A Prandtl Juxtapose..bringing in the tiny guns

As I am interested in figuring out where the air goes after all that prancing about with a Grinder and Vernier caliper, I needed to get me a Pitot tube.. or more accurately a Prandl Pitot-static Tube.  As the ports in a mini head are not all that large the tubes need to be quite small to be of any use. If you are not shure about this drilling holes 393 thousands of an inch (1mm for the more sane metric folks) in 2mm pipe and would like a properly made version I strongly recommend buying one from PTS 🙂 .  Bruce makes a proper concentric version in a very tight 12,5mm bend and while $100 bucks is still good money it is a very good deal considering how fiddly it is to make one this small, and he has to make a living as well !

I did consider buying, but I just wanted to see if I could pull it off. First you have to source a tube that is small enough. As I could only find 2 and 4mm tube I opted for a slightly more pedestrian approach and made two side by side using two 2mm OD tubes , like a shotgun. Ideally a 1.5mm and a 3.0 mm could be used to make a proper Prandtl arrangement.

The two tubes together with a 1 Eurocent coin

the bends look a bit scruffy as I tried to make a super posed version (whaaaaayyy!) and managed to upset the very identical and much tighter bends I had before.. But tubes are soldered together and the static end is plugged with a piece of copper wire I sourced from a air coil inductor I had laying around, cunningly straightened and reduced down to the proper size and knurled and pressed in.

note the plugged static port and the staggered ends with the dynamic port leading. I did not bother with streamlining the end as i hope that the air will form its own new boundaries

To keep the tube from collapsing, bending is done using a mandrel of  lubed Teflon insulated copper wire that is the exact diameter of the inner diameter( taken from some salvaged lab equipment parts), an object of the desired radius and finger power. The metal tends to work harden quickly, so ideally it is best to do  it right the first time 🙂 . As I of course thought I could do it just a bit better, the metal was now very hard to bend and I had to resort to annealing  it , which makes it go very very soft. In the end it more or less worked out like I had planned.

The two outlets are to be connected to a pressure differential device. So far the best candidate is a Testo 510 as it has a fps and m/s in addition to the normal pressure measures as well as a density correction. Only downside is that it is not very cheap ( 155 euro) and does not allow to interfacing with a computer so you have to jot down numbers to map a port or a valve circumference. On the other hand.. it just works, and there is a lot to be said for that.

You could also use a U manometer made of tubing fixed to a piece of wood with two tape measures fixed along side . I might first try that to test is this side by side arrangement actually works. It is very impractical it you want to move the bench from one place to another (and I do that sometimes)


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