between real fires and disturbances.
The company has recently introduced three new variants to the 420 Series embodying Dual Ray technology that, in combination with ISP, offers ultimate precision in smoke detection.
Bosch formerly offered four sensor variants in the 420 Series: the FAH-T 420 (Heat Detector), the FAP-O 420 (Optical Smoke Detector), the FAP-OT 420 (Multisensor Detector Optical, Thermal) and the FAP-OTC 420 (Multisensor Detector Optical, Thermal, Chemical). With the exception of the FAH-T 420, all these detectors feature a single optical smoke detector. The series has now been extended with new detector variants featuring a dual-optical smoke sensor based on the company’s Dual Ray technology.
It is commonly known that distinguishing between steam, dust particles and smoke particles can be a challenge for some detectors. They also find it challenging to detect very light smoke with small particles produced by some open wood fires, particularly what are known as open cellulosic (wood) fire defined in practical tests as TF1 fires. In the past, smoke from such fires could only be reliably detected using multi-criteria sensors or ionisation detectors, the latter consisting of a small amount of radioactive material that detects any invisible smoke particles floating in the air and sets off an alarm.
Some manufacturers have attempted to address this challenge using a combination of thermal sensor and dual-optical sensor based on forward and backward scattering of light from two LED sources of the same wavelength. First described by Gustav Mie in 1908, Mie scattering describes the scattering of light by particles larger than a wavelength. It is responsible for the white light in mist and fog and the white glare around street lamps. Mie scattering is strongly dependent on particle size – the larger the particles, the stronger the intensity of scattered light in the direction of the incident light.
Bosch has adopted a different dual-optical approach in its new precision Dual Ray technology. Although this is also based on the Mie scattering effect, Dual Ray technology takes advantage of the effect to determine smoke density and particle size from the ratio between the intensity of scattered light from two LED sources of differing wavelength (one infrared LED and one blue LED). The smoke density and particle size are used by the detector’s fire-detection algorithm to provide even more reliable differentiation between smoke particles and other particles caused, for example, by disturbances such as dust and steam. This leads to earlier, more reliable fire detection and fewer false alarms.
Three new variants featuring Bosch’s new dual-optical sensor are being added to the FAP-420 Series – the FAP-DO 420 (Dual-Optical Smoke Detector), the FAP-DOT 420 (Multisensor Detector Dual-Optical, Thermal) and the FAP-DOTC 420 (Multisensor Detector Dual-Optical, Thermal, Chemical). Their addition, which brings the total number of detectors in the series to seven, means that the 420 Series now provides optimal choice of detector variants meeting all likely application requirements.
A unique combination
As with the original members of the series, the new variants also feature Bosch’s ISP technology, providing a unique combination of precision Dual Ray technology and the company’s fire-detection algorithm. They are all capable of detecting challenging TF1 test fires – even the FAP-DO 420 which embodies only the dual-optical sensor – and are the first detectors attested by VdS to TF1 and TF8, in addition to the required test fires of EN547.
Moreover, the dual-optical FAP-DO 420 offers a significant cost advantage over some competitor systems which require a multisensor detector (optical and thermal or dual-optical and thermal sensors) to provide reliable detection of TF1 fires. The dual-optical detector can also make full use of the surveillance area at all times, in contrast to multisensor detectors in which the surveillance area may be reduced in certain operating modes (eg, thermal only).
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