AD Hoc Tests on Cerberus Smoke Detector Head

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Technical Note (National Research Council of Canada. Division of Building Research), 1961-08-01

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AD Hoc Tests on Cerberus Smoke Detector Head

McGuire, J. H.; Ruscoe, B.

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DIVISION OF BUILDING RESEARCH

No.

NATIONAL RESEARCH COUNCIL OF CANADA

340

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NOT FOR PUBblCATION FOR ｉｎｔｅｒｎａｾ USE

August 1961 PREPARED BY J. H. McGuir e and B. Ruscoe CHECKED BY _GWS APPROVED BY _NBH PREPARED FOR Record Purposes

SUBJECT AD HOC TESTS ON CERBERUS SMOKE DETECTOR HEAD

According ·to Underwriters Laboratories· 1r).c. of Chicago, the Cerberus smoke detection system responds to a fire, under certain test conditions, much more rapidly than a commercially-available fixed temperature -sensing system. The manufacturer s claim that the Cerberus system will not usually respond to an unpleasantly dense atmosphere of tobacco smoke. The ad hoc tests described in this note were carried out to investigate the validity of this claim and, if it were true, to gain some understanding of the mechanisms responsible for it.

The components and electrical connections within the

Cerberus head are illustrated in Fig. 1. The two ionization chambers are irradiated by self-contained radioactive sources which are principally alpha emitter s. The electric field str ength within the chamber s is such that the current is within the saturation range when the contained gas is atmospheric air,.i. e., the applied field strength is such that recombination of ionized molecules and electrons is small. If heavier particles are

introduced into the lower chamber two effects will result: firstly, the lower'ionic mobilities of the heavier particles will give more likelihood of recombination before the ions and electrons reach their respective electrodes; and secondly, the ion generation per alpha particle will generally be lower.

The above explanation is not strictly correct since other factors such as ionization potential will playa part; nevertheless their signifi-cance will be low. Theoretically, neglecting the ancillary factors already referred to, an ionization chamber could respond to gases of

differ ent molecular weights. The findings dis cus s ed in this note, however, suggest that the Cerberus system is only intended to react to particles of mass lar ger than those of air 'molecules by several order s.

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-The result of these phenomena, so far as the circuit of

Fig. 1 is concerned, is that the voltage distribution across 'the two cells will change, the voltage between the trigger electrode and cathode of the cold cathode triode incr easing. At a pr es cribed voltage, the cold cathode tube will fire and operate a relay in the supply line. This constitutes detection of a fire.

APPARATUS

The only manufactured article purchased was a Cerberus sensing head that included the two ionization chambers and the cold cathode triode. The information sheet, which was also supplied,

stated that the potential difference maintained across the two chambers

was normally 220 volts. It was apparent that all that was needed to complete the system, 50' far as laboratory tests were concerned, was a relay and

an appropriate anode load. Familiarity with other sub-miniature cold cathode triodes suggested that a total anode load of abo ut 150 K would be appropriate to make the circuit behave in the required manner. Since the performance of the sensing system is hardly dependent on the current consumption of the cold cathode tube when it has been fired, it is most likely that the performance of the circuit was characteristic of the performance of any Cerberus detector system. The d-c supply voltage was given by a conventional series stabilizer circuit using a high quality stabilizing tube for reference voltage.

TESTS

The Cerberus detector head was mounted near the ceiling, 2 ft from one corner of a room 7 by 11 by 10 ft high which had been made reasonably air tight. Test fires were located at ground level, 4 ft from the diagonally opposite corner. A fan in the centre of the room directed'towards the fire corner, tended to distribute the products of combustion throughout the room.

,. 3 -TABLE I TEST RESULTS Material Burned Paper Tobacco Weight of Material Burned, gm 30 40 Time to Detection, min. 2 Remarks

Smoke not visible Time to burn -40 min.

Smoke was extr emely dense and atmos-pher e untenable.

Asphalt tile 40 4

Vinyl Asbestos tile 48 4 Tile was repeatedly lit with propane torch.

Linoleum tile 44 3

Fibreboard 36 4

Cotton 40 4

Gasoline 30 3

For each test the total amount of material burning did not exceed the weight given in Table I by more than a factor of 3. When the detector responded the room was entered and the fire extinguished. The weight in Table I hi the weight loss suffered by the material up to this time. These tests were merely of an exploratory nature and are not intended to constitute a scientific analysis of the performance of the Cerberus system. Tobacco smoke failed to actuate the system and it is believed that this failure is associated with the time scale involved. The times listed in column 3 ｡ｲ･ｾ therefore, probably highly significant; no attempt is made ｨ･ｲ･ｾ ｨｯｷ･ｶ･ｲｾ to discuss this significance in detail. Before further consideration is given to the information in Table I, the results of some of the additional tests must be reported. It was found that the detector barely responded to very dense concentrations of ｰｲｯｰ｡ｮ･ｾ

or the products of combustion from a fire involving methanol, and would not respond to carbon dioxide.

4

-DISCUSSION OF RESU LTS

These results imply that the design of the ionization chambers has not been made sufficiently critical so that the system will respond to gases with molecular weights exceeding the average for air by a factor of 2. The system responds to extremely low concentrations of very large particles, with dimensions several order s greater than those of typical

air molecules.

Reverting to Table I, the failure of the system to respond to tobacco smoke can be explained on this hypothesis: Since the rate of

combustion of the tobacco was such that only 40 gm were consumed in 40 min, it is suggested that the heavy particles, to which the system responds, precipitated before detectable concentrations were attained.

No particular significance is attached to the fact that the losses in the weights of the remaining materials at the time of detection were all of the same order. It is merely inferred that the products of combustion of most common fuels involved in building fires include comparable quantities of "large" particles. When the paper was burned combustion was rapid and to a casual observer might have seemed to have been complete, for virtually no smoke could be seen.

SENSITIVITY

All of these tests were carried out with the detector head in the condition in which it was received. The sensitivity-adjusting screw which controlled a shield to one of the radium sources was untouched. The manufacturer· s instructions give details of an electrical measurement which may be taken as a measure of the sensitivity. It consists of

injecting a potential between terminals 1 and 3 on the detector head (Fig. 1). A potential of 28 volts was required to fire the cold cathode triode with the head as received from the factory. From the nature of the circuit, the sensitivity within the limits which are of interest may be considered approximately proportional to the voltage required to operate the triode by this test method . . With the setting that required a potential of 14 volts to ope:J;ate the circuit, it was found that the sensitivity would not be too great for use in many locations. To cause operation of the

circuit by tobacco smoke in practical circumstances it was necessary to bring a smokerl_{s pipe within some 6 in. of the head.}

When the sensitivity was again l'doubled" so that the test

potential was 7 volts, the system proved impractical for use in locations where smoking would be permitted.

Only in this extremely sensitive setting was it found possible to operate the circuit with carbon dioxide. When the detector was held one foot above a methanol flame, the detector would however, respond when the setting was such that the test potential was 14 volts.

5

-CONCLUSIONS

These ad hoc tests imply that the Cerberus system is reasonably sensitive only to particles of dimensions several orders greater than the average for air molecules. This characteristic allows the Cerberus system to be useful and in fact extremely sensitive in areas where there may be heavy concentrations of tobacco smoke. To an approximation it may be said that under any given set of circumstances the system will respond provided a specified weight of material is burned in a reasonably short time, say 5 minutes. Thus, although the atmosphere in a smokers' lounge may be almost unbearable due to the high concen-tration of visible smoke, it will be appreciated that the rate of combustion of tobacco in terms of weight per minute is virtually negligible when

compared with fire conditions.

That' the Cerberus system will not be sensitive to complete combustion need not be considered disturbing. Such complete combustion is only possible in practical circumstances with fuels such as methanol. Any typical building fire which originated with such fuels would involve other fuels to which the Cerberus system does respond at a very early stage.

OTTAWA DIVISION OF BUILDING RESEARCH. NATIONAL RESEARCH COUNCIL CANADA RADIUM 220 VOLTS RADIUM

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SCHEMATIC OF CERBERUS DETECTOR HEAD

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