d. When complete re-set the airflow to the normal value and the unit will
go to the Normal running condition with the Green LED on.
The unit will now function and go into the ALARM condition if the
Fume Hood face velocity falls below the alarm value.
7.1 Calibration Notes :-
1.
Using the Alarm Point Capture method it is necessary to adjust the
face velocity on the fume hood using a mechanical damper ( or fan
speed control if available) to the desired alarm point and this is
sampled by the unit. It is then necessary to re-adjust the face velocity
back to the normal operating value. This method produces a very
accurate alarm point at a fixed value but involves getting access to the
ductwork or fan speed controller
2.
The face velocity readings on the open sash may vary at different
points on the measuring grid by up to 20 fpm. This is quite acceptable
in terms of the fume cupboard performance so long as no individual
point is below the designated Low Air alarm point .The figure entered
for the calibration point can be taken as the average value of all the
measuring grid readings or could be taken as the individual lowest
point on the grid. For most fume hoods this low point is on the bottom
row in the centre and is a convenient position to measure and for
future reference when checking the alarm during annual maintenance.
3.
Take time when measuring the face velocities for the calibration
procedure to allow for the velocities across the open sash to stabilise.
If the velocities are changing or are turbulent during the sampling
period the alarm will detect this and give a low frequency buzz at the
end of the sample indicating that the sample must be repeated.
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