Pitot Tube

Start with Pitot's original instrument, which was one bent tube and nothing else.

1. Bend a length of clear acrylic or brass tube into an L, with a short leg about 40 mm and a long vertical leg 300 mm.
2. Cut the short leg's opening square and deburr it — a ragged or angled mouth reads wrong.
3. Hold it in a flowing stream or a bucket being stirred, with the opening facing directly INTO the flow.
4. Watch the water rise in the vertical leg and stabilise.
5. Measure the height above the free surface.

That height is your reading. The fluid arriving at the opening is brought to rest, and its kinetic energy becomes pressure — enough to lift a column. Pitot did exactly this in the Seine in 1732, and used it to demolish the then-current belief that river water flowed faster at the bottom than the top.

One tube measures total pressure. You need to subtract the pressure that was there anyway.

1. Make a second tube, closed at its upstream end, with several small holes drilled through its SIDE wall, flush and perpendicular to the flow.
2. Mount it parallel to the impact tube, in the same flow.
3. Run both to a U-tube manometer so you read the difference between them directly.
4. Compare the difference reading against the single-tube reading from step 1.

Why the side holes must be perpendicular. A hole facing the flow catches the moving fluid and reads too high; a hole facing downstream reads too low. Only a hole exactly flush with the streamlines senses the static pressure alone. Getting that geometry right is the whole art of a static port, and on aircraft they are placed on the fuselage where the flow is known to be undisturbed.

The instrument reads a pressure. You want a velocity, so establish the relationship yourself.

1. Mount the assembly on a rod and walk it through still air at a measured pace — say 2 metres in 4 seconds.
2. Record the manometer difference at that speed.
3. Repeat at double the speed.
4. Compare: the pressure difference should be roughly FOUR times greater.
5. Tabulate several speeds and plot pressure against velocity.

The instrument's failure mode is worth producing deliberately, because it has killed people.

1. Establish a steady reading at a known speed.
2. Now block the IMPACT opening with tape while the flow continues. Note the reading.
3. Restore it, then block the STATIC ports instead. Note that reading.
4. Try partially blocking each.

Henri Pitot presented his tube to the French Academy of Sciences in 1732, having built it to settle arguments about how rivers flow. His measurements in the Seine showed that water moves FASTER at the surface and slower near the bed, contradicting the accepted view. Henri Darcy refined the instrument in the 1850s into the form still used.

The method is measurement by substitution, and it runs through this whole batch. Velocity is hard to measure directly; pressure is easy. So convert one into the other and measure the easy thing. The same instinct appears in the sine bar from batch 69 — angles are hard, lengths are easy, so trade one for the other. Recognising which quantity your workshop can measure well, and finding a physical relationship that converts the hard quantity into it, is most of instrumentation.

Against its sibling in this catalogue: the Venturi meter measures flow by narrowing the whole pipe and reading the pressure drop, giving a full-pipe average with almost no permanent pressure loss. The Pitot tube measures at a POINT, obstructs almost nothing, and can be traversed across a duct to map a velocity profile. One gives you the total, the other gives you the distribution — different questions, and a serious flow survey uses both.

Its honest limits: it reads at one point, so a single reading in a pipe is only as good as your assumption about the profile; it is sensitive to alignment; it is easily blocked; and at low velocities the pressure difference becomes too small to read reliably. That last limit is why aircraft airspeed indicators are unreliable at very low speed and why other methods are used for slow flows.