An Introduction to Air Sampling Detectors
What are ASD detectors?
ASD is an acronym for ‘air sampling detectors’. They use a network of pipes to draw in air from a specific area, through a central detection unit. Popular brands include VESDA and Honeywell ASD detectors
How they work
The air sampling pipes run parallel to each other along a ceiling, covering a certain area.
A range of holes are equally distributed along each sampling pipe, forming a matrix. This means that air is evenly drawn from the entire area, rather than a concentrated point.
The air samples are then drawn into a detection chamber, which will detect combustion particles in the air.
There are three types of aspirating smoke detector available: light scatter, cloud chamber and particle counting.
Ideal for harsh environments
Aspirating systems are ideal in adverse environments. This is because they can cool air or heat air, and filter out dust.
Most ASD systems are incredibly sensitive, allowing for a fire to be detected in the early stages. Often smoke can be detected before the fire is fully formed. This is because ASD systems can have smoke sensitivity levels as low as 0.05%.
This makes them ideal in areas which contain goods easily damaged by fire, such as tobacco, electronic rooms and highly flammable liquid and gases.
Pipe length and hole sizes
The number of inlet pipes and the length of these pipes will determine the area protected. Make sure that when purchasing, ensure that you get this right.
Each pipe is punctured with sampling holes, through which air is drawn. The size of these holes is important. There are two options:
Option one: Make all holes the same size
This makes it easy for site engineers, but it can cause other problems. The airflow through each sampling hole is affected by air pressure travelling through the pipe. More air will be drawn through holes closest to the detector.
Option two: Make the holes different sizes
To counteract the effects of air pressure and other variables, you can make the air sampling holes different sizes. This means that the same amount of air will be drawn from each point. It does make life harder for site engineers, though.
Light scatter ASD detectors
A high-energy light source is pulsed in the detection chamber. Smoke particles drawn into the chamber will cause the light to scatter. The quantity of scattered light indicates how much smoke is present.
• These detectors are sensitive to slow-burning fires
• They’re ideal when fast detection is needed
• They respond to dust and are susceptible to causing false alarms
• To minimise this risk, dust filters can be used
Cloud chamber ASD detectors
The detection chamber contains water vapour. Vapour will condense around smoke particles, producing droplets of an equal size. The number of these droplets gives an indication of how much smoke is present.
• Unlike light scatter ASD detectors, cloud chamber detectors are resistant to dust
• They respond well to fast, flaming fires
• They show a poor response to slow, smouldering fires compared to some other detectors
• This means that their use for early warning detection is limited
• Cloud chambers consume water, meaning they require regular maintenance
Particle counting ASD detectors
A stream of sampled air is continually drawn through a focused laser beam. Light scattered from smoke particle is measured to show the number of smoke particles that have crossed the laser beam.
• Particle counting systems are sensitive to smouldering fires
• They’re sensitive to overloaded cables
• They’re generally resistant to dust
• Their air flow needs to be regulated
• Fibres or large volumes of dust have been known to cause false alarms