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An Interference Simulator for

Quantitative Determination of

the Susceptibility of Flame

Detectors to False Alarm

A. Scheidweiler „«•>;•

Cerberus Company

Maennedorf, Switzerland

Translated for: Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

November 1975

Final Report

US. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 75-956

AN INTERFERENCE SIMULATOR FOR

QUANTITATIVE DETERMINATION OF

THE SUSCEPTIBILITY OF FLAME

DETECTORS TO FALSE ALARM

A. Scheidweiler

Cerberus Company

Maennedorf, Switzerland

Translated for: Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

November 1975

Final Report

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

PREFACE

This report is a translation of paper prepared

by A. Scheidweiler and presented by him at the

Seventh International Seminar on Problems in Automatic Fire Detection held in Aachen, Germany, on

March 5-6, 1975. The Seminar was sponsored by the

Institute for Electrical Communications Engineering

in Aachen in conjunction with the Gesamthochschule

in Duisberg.

This translation has been prepared to disseminate useful information to interested fire research

personnel on a need-to-know basis and is not an

original work of the Center for Fire Research.

We express our appreciation to the author, Dr.

A. Scheidweiler for his permission and assistance in

making this information available.

We also express our appreciation to Joint

Publication Research Service, Arlington, Virginia,

who provided the translation of the original paper.

AN INTERFERENCE SIMULATOR FOR QUANTITATIVE

DETERMINATION OF THE SUSCEPTIBILITY OF

FLAME DETECTORS TO FALSE ALARM

A. Scheidweiler

All fire detectors are susceptible to false

alarms, i.e., responding to fire-like conditions

when no fire is present. Of all the known fire

detectors, flame detectors are probably the most

false alarm prone of any of the fire detectors,

primarily because these detectors are subjected to

more environmental influences resembling the detectors' operating mode than any other fire detector.

In this paper, the author describes an investigation into the susceptibility of flame detectors

to false alarms. The author describes a test apparatus, constructed and used by his firm, to evaluate

the susceptibility of currently-available flame

detectors to false alarms. In addition, the apparatus can be used to evaluate prototype flame detectors.

The data developed indicates the false alarm

susceptibility of each type of flame sensor. The

data also provides a means of selecting a flame

detector having the greatest resistance to known or

predictable pseudo-fire sources.

Key words: False alarms; false alarm simulator;

fire detection; fire detector; flame detection;

flame detector.

1
My paper has to do with a problem which relates in this

paper only to flame detectors, although in general it plays

an important role in the evaluation of fire detection devices.

This discussion deals with the investigation of susceptibility

to false alarm, where, however, we do not understand this to

mean the occurrence of an alarm because of deficient engineering reliability but rather the deception of a detection device

by interference factors which behave in a manner similar to

the behavior of fire-indicating parameters.

You will certainly agree with the assertion that radiation detectors unquestionably belong among those fire detectors upon whose "intelligence" we impose the most exacting

requirements. The fire parameter to which they respond,

namely electromagnetic flame radiation, is more frequently

simulated by environmental influences than in the case of any

other type of detector. Just consider the effect of solar

radiation, artificial light sources and even heating radiators.

It is true that research and development have made significant

progress in the area of these detectors -- not least of all

in fundamental and pioneering studies in this Institute — yet

no one disputes the fact that when radiation detectors are

employed, even with the most careful planning, one must

still reckon with an increased number of so-called deception

alarms. This circumstance is all the more regretable since

flame detectors could perform a very important function,

particularly in the protection of property, as, for example,

in the control of the triggering process for extinguishing

installations. But it is precisely here, because of the

associated consequences, that it is particularly desirable

that false alarms be avoided.

2
However, it is also justifiable to expect a fire detector

to have high response sensitivity in order that countermeasures

may be applied as early as possible. But, in experience,

with increasing sensitivity there is also the increasing

probability of a false alarm. Hence, it is a question of

finding the right compromise here; i.e., expressed mathematically, the fire detector should exhibit a ratio of recognition

probability to deception probability which is as large as

possible. If in this way one defines a so-called "figure of

merit" as a measure of quality then there at once arises the

question as to how one can obtain quantitative statements

about these probabilities. A mathematical calculation,

initially, would presuppose setting up a catalog of all

possible flame spectra and interference quantities with the

associated probabilities of their occurrence. Unfortunately,

at the present time such information is not yet available and

it will probably be a long time, if ever, before it is available .

Parallel to this problem we have the steadily increasing

need, even today, to be able to make quantitative statements

about the false alarm behavior of flame detectors. Even

though absolute data are not yet possible at the present

time, I am of the opinion that even those test methods are

extremely valuable which permit only a comparative evaluation

of flame detectors relative to one another. In particular,

supplementing tests of engineering reliability, these comparative tests could be built into a test program for radiation

detectors

.

As fire alarm manufacturers who also carry flame detectors in their sales program, we have given thought to this

topic now for a number of years. It is probably unnecessary

to emphasize that the manufacturers must have a vital interest

3
in assuring a high level of quality by means of appropriate

and practical test standards if the conditions are to be

created which will permit fire detectors, and in this case

flame detectors, to receive the confidence which they merit.

A flame detector possesses in general at least one

light-sensitive detection element with a specific spectral

sensitivity. Among the models being marketed at the present

time we find both those which respond principally to the

infrared component of the flame spectrum and also those which

respond to the visible or ultraviolet portion. Their behavior

in response to interference is correspondingly different.

Thus, e.g., a detector using lead sulfide as the infrared

sensitive sensor is most markedly disturbed by heating radiators while it does not react to ultraviolet sources of

interference. Further, almost all modern flame detectors

utilize the flame's characteristic of flickering in a specific

frequency range. Here, too, there exists a possibility of

deception whenever the source of interference displays similar

intensity variations.

Because of the already-mentioned lack of information

needed for an exact calculation we have taken the empirical

path and in so doing have been interested not so much in the

question as to what potential interference sources exist as

in the question as to what sort of interference factors

produce the most sensitive reaction in currently familiar

detectors. The evaluation procedure which I am reporting on

here makes it possible, in the case of each newly developed

detector type, to take into account special interference

sources - i.e., to include the latter in the evaluation

process

.

4
I think it is logical to first explain the method itself

Subsequently, I will introduce to you the measuring apparatus

specially developed for this purpose and will discuss practical examples and measurement results.

In the following discussion it is advantageous to look

upon the detector being tested as a "black box," i.e., to

assume in other words that we know nothing about its physical

and electrical properties. Our sole assumption is that we

are dealing with a radiation detector which is equipped with

sensors which correspond to the present state of engineering

knowledge. In this way one facilitates the practical application of the procedure, i.e., the tester requires no profound knowledge of the theory and working principle of the

test sample.

In the first part of the test procedure an attempt is

made to alarm the detector by radiation from well-defined

sources of interference. The nature of the interferences has

been selected as follows on the basis of existing experience

with present-day flame detectors. A heating plate for the

simulation of infrared interference sources, an incandescent

lamp and a tube of luminous material for the simulation of

artificial visible light sources and a gas discharge tube for

simulating UV radiators. In order to include the alreadymentioned flame flickering in the deception testing, the

intensity of the interfering radiator is varied periodically

in the range 5-20 Hz. For each of the individual sources of

interference the response sensitivity is determined experimentally insofar as possible by variation of the distance

from source to detector with the frequency of the intensity

variations being so selected that the sensitivity is greatest

The thus obtained distance values are recorded and evaluated

as explained later on.

5
It is the purpose of these measurements to discover the

sort of interference sources to which the detector most

sensitively responds. Subsequently the fire parameter sensitivity of the test specimen is determined with the aid of a test flame, again employing distance as a measure of sensitivity. In this measurement, too, the distance to the radiation source is varied until the detector responds. All the

measurements thus obtained are tabulated, as shown in table

1.

Type of Radiation

Interference

Heating plate

Incandescent lamp

Fluorescent light source

UV lamp

Test flame

In the first column there is listed the type of radiation and

in the second the distances d

F

and dg found to be necessary

to produce an alarm. In the third column we find the ratio

of distance values k = d^rd-, which we take as a measure of

F S

the test specimen's discrimination capability. The greater

the computed k-value, the less the effect of the interfering

source involved upon the detector. Naturally, the absolute

value of this ratio depends markedly upon the dimensioning of

the interfering source, i.e., upon the intensity of its

emission but as has already been mentioned this procedure

does not aim at obtaining absolute statements but aims rather

at being able to compare flame detectors with one another

Table 1

Distance Between

Detector and Radiator

d

S

X d

F

:d Sl

d

S

2 d

F

:d S2

d

S

3 d

F

:d S3

d

s

4 d

p

:d s4

d

F

6
relative to their susceptibility to interference. However,

for this purpose it suffices to hold constant the interference sources, once they have been chosen, and the test fire.

The fulfillment of these conditions was the task of the test

apparatus which we developed and which I would like now to

discuss in more detail.

The four interference sources selected are placed in a common housing and can be switched on and off at will.

Figure 1 shows a schematic representation of the apparatus.

The interfering radiation emerges from the sketched openings

and is periodically chopped (i.e., turned on and off) by a rotating disk. The frequency can be easily varied and kept

constant by controlling the revolutions-per-minute . Figure 2

shows the total apparatus with a detector in testing position.

First, the distance is chosen to be of such a size that the

detector still does not respond. The interference source is

always located perpendicularly below the test specimen. Then

the detector is slowly brought closer to the apparatus until

an alarm is triggered. By repeatedly setting back the alarm

and correcting the distance as well as the variation of the

chopper frequency it is possible to determine the measurement

d with adequate precision for the interference source selected.

The same procedure is repeated for the other interference

sources. For measuring the effective fire parameter sensitivity, an alcohol flame was chosen as the test fire. A

cylindrical metal vessel having a diameter small in comparison with its height is filled about three-fourths with alcohol

and then ignited. In this way it is possible to create a well-reproducible flame which burns with sufficient constancy

for a fairly long time.

The measurement procedure described has been used at our

establishment for some time for the comparison of radiation

7
detectors so that we already have a certain degree of experience at our disposal. I should like to briefly present to

you some of these results.

The models investigated consist of products of diversified provenance. I am sure that you will understand our not

stating any model designations, from which it would be possible

to infer the identity of the manufacturer. We merely indicate

which sensor is being employed, i.e., in what spectral sensitivity range it is operating. As may be seen in table 2, in

addition to data regarding flame sensitivity the table also

contains information about the false alarm deception behavior

to be expected. Hence one can also speak of this as a sort

of suitability table for it furnishes the user with a means

of selecting the detector having the greatest resistance to

interference, assuming the interfering influences to be known

or predictable.

The results existing hitherto clearly show that comparisons carried out in accordance with previously conventional

practice (i.e., on the basis of fire parameter sensitivity

alone) can easily be misleading. Alarms having the greatest

flame sensitivity are very often also most susceptible to

interference factors. But this is not brought out during

alarm comparisons in simple fire experiments, which cover

only the response time of the detection devices.

However, today in engineering circles it is undisputed

that the main problem of fire alarm installation is not that

of increasing the sensitivity of the detectors but rather

reduction of their susceptibility to deception. Therefore we

consider extremely valuable any means which helps us to

recognize and to exclude risk of deception alarms. It is in

this sense that I ask you to look upon the work described

8
here as a contribution to the problem of combating deception

alarms, which in the case of flame detectors are of especially

frequent occurrence. We believe that with our method, at

least up until the time when more reliable information exists,

we have at our disposal a valid means of evaluating flame

detectors

.

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Figure 2. Test Apparatus With

Detector in Testing Position.

12
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 75- 956

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

An Interference Simulator for Quantitative

Determination of the Susceptibility of Flame

Detectors to False Alarm

5. Publication Date November 197 5

6. Performing Organization Code

7. AUTHOR(S)

f\7A. OLUC1UWCJ.1C1 Qr"Vif* i rjwp i 1 pt

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4q?Rfi77

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

same as No. 9

13. Type of Report & Period Covered

TP -i v~\ —% i "D f~\ v\ /~\ v* 4~ r inai Keporx.

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) All fire detectors are susceptible to false alarms, i.e., responding

to fire-like conditions when no fire is present. Of all the known fire

detectors, flame detectors are probably the most false alarm prone of any

of the fire detectors, primarily because these detectors are subjected to

more environmental influences resembling the detectors 1 operating mode

than any other fire detector.

In this paper, the author describes an investigation into the susceptibility of flame detectors to false alarms. The author describes a test apparatus, constructed and used by his firm, to evaluate the susceptibility of currently-available flame detectors to false alarms. In

addition, the apparatus can be used to evaluate prototype flame detectors.

The data developed indicates the false alarm susceptibility of each

type of flame sensor. The data also provides a means of selecting a flame detector having the greatest resistance to known or predictable

pseudo-fire sources.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Flase alarms; false alarm simulator; fire detection;

fire detector; flame detection; flame detector.

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