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When doing pitot tube measurements, the pressure is measured in 2 places. At the wall (A), for the static pressure and against the flow (B), for dynamic pressure. Where the difference between these two is required to determine the velocity.

I recently started working in a company where pressure in the machine is measured with the flow (C). To my surprise they have a good read on the velocity in the machines.

I wonder what physical quantity is being measured, since it is not static or dynamic pressure? Is the measurement just a 'reference value' that translates to a velocity only due to the experience of years working with these machines?

Measuring tube perpendicular to flow (A), against the flow (B) and with flow (C)

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  • $\begingroup$ So how do your calculations look for B and C? $\endgroup$
    – Solar Mike
    Commented Jul 7, 2022 at 8:51
  • $\begingroup$ The pitot tube formula (as I know it) is $v^2 = 2*\Delta P / \rho$. When both the static (A) and dynamic pressure (B) are know the delta P is the difference between the two. How the velocity would be calculated using (C) is basically my question, so I cannot really answer that. $\endgroup$
    – Twon
    Commented Jul 7, 2022 at 8:58
  • $\begingroup$ So work it out for B and C, Not A. How do the results differ? $\endgroup$
    – Solar Mike
    Commented Jul 7, 2022 at 8:59
  • $\begingroup$ We currently do not have measurements of all these quantities on the same system. I am trying to set that up to prove to my colleagues that these are not the same thing. I am just trying to understand what physical quantity they have been measuring all this time when using (C). $\endgroup$
    – Twon
    Commented Jul 7, 2022 at 9:01
  • $\begingroup$ So base it on the theory - many many textbooks cover pitot tubes. $\endgroup$
    – Solar Mike
    Commented Jul 7, 2022 at 9:02

2 Answers 2

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The C section is reading the suction. Varying between zero for no velocity to -1 atm for maximum speed which has been calibrated to their range of the experiments. The readings will rang between 1 atm for zero flow to zero atm for the calibrated max flow velocity.

Because the lab has a controlled air temperature and pressure the static pressure is constant and doesn't need measurements.

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Even with the Pitot tube pointing downstream, there's still a streamline joining a zero-velocity location in the tube opening to a location at the full free-stream velocity, so you can still apply Bernoulli's equation to get the velocity from the pressure difference. I think with the tube this way round, I'd be somewhat worried that said streamline might have to cross from inside to outside the turbulent wake of the tube, thus traversing a region where the flow is not irrotational, and therefore Bernoulli's equation doesn't hold; but if you've observed empirically that the velocity values you get are good, I guess that's not a problem.

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