8.2. MEASUREMENT TOOLS AND TECHNIQUES
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sophisticated instruments that can obtain accurate velocities without disturbing the flow. The following sections briefly discuss several of the more
popular methods used to obtain point measurements of fluid velocities.
Pitot Tubes, Impellers, and Lagrangian Floats
In the case of unidirectional flow models there are a number of established
instruments and techniques to measure the flow field velocity. This section
discusses measurement techniques that are limited to unidirectional flow
situations.
Pitot tubes have been utilized in hydraulic model studies for a long
time. They are constructed of hollow tubing which can be as small as
hypodermic needle tubing, and they are placed in the flow and oriented so
the opening at the end of the tube directly faces the main flow. Connecting
the pitot tube to a manometer gives a measure of flow’s total head at that
point. Some pitot tube designs subtract out the static component in the
total head by including an opening in the tube that is shielded from the
moving flow. Once the dynamic component of the total head is isolated, it
can be expressed in terms of flow velocity. Pitot tubes must be calibrated,
and their use must be restricted to within the calibration range. Some pitot
tubes may not be suitable for low-speed flows because of the low flow head,
and they must always be aligned to the flow direction.
The advantages of pitot tubes include low fabrication cost and ease of
installation. The main disadvantage is the sensitivity of the pitot tube to
flow direction. Relatively minor changes in flow direction or small alignment
errors may produce erroneous velocities (Sharp 1981). Another disadvantage is the difficulty in automating the output from pitot tubes; therefore,
pitot tubes are most suited for steady or quasi-steady flow situations.
Another popular unidirectional flow measurement device is the impeller current meter. These instruments come in various sizes and
configurations, but they all feature some type of rotating impeller that
is driven by the fluid flow. Figure 8.2 shows one design for a miniature
impeller. Other designs use cups mounted around the axis of rotation. The
rotational velocity of the impeller varies with the flow velocity, and this
relationship must be established via calibration procedures.
Usually, sensors mounted near the rotating impeller count the number
of revolutions over a given time span (selected by the operator) and then
output this value to a recorder. Appropriate calibration factors are then
used to convert the reading to equivalent velocity. Impeller current meters
respond reasonably well to unsteady flows; however, rapid fluctuations, sue
as larger-scale turbulence, present difficulties because the impeller cannot
accelerate instantaneously, it will not indicate flow reversals, and the time
479
sophisticated instruments that can obtain accurate velocities without disturbing the flow. The following sections briefly discuss several of the more
popular methods used to obtain point measurements of fluid velocities.
Pitot Tubes, Impellers, and Lagrangian Floats
In the case of unidirectional flow models there are a number of established
instruments and techniques to measure the flow field velocity. This section
discusses measurement techniques that are limited to unidirectional flow
situations.
Pitot tubes have been utilized in hydraulic model studies for a long
time. They are constructed of hollow tubing which can be as small as
hypodermic needle tubing, and they are placed in the flow and oriented so
the opening at the end of the tube directly faces the main flow. Connecting
the pitot tube to a manometer gives a measure of flow’s total head at that
point. Some pitot tube designs subtract out the static component in the
total head by including an opening in the tube that is shielded from the
moving flow. Once the dynamic component of the total head is isolated, it
can be expressed in terms of flow velocity. Pitot tubes must be calibrated,
and their use must be restricted to within the calibration range. Some pitot
tubes may not be suitable for low-speed flows because of the low flow head,
and they must always be aligned to the flow direction.
The advantages of pitot tubes include low fabrication cost and ease of
installation. The main disadvantage is the sensitivity of the pitot tube to
flow direction. Relatively minor changes in flow direction or small alignment
errors may produce erroneous velocities (Sharp 1981). Another disadvantage is the difficulty in automating the output from pitot tubes; therefore,
pitot tubes are most suited for steady or quasi-steady flow situations.
Another popular unidirectional flow measurement device is the impeller current meter. These instruments come in various sizes and
configurations, but they all feature some type of rotating impeller that
is driven by the fluid flow. Figure 8.2 shows one design for a miniature
impeller. Other designs use cups mounted around the axis of rotation. The
rotational velocity of the impeller varies with the flow velocity, and this
relationship must be established via calibration procedures.
Usually, sensors mounted near the rotating impeller count the number
of revolutions over a given time span (selected by the operator) and then
output this value to a recorder. Appropriate calibration factors are then
used to convert the reading to equivalent velocity. Impeller current meters
respond reasonably well to unsteady flows; however, rapid fluctuations, sue
as larger-scale turbulence, present difficulties because the impeller cannot
accelerate instantaneously, it will not indicate flow reversals, and the time