EARLY — BUT NOT A FALSE SMOKE ALARM How can the number of false alarms caused by dirty Duct Smoke Detectors be reduced without any reduction in sensit

1. This was an important starting-point when developing this completely new type of the smoke detector system described in this article.

 As the smokes from the fire enters the ductwork system it may be diluted by with clean air from other areas, therefore, it is necessary for any detectors to be very sensitive.

Unfortunately, in the past, high sensitivity has often led to false alarms and as a result confidence in these systems has fallen.

The common reason for false alarms from duct smoke detectors is dust contamination and high airflow rates.

To manage these problems we often been forced to lower the sensitivity of the detectors either by:

Fitting a filter on the detectors to limit the effect of dust and high air velocity. Without a filter the wind velocity may be to high.

With a filter the sensitivity will be greatly reduced and when the filter is clogged with dust the detector may not function at all. Placing the detector outside the duct and diverting the air/smoke mix through tubes to the detector and back again.

What have we achieved here?

If you take only a sample to the detector it may not be reprensentative of the main airflow but may solve the velocity problem.

This leaves the dirt problem, so if the detector is not maintained a false alarm will still result.

If you have difficulties detecting when the detector is in the ventilation duct, with access to all passing smoke, how difficult is it then to detect outside the duct where you only take a small part of the smoke and lead it out for detecting:

• You must know surely that it is possible both to detect smoke and to make it at different levels.
• You can’t mix the sensitivity of detection with different elements that cause false alarms.

 A Duct Smoke Detector called TDD (True Duct Smoke Detector) is able at a very early stage in the rough environment of a ventilation duct without giving false alarms.

TDD is an optical obscuration designed to function inside the ventilation duct.

The sensing part of the detector covers as much as possible of the duct.

A infrared beam is transmitting through the sensing tube in the duct and if smoke present the beam is obscured to some extent and an alarm signal is given.

The TDD-detector has an adjustable level of sensitivity that goes from 3 to 25% (0,1 to 0,9 db) obscurity for all different environments that can exist in a duct.

You may ask, how does the TDD cope with dust?

It surely must affect this detector as well.

Dust is always present in the return air duct as is air velocity.

The air will inevitably bring dust and this will stick to the detector optical parts, however, the detector electronics constantly monitor the obscuration within the sensing tube.

The air brings along dust and particles that of course also catches of the TDD-detector.

Despite it manage to be dirty without making a false alarm and without loosing its sensitivity and as the level changes slowly over a long period of time (Weeks or Months).

The signal is adjusted to compensate for this build up of dust, however, should the obscuration change quite rapidly (Seconds) this is signalled as smoke and will cause an alarm after an initial delay to compensate for drops of water or dust that disturbed from duct wall etc.

Self compensating

Eventually even the TDD will become dirty, how long time this takes is a function of the environment in which the detector is used.

The internal calibration of the detector is provided at an outgoing terminal in order that the level may be seen if connected, for example , to a BMS system.

This signal is 2 to10 volt with 3 volt being clean 10 volt dirty or in alarm and 7 volt being service indication.

Modern smoke detectors are often difficult to clean, some needing special knowledge due to the radioactive nature of the devices used with consequential costs involved in special instrumentation required to recalibrate them after cleaning.

Normal maintenance staff without previous training can clean TDD.

If they clean a pair of glasses they can clean the TDD it is the same technique.

This provides a better economic solution for the low life cost (LCC) for the end users.

Adjustment of TDD to different supervision system

TDD is constructed to be connected to all in the market existing Fire Alarm- and BMS- system, both digital and analogue.

As standard there is two alarm output with an open switch for smoke alarm and one analogue 2-10Volt for an output service alarm.

The output service alarm can for example be connected to system that are computerized and where you can see the dirtiness 0-100% on a BMS or Webb.

The option variant of the detector has 2 output relays for smoke alarm.

One example for how to use the two alarm output can be that alarm output 2 is used for control of fire dampers and valves and gives a local alarm signal while output 1 is used for external alarm signals, suitably for fire defence.

Reset the alarm signal

The detector can be reset by pressing the reset button (front panel) for one second or by interrupting the supply voltage (Remote reset).

The detector can also be reset automatically when the around obscurity ended.