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Air Flow Rate Measurement using a Pitot Static Probe

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. 2 below. This table is printed on the inside of all the FKT series instruments for quick reference. Either the Centroids of Equal Areas or Log-Tchebycheff point distribution may be used.

Our program FlowScan automates the acquisition of survey data for any of the FKT series manometers. It calculates the locations for the survey and integrates the data into flow rates for any rectangular or circular duct.

The FKT 1DP1A-SV is designed with a duct survey mode where it automatically indexes and integrates the pressure readings into mass and volumetric flow rates.

Using our Rapid Averaging Probe (RAP) can significantly speed up the duct survey process because it only needs a fraction of the number of readings compared to point readings using a Pitot-Static probe to get the same results.

Typical pressure connections for a Pitot-static probe

Fig. 1 Typical pressure connections for a Pitot-static probe

A survey using a Pitot-static probe proceeds as follows:

  1. 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). See Fig. 2.
  2. 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.
  3. Partially insert the temperature and RH sensors in an additional hole located close to the previously drilled holes.
  4. 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 Fig. 1.
  5. 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.
  6. 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 than 25%, then another measuring point should be considered as the results may not be representative.
  7. When stabilized, record the desired reading(s).
  8. Move the Pitot Static tube to the next traversing point and repeat 5 and 7 until the traverse is complete.
  9. 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):

Volumetric flow rate

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 (mdot):

Mass flow rate

where: ρ is the density of the gas in the duct.

For specifics regarding validation of surveys, etc, the following references are suggested: (1) ASHRAE. 1988. Practices for measurement, testing, adjusting and balancing of building heating, ventilation, air-conditioning and refrigeration systems. Standard 111-1988, Atlanta, GA and (2) AABC. 1989. National standards, 5th ed., volume measurements. Washington, D.C.

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%.

Point distribution for rectangular and circular duct surveys.

Fig. 2 Arrangement of survey points for rectangular or circular ducts

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