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mail: [email protected]
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SD
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D
Duct Smoke Detector
Issue Number 7.1
30/09/2021
Four Elms Road
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Edenbridge
Kent
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TN8 6AB
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UK
2 of 3
How It Works
The sensor works by using an infra
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red transmitter and receiver fitted to the PCB inside of the housing. The transmitting device sends a beam,
which is reflected from a mirror in the end of the 300mm long probe, and the receiving device then receives this beam when there are no ob-
stacles, like smoke partials in the tube. If smoke is present, then this will disturb the beam, and the receiver will lose the signal and an alarm
will be activated.
Calibration at start
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up
On power up the SD
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D will start an automatic calibration procedure, the power from the TX device will increase in small steps, just until the RX
device receives enough signal to validate the
“
link
”.
This is stored in the volatile memory for any further use, or until a new power on. Now any
obstacles, like smoke will force an alarm.
Periodic calibration
The mirror over time may lose the possibility to reflect the full IR light level due to a contamination build up, such as dust. Periodic calibration is
carried out automatically by the smoke detector every 7 days to compensate for this.
Recommendations
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The SD
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D has a probe length of 300mm and should NOT be used in ducts with a diameter or square size greater than 450mm, unless
the smoke detection is intended as a secondary non
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life safety action with areas served being covered by smoke detection connected
to a fire alarm system that is able to shut
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off the ventilation fans, as it will not monitor the whole of the duct.
•
Must be mounted AFTER filters and humidification plant, this will prevent any spurious alarms caused by dust or water vapour.
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Units should NOT be mounted where ambient sunlight will fall on the photocell.
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Units should never be mounted at or near sharp bends in ductwork, as turbulence and side currents can carry the smoke away from
the probe.
•
The SD
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D is an optical beam detector and therefore, air velocity has little effect on performance. However, careful positioning of the
detector is required if optimum detection is to be achieved. The detector should always be fitted with the holes in the sensing tube par-
allel to the airflow.
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Units should not be mounted close to supply or extract fans.
•
With large installations, where several ducts collect into a common duct, consideration must be given to dilution of smoke and rate of
airflow. For example, 10 small ducts may feed at different points into one large main extract duct, and if only one smoke detector is
fitted in the main extract duct, the position can arise where one duct is feeding smoke and nine are feeding clean air. The amount of
smoke is therefore diluted by 90%, and if this is coupled with a high flow rate, the smoke may not be detected for the minimum operat-
ing period of 1 second, resulting in no alarm being generated. It may be necessary to mount 10 detectors in each of the small ducts to
guarantee correct operation.
