Alnor APM 150, Owner'S Manual, Page: 19 / 78

15
15
MAINTENANCE
Nylon Hoods
The hoods may be hand washed periodically using
cool water and mild detergent. When washing, do
not let the material come in contact with sharp
corners or other objects which may tear the hood.
Manifold
The manifold should be checked before using to
be certain that the sensing holes have not become
clogged with dirt or dust particles. Inspect for
leaks which may be caused by a poor connection
to the sensor housing through the long pieces of
flexible tubing. Also check to make sure that the
O-rings used to seal the tubes to the center hub are
in place.
The manifold may be cleaned using mild detergent
and warm water. Do
not
immerse the manifold in
water. It is recommended that cleaning be done
with the manifold in place, and with care.
APPENDIX A:
TRAVERSING A DUCT TO DETERMINE AVERAGE AIR VELOCITY OR
VOLUME (ONLY WITH A 175 OR 275 PROBE ATTACHED)
The following techniques can be used to measure
air flow inside ducts using a velocity probe.
Where to Take the Measurement
In order to make air velocity measurements in a
duct, it is best to measure at least 7.5 duct
diameters downstream and at least 3 duct
diameters from any turns or flow obstructions. It is
possible to do a traverse as little as 2 duct
diameters downstream or 1 duct diameter upstream
from obstructions, but measurement accuracy will
be impaired. When measuring rectangular ducts,
use the following formula to find the equivalent
diameter of the duct when calculating how far 7.5
diameters downstream or 3 diameters upstream is.
Equivalent Diameter =
4HV/Pi
Where: H = horizontal duct dimension
V = vertical duct dimension
Pi = 3.14
It is also possible to take a single reading to
measure air velocity or air volume flow in a duct,
measuring in the center of the duct and
multiplying the reading by 0.9 to correct for the
higher velocity at the duct’s center. If conditions
are very good, accuracy of
±
5 or
±
10 percent can
be obtained this way. This method is not reliable,
however, and should only be used where small
duct size or other conditions do not permit a full
traverse.
Traversing a Round Duct
Using the log-Tchebycheff method, the duct is
divided into concentric circles, each containing
equal area. An equal number of readings is taken
from each circular area, thus obtaining the best
average. Commonly, three concentric circles (six
measuring points per diameter) are used for ducts
with diameters of 10 inches or smaller. Four or
five concentric circles (eight or ten measuring
points per diameter) are used for ducts with
diameters of 10 inches or more.