1. Overview
In this article, use of a Pitot Static tube, in conjunction with a manometer will be explained. Reference will be made to the FlowKineticsTM LLC FKT series manometers, as these instruments greatly simplify velocity acquisition. The Pitot Static tube allows the direct measurement of dynamic pressure allowing calculation of the gas velocity in ducts, pipes wind tunnels etc.

2. Measurement of Velocity

The Pitot Static tube measures the total pressure (or impact pressure) at the nose of the Pitot tube and the static pressure of the gas stream at side ports. The difference of these pressures, i.e. the dynamic or velocity pressure (Pdynamic) varies with the square of the gas velocity.

When selecting a Pitot Static tube to be used in conjunction with the FKT Series (or any manometer for that matter), it is necessary to select a tube with a constant close to unity, if errors in velocity are to be avoided. If data for a particular Pitot tube is not available,

the constant C may be estimated. This constant is dependent on the spacing of the Pitot tubes' static pressure ports (see Fig. 1) from the base of the Pitot tube's tip and the stem's center line. Prandtl type Pitot tubes typically have constants C close to 1. Figure 2 shows the effect and error of the location of the static pressure tappings on the static pressure error.

The lower line gives the static pressure error associated with the distance of the static ports from the base of the tip, expressed in diameters. The upper line presents the static pressure error due to the distance of the static ports (expressed in diameters) from the stem center-line.

Taking Measurements with the FKT Series

To measure velocity with the instrument with the greatest accuracy, it is necessary to measure the target gases absolute pressure and temperature as well as Relative Humidity, to allow the FKT Series to calculate the correct gas density. This is achieved by connecting a length of Silicon or Tygon® tubing from the Pabs port to a wall static pressure tap (or averaging ring) at the measurement point location. Alternatively, the Pabs port may be connected to the static port of a Pitot Static tube, provided C » 1 for the tube. Temperature/RH is measured by partially inserting the temp/RH sensor into the duct/wind tunnel etc.

Measurement starts with attachment of Silicon or Tygon® tubing to the Pitot Static tube and the pressure transducer of choice. The "P+" connection barb of the transducer is connected to the Total pressure port of the Pitot tube, and the Static pressure port of the Pitot tube is connected to the transducers "P-" barb connection, see Fig. 1 and the picture below. The appropriate transducer for the expected velocity range should be used for maximum accuracy. However, if in doubt as to the expected velocities, use the largest pressure range available to avoid overloading. If using the FKT 2DP1A-C Series (which accounts for compressibility and displays accurate velocities up to approximately 250m/s), the ratio of the specific heats, g, must be set.

The Pitot Static tube can then be carefully inserted into the gas flow. It may be necessary to drill holes into the ducting for insertion. The absolute pressure and temperature/RH must be measured simultaneously with the differential pressure measured by the Pitot Static tube for best accuracy. A "T" tubing barb can be used to connect the static port of the Pitot Static tube to the P- port of the differential pressure transducer as well as the Pabs absolute pressure transducer, see the sketch below. A Pitot Static tube with C of approximately unity should be used when this type of connection is employed.

In many applications, the ambient density may be close to the target gas density. This can readily be determined using the FKT Series by recording the ambient density (displayed continuously), followed by the target gases density. The density will be calculated and autonomously presented by the FKT Series through measurement of absolute pressure, temperature and RH. If the density is comparable, then simultaneous measurement of target flow density is unnecessary, i.e. the temp/RH sensor can be left in its housing.

3. Pitot Static tube duct surveys
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:

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).
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/RH sensor 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 sketch above.
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 then 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.