Response Characteristics of Smoke Detectors in Early Stage of Fire.

Abstract

This paper is concerned with the effect of different smoke, as well as of smoke movement, to the response characteristics of smoke detectors. The method is employed essentially the same test fires as those performed by TH Aachen and UL. The fuel is selected bearing Japanese fire statistics and other test fires in mind. In this report, a smoke density is indicated by the extinction coefficient per meter for natural logarithm.
The results are summarized as follows :
1) The general tendencies of effect of different smoke to the response characteristics of smoke detectors are mainly obtained by 7×7×2.7m small room tests :
Light scattering type detectors operate at the rated smoke density for cellulosic smoke, and at a density equals to 4 times as dense as the rated density for sooty smoke from burning kerosene and polystyrene, and intermediate density for smouldering cellulosic smoke.
Ionization chamber type detectors are sensitive to flaming combustion, and insensitive to smouldering smoke. It is noted these detectors operate by transient masses having an abundance of invisible combustion products in the moment cellulosic strips flame up. Smoke density at the time of operation of a certain detector is very higher value than the others because of the difference of construction.
A detector sensitive to water vapour in smokes operates by cellulosic combustion products and by even those of alcohol, but does not detect those of plastics.
A few fixed-temperature type thermostats having very short thermal time constants are still more insensitive than rate-of-rise type thermostats.
2) For the purpose of getting effect of movement of smoke experiments are performed in a building having 20 m corridor and stairway extended over 3 stories.
Smoke density at the time of operation of detectors increases, the farther the distance from a origin of a fire is.
3) For small room tests, the mean vertical velocity of smoke is found to be between 20-180 cm/s, and the mean horizontal velocity between 5～150 cm/s. For the larger scale tests, the velocity of smoke front in corridor is observed to be between 1.3～14 (average 4) cm/s.
4) A physiological limit with smoke exposure will be set at smoke density of 0.02/m for cotton fires,0.28/m for wooden fires,0.72/m for gasoline fires.
5) By the smoke box tests under laboratory condition,it is found that ionization chamber type detectors can operate when PVC is heated at a temperature above 240°C in a humid atmosphere,and at a temperature above 300°C in a dry atmosphere.
Further studies on smoke particle size distribution and on the physical properties are needed to illustrate unstable relation between smoke density and the output of detectors. In order to produce more successful results, it is necessary for test fires to improve the method of ignition and the promotion of combustion and to select fuel.