VDOT-ASD-400 Aspiration Detector
Velocity Detection
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VDOT-ASD-400 Aspirating Smoke Detector User’s Manual V1
Balance is generally expressed as a percentage. If all the sample points have the same amount of air
entering each sample hole then this is invariably described as a system with 100% balance.
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Relative sensitivity
The relative sensitivity of each air-sampling hole (assuming that all sampling points have been
calculated for an equivalent sensitivity i.e. 100% balance) can be calculated as a simple function of
the detector sensitivity and the number of sample points. For example, a 0.1% high sensitivity
detection device connected to pipe-work containing 40 equivalent air sampling points can be
regarded as a 4%/m system which can be considered as a Class C sensitivity system. Unless
otherwise stated in approval documentation a figure of better than 5%/m sensitivity per hole may
be applied.
Design Guidelines
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The ideal diameter of sampling pipes is OD: 25mm / ID: 21mm. Other sizes will often work but
will provide different response times.
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Maximum pipework limitation
The limitation of each pipe inlet of VDOT-ASD-400 when the aspirator speed set to 10 is 100m
pipe length with up to 18 holes alone the pipe (maximum pipe length) or 60m pipe length with
up to 25 holes along the pipe (maximum holes) under the requirement of 120s maximum
transport time. There are four pipe inlets for VDOT-ASD-400, so the total maximum pipe length
is 4x100m/4x18holes or 4x60m/4x25holes.
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It is not possible to maximize the pipe length and the number of sampling points at the
same time. To maximize the pipe length, the number of sampling holes must be reduced.
On the other hand, to maximize the number of the sampling holes on the pipe, the pipe
length must be reduced.
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Ideally, if the total length of sampling pipe is greater than 50 meters, then branch pipes should
be used. When using branched sampling pipes, care should be taken to achieve a reasonable
degree of balance (say within 10% of airflow) to ensure even suction from the pipes.
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Sampling pipes must have capped ends. The end cap should be drilled with a sampling hole
normally between 4 or 6mm diameter and free from burrs.
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Sampling holes should normally be 2-4mm diameter or as calculated by ASDFlow and free from
burrs.
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This guide holds true for average sampling pipe lengths, but if using long pipes (typically more
than 60 meters total), performance may be improved by making the sampling holes near the
ends slightly larger than those nearer the detector.
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Although by no means essential, it must be recommended that if in doubt, ASDFlow be used to
