Using a Pitot-Static Probe

Overview

On this page, use of a Pitot Static tube, in conjunction with our FKT series meters will be explained. However the methodology presented may be used in conjunction with any differential pressure measuring device. A Pitot Static tube allows the direct measurement of dynamic pressure allowing calculation of the gas velocity in ducts, pipes, wind tunnels etc. A typical Pitot Static tube is shown in Fig. 1.

Fig. 1 Generic Pitot-Static Pitot Configuration

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. Thus the gas velocity may be expressed as:

(1)

where ρ is the gas density and C is a correction constant dependent on the design of the Pitot Static tube. NOTE: This equation is typically valid for incompressible (constant density) flow. High velocities (V) will lead to increasing errors as shown in Table 1.

Table 1. Velocity error due to compressibility

V,m/s (ft/s)	25 (82)	50 (164)	75 (246)	100 (328)	125 (410)	150 (492)	200 (656)	250 (820)
Error, %	+0.07	+0.27	+0.59	+1.06	+1.66	+2.40	+4.28	+6.74

When selecting a Pitot Static tube to be used in conjunction with the FKT Series, 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. The use of Fig. 2 to find the constant C for a given Pitot Static tube will be illustrated with an example.

Example:

A standard round nose Pitot Static tube has static orifices located 2D from the base of the tip and 10D from the stem’s center-line. What is the correction constant C? From Fig. 2, the tip error is –1.4% and the stem error is +0.8%. The net error is –0.6%. Thus the indicated dynamic pressure will be too high. The correct dynamic pressure and velocity is then:

Pdynamic-correct=(1-0.6%/100%)=0.994 and

Vcorrect = (0.994)1/2Vindicated = 0.997Vindicated

Thus by inspection, for this tube, C = 0.997.

To simplify determination of the constant C, Table 2 may also be used, which shows the constant for various Pitot tube geometric variations (for a standard round junction tube).

Table 2 Pitot Static tube correction constant C

Dist from Tip, x/D	2	2.5	3	3.5	4	6	8	10	12	14	16
Dist from Stem, x/D
2	1.023	1.025	1.026	1.028	1.029	1.030	1.030	1.030	1.030	1.030	1.030
4	1.006	1.007	1.009	1.010	1.012	1.013	1.013	1.013	1.013	1.013	1.013
6	1.001	1.002	1.004	1.005	1.007	1.008	1.008	1.008	1.008	1.008	1.008
8	0.998	1.000	1.001	1.003	1.005	1.005	1.005	1.005	1.005	1.005	1.005
10	0.997	0.999	1.000	1.002	1.003	1.004	1.004	1.004	1.004	1.004	1.004
12	0.996	0.998	0.999	1.001	1.002	1.003	1.003	1.003	1.003	1.003	1.003
14	0.996	0.997	0.999	1.000	1.002	1.003	1.003	1.003	1.003	1.003	1.003
16	0.995	0.997	0.998	1.000	1.001	1.002	1.002	1.002	1.002	1.002	1.002
18	0.995	0.996	0.998	1.000	1.001	1.002	1.002	1.002	1.002	1.002	1.002
20	0.995	0.996	0.998	0.999	1.001	1.002	1.002	1.002	1.002	1.002	1.002

The velocity indicated by the FKT Series would then be corrected by multiplication by C (for a non-unity Pitot Static tube).

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, temperature and relative humidity (RH), to allow the FKT meter to calculate the correct gas density. This is achieved by connecting a length of tubing from the Pabs port to the static port of a Pitot Static tube, provided C is approximately 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 tubing to the Pitot Static tube and the differential pressure transducer of choice (3 are available in the FKT 3DP1A, two in the FKT 2DP1A-C and one in FKT 1DP1A-SV). The “P+” connection 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-” connection, see figure above. 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.

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, temperature and RH must be measured simultaneously with the differential pressure measured by the Pitot Static tube for best accuracy. A “T” tubing barb (supplied) 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 through measurement of absolute pressure, temperature and RH. If the density is comparable, then simultaneous measurement of target flow density is unnecessary and the temperature and RH sensors can be left out of the test area.

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 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 RAP probe 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.

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).
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 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.

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: A_duct is the duct cross sectional area.

n is the number of points (total number of points surveyed).

V_i 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 ():

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%.

Fig. 3. Arrangement of survey points for rectangular or circular ducts