USING A PITOT STATIC TUBE
FOR VELOCITY AND FLOW RATE MEASUREMENT
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
A Pitot Static tube is shown in Fig. 1.
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Fig. 1 Generic Pitot
Static tube configuration.
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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:
where r 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 2.
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,
Distance from Pitot Static Tube base of
tip,
or centre-line of vertical stem to
static holes, Diameter (xD)
Fig.2 Effect of static pressure
hole location from Pitot Static Tube
stem or tip
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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:
To simplify determination of the constant C,
Table 3 may also be used, which shows the constant
for various Pitot tube geometric variations
(for a standard round junction tube).
Table 3 Pitot Static
tube correction constant C
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The velocity indicated by the FKT Series manometer
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 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.
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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.
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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:
