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One of the simplest systems used to measure air velocity is the Pitot-static probe, and one of the simplest ways to drive a Pitot-static probe is with a differential pressure sensor. However, all measurements are uncertain- and because of the nonlinearity in the response of the sensor to the velocity, estimating Pitot probe uncertainty (or the uncertainty of any pressure-based velocity probe system) is not always a straightforward matter.

For example, let us say that you are expecting to measure air speeds up to 20 m/s with your probe. You would therefore select a typical high-sensitivity differential pressure sensor with a full-scale range of 250 Pa, and perhaps an uncertainty of ±0.5% full-scale. This translates to an uncertainty in velocity of about ±0.25% at maximum speed- but more than ±7% at the lowest speeds! For more information, our technical note has a detailed discussion of the uncertainties associated with pressure-based velocity probes.

The same issue of uncertainty also affects multi-hole velocity probes. These probes resolve directionality by comparing the pressures at the different holes, so the uncertainty in both flow speed and angularity will increase with decreasing speed.

For high accuracy measurements at very-low speeds, you may want to consider an alternative technology.