Fire Lab

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
10 % of false alarms are caused by dust and another 10 % are caused by water steam.

For this reason several dust and water fog and steam sources have been analyzed during a big field campaign. The focus was on the dust properties caused by construction works as well as on water steam properties in the surrounding of an installed smoke detector. In a comprehensive test campaign typical size distributions of water droplets in common false alarm scenarios, as showering have been analyzed.
 

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.

Designing a test procedure for the evaluation of the behavior of smoke detectors in non-fire situations the focus should be on dust particles smaller than 100 µm because it is well known that particles > 100 µm settle down very fast. The analysis of dust properties caused by construction works in comparison with standardized test dusts showed that e.g. the quartz-free vacuum cleaner test dust DMT Dolomite 90 as test dust matches the specifications. 99 % of the particles are smaller than 100 µm, 90% are smaller than 50 µm and 10% are smaller than 2 µm. DMT Dolomite 90 test dust consists of particles in the same range as the ISO test dusts. It covers the whole dust range of "ISO ultrafine" (A1) and "ISO fine" (A2) but shows the advantage of being quartz-free.

The developed test apparatus is a very helpful and important tool for developers as well as for system designers giving the possibility of performing reproducible and well characterized dust tests. Having now a quantitative decision criterion to find the optimal detector for a specific scenario, it will find its application in the field of detector development, detector calibration.

The test apparatus consists of a test duct, an aerosol generator (Palas RBG 1000), a bipolar corona discharger to neutralize the charge of the generated dust and to reduce the dust accumulation at the channel walls, the obscuration meter (Lorenz AML) and a LabView software for controlling and timing of the aerosol generator. The duct consists of two concentric stainless steel rings with a diameter of 12 in and 24 in respectively and a linear connection between the left and right semi-circles, the cross-section is 6 in × 6 in. The flow velocity of 1 m/s is generated by an encapsulated motor with a mounted airscrew.


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.

The aerosol characteristic of different water fog and steam sources have been analyzed in a field campaign finding the proper source to reproduce the showering steam situation. False alarms triggered in foggy environments could be the effect of two processes: (i) the condensation of water on the optics due to a high dew point temperature of the optics or (ii) the light scattering process of the fog aerosol. Focus of the test apparatus is the second process, i.e. the production of a foggy aerosol. As aerosol generator a high end airbrush pistol as source for water mist showed the best match to the requirements. The peak value of the produced particle size distribution matches the measured values of the performed measuring campaign. The airbrush works with compressed air and distilled water to prevent calcification of the airbrush nozzle.

The developed test apparatus is similar to the test apparatus for dust tests. The duct consists of two concentric stainless steel rings with a diameter of 12 in and 24 in respectively. The cross-section of the duct is 6 in × 6 in. The flow velocity of 1 m/s is generated by an encapsulated motor with a mounted airscrew. The whole apparatus consists of an obscuration meter (Lorenz AML), humidity and temperature sensors and a LabView software for controlling and timing. For reproducible test conditions a linear increasing water steam ramp of 0.5 dB m-1 min-1 was implemented. Depending on user requirements also special scenarios like very fast concentration slopes comparable to a realistic shower scenario can be performed.


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.