|Universität Duisburg-Essen, Campus Duisburg|
Non-Fire Sensitivity Testing of Optical Smoke Detectors
Optical smoke detectors are designed to detect small concentrations of smoke to ensure a fast and reliable
detection of arising fires. Unfortunately the complex task of avoiding false alarms is not completely addressed.
In contrast to the well standardized methods for the evaluation of the detection capability of a smoke detector,
there is a lack of reproducible and representative test methods concerning the false alarm susceptibility with
regard to nuisance aerosols. Even though the standard of optical smoke detectors is very high, false alarms
caused by dust and water steam consistently happen. A recent German study says that about
Test Apparatus for Dust Tests
Depending on the tool for work, particles were blown into the air with high speed during all performed tests,
leading to many particles being bigger than 100 µm. The measured particle sizes and their median were smaller
than 360 µm. Based on the mass 90 % of the particles are smaller than 200 µm. For the evaluation of false
alarms of optical smoke detectors caused by dust those distributions measured in a bigger distance (about 4 m)
from the source of dust are relevant, as they are closer to real life situations.
Based on the EN54 test-standard for the directionality or the response behavior of a smoke detector the increase of the aerosol concentration was set in the range 0.015 = ?m / ?t = 0.1 (dB m-1 min-1). Due to the implemented feed control, a linear slope of the dust concentration in the test apparatus could be achieved, similar to smoke tests according to EN54. Typically the increase is about 0.06 dB m-1 min-1 to simulate a slowly increasing pollution. On the other hand, construction works close to an optical smoke detector may cause a fast increasing dust exposure. For this reason a second linear increasing ramp of 0.5 dB m-1 min-1 was implemented.
The development of a test apparatus for the evaluation of the behavior of smoke detectors in non-fire situations is a first step to provide an important tool for developers as well as system designers. Due to the fact that dust is the major source of false alarms in airborne applications the developed test apparatus became part of a test standard for optical smoke detectors in aircraft applications. The main topics of the developed test apparatus have been adopted by the Aerospace Standard AS 8036. The goal is that no alarm or at least a much later alarm shall occur as a result of normal dust present at the detectors location. This standard specifies minimum performance standards for optical smoke detectors intended for use in protecting aircraft cargo compartments, galleries, electronic equipment bays and other similar installations.
Test Apparatus for Water Fog Tests
This chapter describes an investigation of typical false alarm scenarios for smoke detectors produced by
water aerosols and introduces an apparatus for testing detectors in a high humidity environment with suspended fog.
In high humidity environments evaporation and saturation processes play an important role for the life time of water droplets and are mainly determined through temperature and relative humidity. The life time of droplets increases at higher humidity and temperature values and thus have a strong influence on the control. This climatic control is implemented in the LabView software of the duct and it provides constant start conditions for each test, e.g. a relative humidity about 50 % to 60 %.
So two novel test systems for the evaluation of the behavior of optical smoke detectors in non-fire situations were designed, set up and analyzed. Both devices allow tests with a controlled linear increase of dust concentration and water droplet concentration respectively in a reproducible manner. But the ramps are not limited to the linear behavior. Due to the flexible control system also other ramp shapes can be performed.
Test Apparatus for Spray Tests
The third newly developed test device is a test apparatus for spray tests due to the fact that insecticide
aerosols are major source of false alarms in airborne applications. The developed test devices became part
of Aerospace Standard AS 8036 for optical smoke detectors in aircraft applications. The conical construction
is the result of the analysis of the spraying angle of different sprays. To fit best to the false-alarm
sources (sprays) in aviation, typically insecticide sprays and deodorant sprays are used as aerosol source.
The spray nozzle is controlled by LabView via a servo motor. The aerosol concentration is measured by the
MIREX smoke measuring device.