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  • 8/10/2019 The Final Decade of the Twentieth Century Saw Many

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    Detectors which combine more than one smoke sensor are treated by the regulating bodies as

    smoke detectors having more than one type of sensor.

    Multi-sensor detectors that incorporate CO sensing as well are planned .The argument for fire detectors with two smoke sensors (ionisation, optical) and a heat sensing

    element is that they offer the best of both worlds, as far as smoke detection is concerned, i.e, the

    detection of large particle smoke with the optical chamber and the detection of small particlesmoke with the ionisation chamber.The argument for multi-sensor detectors with one smoke sensor (optical) and a heat sensing

    element is that the optical-heat combination performs, in practice, as well as an ionisation sensor,

    thus making the ionisation sensor redundant.

    Discovery multi-sensor detector Whatever the merits of multi-sensor detectors with ionisation

    smoke chambers as well as optical chambers, it is obvious that the inclusion of the ionisationsensor means that the detectors are subject to the same regulations of storage, transport and usage

    as simple ionisation detectors.

    Mode of operation In single-sensor detectors a signal is generated which corresponds to the

    amount of smoke or heat present around the detector. If the signal increases conventional detectors

    change to the alarm state at a pre-set threshold while analogue detectors transmit the signal forprocessing either within the detector or in the control panel.

    A multi-sensor detector has two or three signal inputs and these are processed in the detector using

    an algorithm written by the manufacturer for that particular type of detector and its associatedequipment.

    Different operating modes may be offered for particular environments. The range of modes and the

    method of selecting a mode will differ from one manufacturer to another. As an example, the

    Apollo Discovery multi-sensor has five operating modes that are selected by the system designeror commissioning engineer via the control panel. This detector has an optical smoke sensor and a

    heat sensor, and the detector can be

    selected to operate as a smoke detector only, a heat detector only or as one of three combinations of

    both. If one of these three combinations has been selected, the signals from each are considered inrelation

    to the other, for instance, the smoke detector can become more sensitive if the heat detector reportsan increase in heat.

    Manufacturers will supply guidance on which mode to choose or they may make engineers

    available who will programme the detector appropriately.

    Performance of multi-sensor detectors The performance of multi-sensor detectors is obviously ofparamount importance. If we say that we are seeking enhanced performance, we mean that we

    want continued reliable detection of real fires with increased resistance to false alarms- Tests were

    carried out at the EN54 fire test laboratory at Apollo's factory in Havant. The graphs show two

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    typical fires: one is overheating with large particle smoke (figure 1), the other is a flaming fire with

    small particle smoke (figure 2). These two fires represent opposite extremes of the test fire scale.

    The smouldering fire is detected initially by the optical detector, which reaches the alarm level injust under 21 minutes. The multi-detector sensor, however, requires only one minute longer to

    reach the same level. The ionisation detector, by contrast, fails to reach the alarm level in the time

    available for the test.At the other end of the scale is the n- heptane fire. The ionisation detector needs only 15 seconds toreach the alarm level. The multi-sensor detector takes about 1 min 30secs to reach the alarm level,

    but the optical detector only just manages to reach alarm in the time allotted for the test. Note that

    the response of the ionisation detector is such that it is easy to understand why ionisation detectorscan give false alarms.

    Taken together, it will be seen that the multi-sensor detector is the only one that performs

    satisfactorily at both ends of the fire scale.

    As far as the increased resistance to false alarms goes, the author is unaware of any publicationwhich documents a controlled experiment to determine whether or not multi-sensor detectors

    reduce the rate of false alarms. There is, however, anecdotal evidence that systems which have had

    a tendency to false alarms with single-sensor detectors have shown significant reductions in thefalse alarm rate, once multi-sensor detectors have been installed instead. This is, of course, exactly

    what the theory would predict, since multi-sensor detectors avoid the sensitivity extremes of both

    ionisation and optical detectors.

    ConclusionThere are no apparent disadvantages in the multi-sensor detector.

    They perform well; they use known and proven technology and they can be confidently retro-fitted

    where ionization detectors are no longer tolerated or permitted.Moreover, they can be expanded in future to include the new CO sensors that are emerging.


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