# Minimum length of the Pitot tube?

#### Borek

Mentor
I have a thin tube (4 mm diameter) sticking out of a small box (a bit larger than a matchbox). I want to use this tube to measure the dynamic pressure. How long must the tube be for the results to be reasonably accurate?

Intuition tells me the working end of the tube should be kept in the unobstructed flow, but I have no idea what it means in practical terms (and I want to keep the tube as short as possible).

I don't think I will be able to bend the tube without collapsing it (not even sure if that is a correct word in the context).

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#### ChemAir

Gold Member
Reasonably accurate is probably the relevant term here.

I have a couple of pitot tubes for duct air flow measurement (L shaped). The one with a 3.8mm hole at the end is about 180 mm from the vertical section that protrudes through the duct. The comparison reference is a row of radial holes that is ~60mm from the velocity end (120mm from the vertical part). In this case, 120mm is far enough away from the vertical tube to likely not influence the measurement significantly. A square box would require more, but it depends on exactly what is going on, and what else can influence the measurement

If I was doing it, I'd probably trial and error it, starting with the maximum I could live with, hoping to work it down. Without that constraint, I'd probably lean toward 180-200mm since I have one with those dimensions that works, and go from there.

Pitot tube air flow measurements are noisy.

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Gold Member
Trial and error is indeed a pretty good idea here. Another option would be to estimate the minimum length by estimating the flow field in front of your box through any number of methods, though they would largely be a pain to do.

So yes, I'd go back to the trial and error idea. It's low-tech but it's foolproof.

Also note, a Pitot probe measures total pressure, not dynamic pressure.

ChemAir said:
Pitot tube air flow measurements are noisy.
What does this even mean? Pitot measurements are not necessarily noisy. They would be if you don't take the proper precautions, like any other measurement, but this is not something endemic to Pitot measurements.

#### Borek

Mentor
I'd probably trial and error it
Trial and error is indeed a pretty good idea here.
Oh, well, I hoped for some rule of thumb, but if it takes working with a knife (soft aluminum) - so be it.

Also note, a Pitot probe measures total pressure, not dynamic pressure.
Good point, lousy language on my side.

Thanks guys. Not exactly the answer I hoped for but at least one I understand

#### ChemAir

Gold Member
What does this even mean? Pitot measurements are not necessarily noisy. They would be if you don't take the proper precautions, like any other measurement, but this is not something endemic to Pitot measurements.
Definitely not endemic to pitot tubes, agreed. Just mentioning (poorly) that a noisy signal is not necessarily a failure.

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"Minimum length of the Pitot tube?"

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