Tuesday, September 17, 2013

Drilling DIY pitot static annulus pressure taps

During manufacturing operations of DIY pitot a sensible as important phase is the drilling of pressure taps, I introduce some common issue for the 8mm pitot probe.

Making of the DIY pitot tube is straightforward, anyway special care should be taken in the annulus static pressure port drilling operations. As the involved dimensions are reduced some manufacturing problem can arise, have a look to a the pitot probe static ports here below
Figure P10.1 Finished annulus static ports, JLJ used pitot.
Holes are 1 mm diameter and spaced 45° to have eight holes total on an annulus shape. To drill that holes by hand can be really thrilling and sometimes the result is unusable so a lot of swarf crap should be expected. To contrast the fate a more strong approach can be used, the drilling jig. Here below an example of well proven and comfortable setup
Figure P10.2 Pitot tube on a DIY dead center drilling jig in action, the tube is wrapped into masking tape to avoid scratches during drilling

In this drilling jig two main components are assembled on a railed rigid frame, an adjustable dead center and a mandrel. Dead center will be pushed tight against the pitot tube to ensure that the component will not bend when the drill bit plunges into the piece. The used mandrel have the possibility to rotate with the clamps locked on the tube; when turning the tube by hand a dial provide angle so it's possible to work fast and precisely. Usually this kind of mandrels can by bought in hardware stores as they are used, for example, for lathe or milling operations. With such a kind of setup you can simply put the jig on your vertical drill or lathe table vice, drill the first hole on the center line of the tube and go ahead simply rotating the mandrel of 45° per time. If your equipment is correctly aligned every hole will be perpendicular to the tube surface, with next to zero effort.

Consider the following figure depicting the air path around mechanical burrs.

Figure P10.3 Possible streamlines around burrs

Recalling the Navier-Stokes equations and in particular the equations for incompressible fluid flow it's evident the strictly correlation between machining burrs, that cause a local change in stream velocity, and pressure tap error in pressure readings. Given V as the current flow speed, \(\rho\) as the fluid density and \(p\)as the stream line pressure the Euler equation is \(dp=-rhoVdv \) , that correlate directly the change in stream speed and pressure.

Look figure 10.3 that shows a cut view of a pressure tap with two different kind of idealized burrs, one upstream the other downstream. Both the defects leads to a variation in the stream speed so a variation in pressure at the tap point. So the pressure tap must be deburred but the tap opening on the tube shall be flush to the outside diameter.

Figure P10.4 Pressure tap deburred in a wrong way, unwanted radius introduced

Reaming a hole, as per figure 10.4 is to avoid in fact the correct hole intersection with the tube must me sharp, no radius allowed.

The use of a tapered reamer is common in mechanical work but inappropriate for pressure tap making. If the probe ports are not properly prepared the calibration factor will be notably dependent on Reynolds number. If speed increase also the reading error will increase. By many authors the static pressure error is the main source for speed reading error, total pressure port is often considerate as ideal, my experience lead to agree this approach.

To get good measuring results do not use a tapered reamer or drill bits to remove the burrs. A most precise way to attain flush deburred holes is to drill a preliminary hole of about 0,8 mm dia and after use a flat end mill to enlarge the hole to nominal 1mm dia. A pretty result can be attained, on carbon pitot tubes, directly drilling the 1 mm dia holes and using a wood piece to remove by hand the burrs.
Have fun,
JLJ 

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