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I would like to measure air flow speeds in a duct system for dust collection in a workshop. Relevant air speeds in the ducts are around 10-30 m/s. The ducts are circular tubes with a diameter of 160 mm and thus the Re numbers will be around 100,000 to 300,000.

A pitot-static tube (a.k.a prandtl tube) in combination with an accurate-enough manometer appears to be the conventional measurement equipment for this application. My question is, do I really need a profesional-quality pitot-static tube for about 80-120 USD, or would it be sufficient with a 5 USD tube seemingly intended for RC planes?

In other words, what is the difference between a cheap and an expensive pitot tube? Can we expect substantial differences in performance (e.g., robustness, accuracy, ...), and if so what would be the relevant parameters? Or is it rather about which guarantees are made by the manufacturer? The factor 20 price difference suggests something fundamental could be different.

Does tip geometry play an important role? Is it important that static and dynamic pressure are measured in the same device? Is it important how the holes for dynamic and static pressure are located? Are the professional-quality obviously better in any such parameters?

Could I even do the measurement by measuring the differential pressure between (1) a hose with an opening perpendicular to the flow and (2) a thin, bent brass pipe with its opening oriented against the flow?

I am asking partly because I don't want to waste 100 USD, and partly because I want to understand the engineering considerations beyond the fundamentals of Bernoulli's equation.

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  • $\begingroup$ IMHO this is not the right forum to answer this type of question. Any reasonable comparison will need the specific products and also the specifications that are set in the problem. Otherwise, it is very similar to asking "What's the difference between a 5k car and 100k car?" $\endgroup$
    – NMech ♦
    Feb 1, 2022 at 14:10
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    $\begingroup$ Which one comes with a calibration curve? $\endgroup$
    – Solar Mike
    Feb 1, 2022 at 14:30
  • $\begingroup$ Thanks for your comment. Maybe I should try to clarify my question in some way. I don't yet even understand what specifications would be relevant to discuss, whereas in the car example I can easily come up with a large number of potential differences in performance, design, etc. $\endgroup$
    – rasmuse
    Feb 1, 2022 at 14:32

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The things I know about are:

  • calibration (accuracy, time constants, accuracy variations due to AOA, accuracy variations due to speed/Reynolds number)
  • proper geometry to minmize variations due to AOA
  • proper geometry and positioning for static ports to measure undisturbed flow
  • Things like orifice size affects response time due to Reynold's number/viscosity and what speeds/Reynolds numbers it will be accurate at

Remember some of these tubes are lab grade instruments.

You don't need to measure static and dynamic pressure in the same device, but it sure is convenient and theoretically more accurate if you can do it (because otherwise static ports must be placed at another appropriate location where static pressure could also be different.

I think I trust a home made pitot tube more than a pitot-static tube where aerodynamics matter more, You'll probably be fine with a dust collection setup where flow is always parallel and you can be off by 30% and probably won't know the difference. Hobbies tubes just follow TLAR. Which means hobbiest pitot tubes just slap a coned or domed nose on a tube so it looks prettier. Hobbiest Prandtl tubes, in particular, are quite ugly because the construction difficulties of concentric tubes.

Could I even do the measurement by measuring the differential pressure between (1) a hose with an opening perpendicular to the flow and (2) a thin, bent brass pipe with its opening oriented against the flow?"

I assume you are talking about 90 degree bent probes that enter the flow stream from the duct walls.

Just having a straight open ended tube for static pressure protruding into the duct trips up the air around the static inlet. You need to also bend it pointing into the stream so it is parallel to the flow so as not to disturb it. Then you plug the end with a geometry to not trip the air before to flows down the outside of the parallel tube segment. I think a hemisphere is more tolerant to AOA differences but the sharp end of a tear drop works better if the flow is always head on. I don't know if you can rely on this though because flows have disturbances and hitting the sharp point off angle will trip the flow.

Then you have holes along the length of the parallel segment facing 90 degrees to the flow.

Some Prandtl-tubes have a boundary trip behind the nose after the dynamic inlet for the static port. Others don't. I don't know when you want or do not want to trip it but it's a careful thing so if you have a separate static probe just place it away from disturbances and don't trip it. I think the trip might be there to introduce turbulence to prevent something like a laminar bubble or flow separation on top the static port if it is expected. That should be worse than just having turbulent but attached flow at the static port.

Those are my armchair conclusions after doing the same research a few years ago that you are doing now.

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