❏ Heat detectors monitor temperature differences and

❏ Ionisation detectors register smallest smoke particles and invisible aerosols.

Modern multi-sensor detectors, al-so known as multi-criteria fire de-tectors, involve a combination of op-tical detectors, thermal detectors,ionisation smoke detectors or a gassensor and are able to detect inter-ference factors. The detectors mayalso contain an integrated flashinglight, audible or voice alarm.

The purpose of fire detection sys-tems is to detect a fire in its earlieststage and activate the alarm. Fromits initial phase, a fire is subject tovarious conditions, which have dif-ferent effects on its development.The interaction of fire cause, com-bustible material and air supply canproduce a smouldering fire, an openfire or a combination of the two. Thecombustion process generates tem-peratures, fire gases and smoke. Thelatter can take a variety of forms,from invisible aerosols to black soo-ty smoke, depending on the com-bustible material and the progres-sion of the fire. Fire detectors mustbe able to detect these different firephenomena characteristics of the fire development quickly and reli-ably. A number of different detec-tion technologies are applied. Theyinclude the following:

❏ Point detectors❏ Flame detectors❏ Line type smoke detectors❏ Line type heat detectors❏ Aspirating smoke

detection systems

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Modern fire detection systems notonly apply increasingly intelligentmethods to detect fires but also imi-tate defined processes automatical-ly and provide emergency serviceson the ground with important infor-mation.

Early and reliable fire detectionusing appropriate detectors has toppriority. The detection criteria forautomatic detectors are deter-mined by the combustion productsproduced by the fire.

Detection methods of different de-tector types:

❏ Optical detectors detect visible smoke particles generated by the fire,

❏ Manual call points

This article looks at point detectorsand their development.

The most popular types of automa-tic point fire detectors can roughlybe divided into two groups: smokedetectors and heat detectors. Theyinclude conventional detectors thatmonitor limit values, intelligentbus-powered detectors and multi-criteria fire detectors with severalintegrated sensors and intelligentmeasurement value processing.

Limit monitors have a preset thres-hold for the detection limit. Whenthis threshold is exceeded, an alarmsignal is sent to the control centre.Intelligent fire detectors usually in-volve processor-based measure-ment value processing and send themeasured values or the status to thecontrol centre via a bus protocol.

The most popular smoke detectorsoperate on either a scattered lightprinciple or an ionisation principle.They involve different physical pro-cesses and consequently also re-spond to different aerosols.

2.1 Mono-criterion detectors

2.1.1 Heat detectors

Heat detectors detect the rise intemperature that is caused by a fire.In practice, heat detectors are used

Figure 1: Fire sensitivitytest of smoke

detectors

Early detection of fire phenomena characteristics

Application and experience of multi-sensor detectorsAUTHOR: DIPL.-ING., DIPL.-KFM. WALDEMAR OLLIK

The author of this article, Dipl.-Ing., Dipl.-Kfm. Waldemar Ollik,is Product Manager Fire at Novar GmbH,Neuss, Germany.Contact:waldemar.ollik@honeywell.com

1 Introduction

2 An overview of the types, construction and special features of multi-sensor detectors

Abbildungen 4 (links) und 5 (unten)

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in areas that cannot be monitoredby smoke detectors. This type of de-tector can be divided into twogroups that use different evaluationmethods:

❏ Heat detectors with a fixed temperature function

❏ Heat detectors with differentialresponse and a fixed temperature function

Heat detectors with a fixed tempe-rature function activate the alarmwhen a pre-set temperature has been exceeded. The response tem-perature must be significantly higher than the normal ambienttemperature to prevent the detec-tor from responding to temperaturerises caused by normal heating or di-rect sunlight.

Heat detectors with differential re-sponse also evaluate the rate oftemperature rise in addition to itsmaximum value.

2.1.1.1 Rate-of-rise detectors:

Advantages: Rate-of-rise detectorsare used where operating condi-tions cause small or slow tempera-ture fluctuations. They are there-fore suitable for areas where smokeor similar aerosols may be presentfor operational reasons but whichare also at risk of open and rapidlyspreading fires in the event of analarm.

Disadvantages: In the event of a fire, heat detectors respond only iftemperatures rise very quickly or ifthe static response temperature hasbeen exceeded. Heat detectors areunable to detect glowing or smoul-dering fires, for example.

2.1.1.2 Maximum temperature detectors:

Advantages: These detectors areused in areas where temperaturesfluctuate greatly and where the ex-ceeding of a temperature thresholdis to be interpreted as an alarm.

Disadvantages: If the measuredtemperature exceeds a specified value for a certain length of time,the alarm signal is initiated.

2.1.2 Ionisation detectors

The air in the sensor chamber is ionised by a mildly radioactive com-pound, the alpha emitter americium241 with an activity of less than 5 k Bq. A voltage is applied to the pinelectrode to produce a defined cur-rent flow. Smallest fire aerosolsattach to the ions and reduce thecurrent flow. The changed signal isevaluated for the alarm decision.

Ionisation smoke detectors canidentify both dark and light aerosolseffectively. They are also able to de-tect invisible fire aerosols.

Their disadvantages include the factthat the measured value is highly de-pendent on airflow and that the sys-tem requires a radioactive ionisationsource, typically a radionuclide.

2.1.3 Optical smoke detectors

Optical smoke detectors using ascattered light principle consist of atransmitter LED and a receiver pho-to diode. The diodes are arranged ata defined angle to each other andoptically separated by a screen.

In the event of a fire, part of the lightbeam from the transmitter LED isscattered diffusely onto the receiverwhen fire aerosol particles enter thedetector. The scattered light causesan increase in the signal at the re-ceiver. The incoming signal is eva-luated.

Optical detectors respond particu-larly well to visible smoke and areable to detect a fire reliably even athigher wind speeds. The detector isespecially suitable for the detectionof smouldering fires, light smoke,open fires such as plastic fires orsmoke-emitting liquid fires.

The disadvantage of optical detec-tors is their sensitivity to fog or moisture, which promote the for-mation of droplets. Insects enteringthe detector can trigger falsealarms. Optical sensors can only de-tect visible aerosols.

The advantages and disadvantagesof fire detectors with a single sensorled to demands for a multi-criteriafire detector relatively early on.

However, this only became feasibleas a result of the rapid developmentof microprocessor technology in thecomponents industry and the intro-duction of SMD technology in theproduction sector.

2.2 Multi-sensor detectors

2.2.1 OTI detectors

One of the first multi-sensor detec-tors was the OTI multi-sensor detec-tor, which contained three differentsensors. It was introduced at the same time as the OT multi-sensordetector at the Security Fair in 1990.

The OTI multi-sensor detector con-sists of an optical smoke detector, aheat detector and an ionisation de-tector (OTI = optical, thermal and ionisation sensors). The conditionsof the individual sensors are recor-ded and compared to each otherusing a complex algorithm beforean alarm decision is made. This idea

Figure 2: Ionisation smoke detector

Figure 3: Optical smokedetector

+ E l e c t r o d e(pin)

Ions

Aerosol particles

IsolatorReference chamber

Measurement chamber

Ionisation compound

α radiationIon deposits

Smoke particlesIndividual displayand test diode

Scattered

Receiver opt. part

Trans-mitteropt. part

light

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was borne from the fact that the wider the range of informationcollected the more reliable the deci-sion as to whether a fire has actual-ly developed would be.

2.2.2 O2T detectors

Another milestone in the develop-ment of multi-sensor detectors isthe O2T detector, which was laun-ched in 2000. Like OTI detectors, O2Tdetectors contain three differentsensors: two optical smoke detec-tors (O2) and a thermal sensor (T).

The advantage of O2T detectorscompared to conventional scatteredlight detectors is their two-angletechnology.

All popular scattered light detectorsuse infrared light with a wavelengthrange between 800 nm and 1 μm.The light is emitted by an IR trans-mitter diode and the scattered lightis measured by a receiver that hasbeen adjusted to this wavelength.Scattered light detectors basicallydiffer in the construction of their ex-ternal casing and the selection ofthe scattering angle. Most scatteredlight detectors operate within a

scattering angle range between 90°and 180°. This range is referred to asforward scattering while the rangebetween 0° and 90° is referred to asbackward scattering. The closer theangle gets to 180°, the higher thewanted signal. A large scatteringangle produces a higher signal yieldfor light smoke, however, the signalyield for dark smoke is lower by afactor of ten.

Scattering angles of less than 90°,i.e. backward scattering, reduce thetotal signal yield. However, the dif-ference between dark and lightsmoke is now considerably less thana factor of ten. Accordingly, the dif-ferences between light and dark types of smoke are getting smaller.Particles, dust or salt crystals gene-rate a significant backscattering sig-nal in detectors using backwardscattering. This phenomenon cau-ses problems with these detectorsand may result in false alarms.

Most optical smoke detectors use ascattering angle over 90°.

For this reason, modern scatteredlight detectors are not able to detectdark aerosols, such as those genera-

ted by liquid fires, e.g. diesel, oil, pe-trol and heavy or medium-weighthydrocarbon compounds, as effec-tively as light aerosols, such as those generated by a glowing orsmouldering fire involving wood orcotton, for example. Scattered lightdetectors are very sensitive to lightsmoke, e.g. smouldering fires, to en-sure that the alarm is activated in time. As a result, light aerosols, suchas steam, cigarette smoke, vapoursreleased by operating processes,dust, process-based aerosols, ex-haust gases and hot fat vapours, cantrigger a false alarm.

Whatever the chosen scatteringangle, it is impossible to distinguishbetween different types of smokeon the basis of a singular signal alone. However, this distinction isnecessary if the false alarm rate is tobe improved. Conventional me-thods offer no solution since the detector has to be adjusted to theaerosol that is the most difficult todetect.

The O2T multi-sensor detector solvesthis problem by using two scatteredlight signals that operate at differ-ent angles. The lights are arrangedso that one scattered light path isvery effective at detecting light aerosols and the other is very effec-tive at detecting dark aerosols. Thedifferent types of smoke can be distinguished by correlating themeasured results of each path. Thetype of smoke can then be deter-mined by a microprocessor that isintegrated in the detector. The re-sult is practically constant sensitivi-ty in the presence of different aero-sols and much lower false alarm rates since the resultant sensitivityis much lower than that of conven-tional scattered light detectors.

The application of two-angle tech-nology in O2T detectors makes itpossible to identify certain sourcesof false alarms, such as steam ordust and vapours from operatingprocesses, and clearly distinguishthem from smoke.

Interference factors can be furtherreduced by the applied algorithm.Known interference factors caneven be eliminated completely by

Figure 5: O2T multi-

sensor detector

Figure 4: OTI multi-

sensor detectorOptical sensor chamber

IsolatorReference chamber

Ionisation chamber

Heat detector

Receiver opt. part Transmitter

opt. part

Transmitter

Trans-mitterReceiver

Smoke particles

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parameterisation. These featuresare not available with conventionalscattered light detectors, even those fitted with an additional ther-mal component (OT detector), be-cause of their "optical one-dimen-sionality".

2.2.3 OTG detectors

Almost all fires generate the invi-sible and odourless toxic gas carbonmonoxide (CO) during their initialphase. Smoke poisoning is the mostcommon injury among fire victims.

95% of all fire victims are injured du-ring the smouldering phase of the fi-re. This risk is particularly high at

night, and most fire victims suffo-cate in their sleep. Every examina-tion of fire victims has so far shownthat carbon monoxide was the pri-mary cause of death.

In addition to smoke and thermalsensors, OTG multi-sensor detectorsalso contain an integrated CO de-tector. Through the early detectionof fire gases, the OTG detector is ab-le to detect a fire before it becomesvisible. The system can also activatethe alarm when it detects a life-threatening concentration of odour-less carbon monoxide.

2.2.4 OTblue detectors

Instead of an infrared light source,this type of detector uses a blue diode that emits very short wavelight. The much shorter wavelengthmakes it possible to detect muchsmaller particles that are invisible tothe human eye.

OTblue detectors can be used in areasthat are currently monitored by ionisation detectors. They are ableto detect liquid fires, open wood fires, invisible aerosols and evenparticles that previously could onlybe detected by ionisation detectors.Compared to ionisation detectors,they are far less susceptible to inter-

Figure 6 (farleft): Greatlyimproved signaldetection forlight aerosolsdue to the largeamount of for-ward scattering

Figure 7 (left):Greatly im-proved signaldetection fordark aerosolsdue to the largeamount ofbackward scat-tering

Test fire: Steam

In contrast to conventio-nal scattered light detec-tors, O2T detectors do nottrigger an alarm whenexposed to steam.

They also detect darksmoke reliably and ac-tivate the alarm earlierthan other detectors.

O2T detectors detect firesemitting light smokemuch earlier and more re-liably than conventionalscattered light detectors.

Test fire: n-Heptane Test fire: Cotton

Figure 8: Detection byO2T multi-sensor detectorsduring a testwith differenttest fires

Smoke particles Smoke particlesTransmitter

TransmitterReceiverReceiver

Transmitter

Transmitter

Forward scattering Backward scattering

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This section compares multi-criteriafire detectors to optical detectorssince they represent the most popu-lar type among mono-criterion de-tectors.

Figures 11 and 12 show the steady rise in market share of multi-criteriafire detectors in Germany and inter-nationally. Between 2002 and 2006,their share rose from approximate-ly 24% to approximately 32%.

The level of distribution on the Ger-man market (figure 12) is much hig-her. The share of multi-criteria firedetectors increased from approxi-mately 38% to approximately 57%

ference factors, such as airflow andmoisture and do not require a radio-active source.

The following example illustratesthe advantages of optical smoke de-tectors with a blue LED over thosewith conventional infrared LEDs.

The development of multi-criteriafire detectors in Germany can be illustrated on the basis of figuresfrom Novar GmbH.

The figures refer only to intelligentbus-powered fire detectors. Con-ventional limit monitors are not in-cluded in the statistics.

between 2002 and 2007. The mar-ket share is not just higher at pre-sent but also continues to show higher growth rates.

The differences in the market sharesof multi-criteria fire detectors inGermany and abroad (figure 11) arepartially due to the fact that differ-ent and less stringent standards ap-ply abroad. For example, manycountries do not require the fire de-tection systems to be linked to thefire brigade. Higher costs are an-other important factor that makesmono-criterion detectors the pre-ferred option.

Figure 10: Advantages of optical smoke detectors with a blue diode com-pared to those with conventional infrared LEDs

Figure 9: OTblue detector

Figure 11:Worldwide

distribution ofmulti-criteria fire detectorscompared to

optical detec-tors (Novar

GmbH)

Red = Multi-criteria

fire detectorsBlue = Optical

detectors

Paraffin oil mist=> Large particles

– Similar response to blue and infrared light

– Slightly more sensitive to infrared light

Smouldering cotton fire => Small particles

– Significant differences in response

– Three times more sensitive to blue light

– This ratio becomes even more evident with even smaller particles

3 The development of multi-criteria fire detectors compared to mono-criterion detectors

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Technical progress in microproces-sor technology with regard to per-formance and cost has led to the in-tegration of microprocessors in firedetectors. As a result, different sen-sors could be combined in a singlemulti-criteria fire detector for thefirst time. This also led to the intro-duction of intelligent measurementvalue processing in point fire detec-tors.

The resulting advantages comparedto conventional detectors includethe following:

❏ Interference impulses can be sup-pressed or filtered out by pre-filte-ring raw measured values using dif-ferent methods❏ Processor-controlled decentra-lised signal processing❏ Alarm decisions based on intelli-gent algorithms❏ Pattern recognition of fire pheno-mena characteristics ❏ Automatic detector adjustmentto various ambient conditions❏ Tracking of response values❏ Optimum self-diagnostics of elec-tronic and sensor systems ❏ Early detection of sensor conta-mination ––> prevention of falsealarms❏ Automatic maintenance requests

❏ Parameterability and hence opti-mum sensor adjustment to ambientconditions❏ Time-controlled switching be-tween different parameter sets de-pending on operating conditions❏ Sensors can be evaluated indivi-dually or collectively❏ Virtually constant sensitivitywith regard to detecting differentcombustible materials ❏ Partial suppression of interfer-ence factors

The advantages of multi-criteria firedetectors are demonstrated by ear-ly fire protection and a significantreduction in false alarms.

Is there any supporting evidence forthis claim? Unfortunately, there areno statistical evaluations that per-mit an assessment of multi-criteriafire detectors with regard to the de-velopment of false alarms.

According to fire brigade statisticson false alarms and nuisancealarms, the number of false alarmshas remained constant or has evenfallen to some extent despite a con-tinuous increase in the applicationof fire detection systems (source:"False Alarm Statistics on Fire De-tection Systems", Jürgen Weiß, VdSconference on 9 December 2004).Multi-criteria fire detectors havecertainly played an important role in

this positive development. The im-proved reliability of fire detectionsystems can be divided into differ-ent categories:

❏ Equipment and installation stan-dards or specifications (DIN EN 54equipment standard, DIN VDE 0833system development, DIN 14675Planning – Installation – Mainten-ance)❏ Higher available technology ofthe overall system❏ More professional system plann-ing and installation❏ High-quality maintenance tasksincrease the reliability of the systemover its entire operating time

Areas of application for O2T detectors

Typical areas of application for O2Tdetectors include:

❏ Factories with high ceilings anddust or aerosol formation duringproduction processes and machineusage, e.g. forklift trucks (figure 13)❏ Big events with changing stageequipment and different activities,e.g. use of fog machines and smokedevelopment (figure 14)❏ Industrial kitchens with steamand cooking vapours.

Figure 12: Distribution ofmulti-criteria fire detectorscompared tooptical detec-tors in Germany(Novar GmbH)Red = Multi-criteria fire detectorsBlue = Opticaldetectors

5 Practical examples

4 Advantages of multi-criteria fire detectors

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Each object is subject to specific in-terference factors that can causeconventional scattered light detec-tors to trigger a false alarm, e.g.steam from paper rolls in printingplants, paper mills, shower cubiclesin hotel rooms and microparticlesfrom humidifiers in museums.

Typical areas of application for OTGdetectors include:

❏ Hospitals or nursing homes wheredeveloping gases must be detectedas early as possible because bedrid-den or disabled patients may not beable to react quickly (figure 15). ❏ Underground garages that requi-re an alarm for excessive gas con-centrations due to high parkingdensity or inadequate or defectiveventilation systems.

The introduction of multi-criteria fire detectors was an important steptowards even more reliable fire de-tection with point detectors.

We know that fires not only gene-rate smoke, heat and flames but al-so produce fire gases. Since fire ga-

ses are produced during the earlystage of the fire before smoke andtemperatures develop, there is a po-tential here to detect fires immedi-ately after ignition and consequent-ly gain even more valuable firefighting time.

The first systems with gas sensorsare already available, e.g. OTG mul-ti-criteria fire detectors with elec-trochemical cells that detect carbonmonoxide (CO) as described above.Other gases can be measured withthe appropriate sensors but the re-sult is only the selective detection ofindividual fire gases. Fires are basedon certain fire gas mixtures, or firegas patterns, which contain differ-ent gases in different concentra-tions. The aim of a multi-gas sensorsystem, the electronic nose, is toclearly identify these gases.

There are many cases where natureshows us how it is done:

❏ Humans can sense and distin-guish odours. People are able tosmell danger such as fire withouthaving to see it.❏ The Black Fire Beetle (Melanophi-la acuminata) has a natural multi-criteria fire detector. This beetle isable to smell guaiacol over large distances and heads towards the lo-cation of the fire using IR sensors.

Learning to smell fire gases is one ofmany challenges facing the deve-lopment of future fire detectors.

Figure 13: Factories with

high ceilingsand dust or

aerosol formati-on during

production processes and

machine usage,e.g. forklift

trucks

Figure 14: Big events often involve changing stage equipment and the use offog machines and smoke development

Figure 15: In hospitals and

nursing homesthe formation

of gas develop-ment has to be

detected as ear-ly as possible

because of longreaction times

6 Potential of future developments