FB-AP Multisensor
The FB-AP Series multisensor detector contains an optical smoke sensor and a thermistor temperature sensor whose outputs are combined to give the final analog value. The way in which the signals from the two sensors are combined depends on the response mode selected. The five modes provide response behavior which incorporates pure heat detection, pure smoke detection and a combination of both. The multisensor is therefore useful over the widest range of applications. The multisensor construction is similar to that of the optical detector but uses a different lid and optical mouldings to accommodate the thermistor temperature sensor. The sectional view (Fig 5) shows the arrangement of the optical chamber and the thermistor. The signals from the optical smoke sensing element and the temperature sensor are independent, and represent the smoke level and the air temperature respectively in the vicinity of the detector. The detector’s microcontroller processes the two signals according to the mode selected. When the detector is operating as a multisensor (i.e. modes 1, 3 and 4) the temperature signal processing extracts only rate-of-rise information for combination with the optical signal. In these modes the detector will not respond to a slow temperature increase – even if the temperature reaches a high level. A large sudden change in temperature can, however, cause an alarm without the presence of smoke, if sustained for 20 seconds.
The processing algorithms in modes 1 to 4 incorporate drift compensation. The characteristics of the five response modes are summarized below.
Mode 1 has very high smoke sensitivity combined with high temperature sensitivity. This gives a high overall sensitivity to both smoldering and flaming fires.
Mode 2 has a smoke sensitivity similar to that of a normal optical smoke detector but has no response to temperature. This mode is therefore equivalent to a standard optical detector. It is suitable for applications in which wide temperature changes occur under normal conditions.
Mode 3 has moderate smoke sensitivity combined with a moderate sensitivity to heat. This combination is considered the optimum for most general applications since it offers good response to both smoldering and flaming fires.
Mode 4 has lower than normal smoke sensitivity combined with high heat sensitivity. This makes it suitable for applications in which a certain amount of fumes or smoke is considered normal.
Mode 5 has no smoke sensitivity at all, but gives a pure heat detector response meeting the response time requirements for a Class A1 detector in the European standard EN54–5:2000. In this mode the detector will respond to slowly changing temperatures and has a “fixed temperature” alarm threshold at 58°C. The analogue value in this mode will give the approximate air temperature over the range 15°C to 55°C.
In mode 5, the smoke sensor is still active though it does not contribute to the analog signal. As a consequence, if the detector is used in a dirty or smoky environment the optical sensor drift flag may be activated in the heat-only mode.
Note: in situ testing of the multisensor detector should be carried out as for smoke detectors in response modes 1-4 and for heat detectors in response modes 5.
Design Note: if the multisensor is to be used in mode 5, heat detector spacing/coverage should be applied.