Pitot tubes operate on a similar principle to differential pressure transmitters to ISO 5167. The tube projects into the pipeline and is divided into two pressure chambers (upstream and downstream), each with several pressure tapping holes. On the upstream side the total pressure is applied, on the downstream side a negative pressure is generated. The pressure difference is measured by a differential pressure transducer outside the pipeline and thus the flow rate is determined. The pressure holes distributed over the flow cross-section give a good averaging of the pressure signal. This reduces the requirement for a uniform flow profile, i.e. the required length of the stilling section.
Typical pressure tapping fitting designs are:
The measurement uncertainty is approx. 1% of the measured signal.
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