Dwyer 670 - Manual (Page 15)

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2.2 Calibration Notes:
1. The two calibration methods are intended to give the installer two options for calibrating the
alarm.
2. 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.
3. The second method of Two Point Capture is slightly more involved but does not require any
access to the ductwork or fan speed controller and is generally a quicker solution.
The principal behind this method is as follows:
The output from the airflow sensor is linear over the normal operating range of the face
velocities on fume hoods. If we therefore capture the airflow sensor output at two known
points we can then calculate the alarm point and give an alarm when the airflow falls below
this value. However because the 670 does not have a digital display of the face velocity we
can not enter particular values into the alarm. For this reason we take the normal operating
value of the face velocity with the sash open to the max safe working opening and call this
100% -- this point is captured during the first sample. Then by closing the sash it is possible
to create a face velocity which is one and a half times the normal operating value, i.e., 150%
- this point is captured during the second sample. We then use these two values to calculate
the sensor output at 80% and this becomes the Low Air alarm point. The actual value of the
normal operating face velocity is not important and in each case the alarm point will be 80%
of this value.
For example:
A normal face velocity of 100 fpm would give an alarm point of 80 fpm
A normal face velocity of 80 fpm would give an alarm point of 64 fpm
4. 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 center and is a convenient position to measure and for future
reference when checking the alarm during annual maintenance.
5. Take time when measuring the face velocities for the calibration procedure to allow for the
velocities across the open sash to stabilize. 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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