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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