Air Speed Measurement using a Pitot Static Probe

Overview

On this page, use of a Pitot Static tube, in conjunction with our line of 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 speed in ducts, pipes, wind tunnels etc. A typical Pitot Static tube is shown in Fig. 1.

Measurement of Speed

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 speed pressure (Pdynamic) varies with the square of the gas speed. Thus the gas speed 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 speeds (V) will lead to increasing errors as shown in Table 1.

Table 1. Speed 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 a speed meter, it is necessary to select a tube with a constant close to unity, if errors in speed 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 speed 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 speed indicated by the meters would then be corrected by multiplication by C (for a non-unity Pitot Static tube).

Taking Measurements with the FKT Series Meter

To measure speed 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 speed range should be used for maximum accuracy. However, if in doubt as to the expected speeds, 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.