Comparison of Pitot Traverses Taken at Varying Distances Downstream of Obstructions

Abstract

This study determined the deviations between pitot traverses taken under "ideal" conditions--at least seven duct diameter's lengths (i.e., distance = 7D) from obstructions, elbows, junction fittings, and other disturbances to flows--with those taken downstream from commonplace disturbances. Two perpendicular 10-point, log-linear velocity pressure traverses were taken at various distances downstream of tested upstream conditions. Upstream conditions included a plain duct opening, a junction fitting, a single 90 degrees elbow, and two elbows rotated 90 degrees from each other into two orthogonal planes. Airflows determined from those values were compared with the values measured more than 40D downstream of the same obstructions under ideal conditions. The ideal measurements were taken on three traverse diameters in the same plane separated by 120 degrees in honed drawn-over-mandrel tubing. In all cases the pitot tubes were held in place by devices that effectively eliminated alignment errors and insertion depth errors. Duct velocities ranged from 1500 to 4500 ft/min. Results were surprisingly good if one employed two perpendicular traverses. When the averages of two perpendicular traverses was taken, deviations from ideal value were 6% or less even for traverses taken as close as 2D distance from the upstream disturbances. At 3D distance, deviations seldom exceeded 5%. With single diameter traverses, errors seldom exceeded 5% at 6D or more downstream from the disturbance. Interestingly, percentage deviations were about the same at high and low velocities. This study demonstrated that two perpendicular pitot traverses can be taken as close as 3D from these disturbances with acceptable (< or = 5%) deviations from measurements taken under ideal conditions.

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“…The pitot tube was connected via tubing to a handheld micromanometer (TSI, Inc., Airflow Meter 8386A, Shoreview, MN) for actual pressure measurements. This pitot traverse measurement location was sufficiently downstream of duct-segment fittings to be consistent with the 7 to 10 duct diameters recommended by the ACGIH Industrial Ventilation Committee [ACGIH 2007b] and well beyond the 3-duct-diameter minimum recommended by Guffey and Booth [1999] for two perpendicular pitot traverses. Six inches downstream from the pitot tube sampling location, a 3/16-in hole was drilled into the bottom of the duct to allow positioning of the omni-directional inlet of an aerosol spectrometer (Grimm Dust Monitors, Models 1.10x, Labortechnik GmbH and CoKG, Ainring, Germany) within the midstream of the duct airflow.…”

Evaluating the effects of elbows and duct size on the accuracy of hand-held Pitot traverse flow measurements

“…Since air pressure measuring devices, such as manometers, are much easier to calibrate than velocity measurement devices, (Guffey, 1999) it is more practical to determine velocities from measured velocity pressure instead of measuring velocities directly. Also, according to Cheremisinoff, (1988) there is no flow meter commercially available that can directly sense weighted mean velocities.…”

“…Obstructions to duct flow can be caused by elbows in the duct, material that is lodged inside the duct, or dents and any other deformations in the duct. In a uniformly round duct at ideal locations, velocity will be greatest at the center of the duct and decrease towards the inner surface of the duct (Guffey, 1999). This is due to friction slowing the air flowing close to the wall.…”

“…At locations that are 50D or greater downstream from these areas of disturbed air flow, the effects are negligible. As demonstrated by Guffey and Booth (1999), at distances closer than 5D, these errors can sometimes exceed 5% even when mechanical devices are used to carefully insert the Pitot tubes.…”

“…Research has shown that the accuracy of mean velocities determined from device-held Pitot traverses are within 2% under these good conditions (Guffey and Booth, 1999). Two perpendicular traverses at the same cross-section are generally more equal to each other far downstream of disturbances (Cheremisinoff, 1988) so that only one traverse is needed to obtain errors less than 3% (Guffey and Booth, 1999).…”