If average duct velocities, or mass or volumetric flow rates are required, it
is necessary to perform a Pitot traverse of the duct. This involves taking
measurements at various positions across the duct. Before a traverse is
conducted, it is necessary to select a suitable location to perform the survey.
If possible, avoid traverses close to fans, dampers pipe bends, expansions etc.
Try to survey at least 8 duct diameters downstream of the aforementioned
elements and 2 duct diameters upstream of these elements. The survey is
performed with the aid of Fig. 3. Either the Centroids of Equal Areas or
Log-Tchebycheff point distribution may be used. A survey proceeds as follows:
- Decide on the number of survey points and then mark these on
the Pitot tube using a marker or adjustable spring clips (present on some
Pitot Static tubes).
- At the selected survey location, drill two perpendicular holes
in the duct (for a round duct) or the desired number of holes for a
rectangular duct, ensuring sufficient hole clearance to safely insert the
Pitot Static tube.
- Partially insert the temperature/RH sensor in an additional
hole located close to the previously drilled holes.
- Connect Pabs to a static pressure tap/ring close to the survey
location, or use a "T" barb to connect to the static Pitot tube
port, see sketch above.
- Carefully insert the Pitot Static tube into the duct and
position at the first traverse location. Ensure that the Pitot Static tube
is aligned with the axis of the duct using the alignment guide on the tube
as a reference.
- Wait for the readout on the display to stabilize. If the
readout continues to oscillate increase the damping (DAMP). If the
magnitude of the oscillations is greater then 25%, then another measuring
point should be considered as the results may not be representative.
- When stabilized, record the desired reading(s).
- Move the Pitot Static tube to the next traversing point and
repeat 5 and 7 until the traverse is complete.
- Repeat points 5-8 for the other traverse locations.
Once the traverse has been completed, the
volumetric and mass flow rate through the duct can be calculated as follows:
Volumetric flow rate (Q):
where:
Aduct is the duct cross sectional area.
n is the number of points (total number of points surveyed).
Vi is the indicated velocity at each measurement point.
Thus, using a Centroids of Equal Areas or Log-Tchebycheff point distribution
allows the velocity measurements to simply be summed and averaged.
Mass flow rate (m):
where: p is the density of the gas in the duct.
NOTE: Assuming fully developed turbulent
flow with low air swirl (rotation), i.e. after a
long section of duct, the average duct velocity may be estimated using a single
Pitot reading at the center of the duct. The average velocity is then
approximately 0.9 of this reading with an accuracy of ±5%.
