Pitot Tube
About measuring with pitot tube
The pitot static tube is mainly used for
making temporary measurements of flow, although it is also used in some
instances for permanent flow monitoring. It measures the local velocity
of flow at a particular point within a pipe rather than the average flow
velocity as measured by other types of flowmeter. This may be very
useful where there is a requirement to measure local flow rates across
the cross-section of a pipe in the case of non-uniform flow. Multiple
pitot tubes are normally used to do this.
The instrument depends on the principle that a tube placed with its open
end in a stream of fluid, will bring to rest that part of the fluid
which impinges on it, and the loss of kinetic energy will be converted
to a measurable increase in pressure inside the tube. This pressure, as
well as the static pressure of the undisturbed free stream of flow, is
measured.
Pitot tubes have the advantage that they cause negligible pressure loss
in the flow. They are also cheap, and the installation procedure
consists of the very simple process of pushing them down a small hole
drilled in the flow-carrying pipe.
Their main failing is that the measurement
accuracy given is normally only about ±5%, and sensitive pressure
measuring devices are needed to achieve even this limited level of
accuracy, as the pressure difference created is very small. More
recently, measurement capabilities with the uncertainty down to ± 1%
have been claimed for specially designed pitot tubes.
The annubar is a development of the pitot tube which has multiple
sensing ports distributed across the cross-section of the pipe. It thus
provides an approximate measurement of the mean flow rate across the
pipe.

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