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Making a total pressure tube

  1. Jan 20, 2010 #1
    Hello

    I am attempting to make a total pressure tube/probe to use on small scale wind tunnel testing.

    Like a pitot static probe, this total pressure probe has to have a 90 degree bend like an L shape.

    I have several copper tubes of around 3 mm OD.

    I tried bending them by heating the metal using a bunsen burner, and then bending.
    However, I get quite a large radius when bending. If I attempt a small radius (like a proper 90 degree bend) then the tube fails, it ends up having a dent on it at the bend.

    Is there anyway I can bend it precisely with a small radius?

    It has to be some kind of DIY technique because I do not have any bending tools and neither does the University that I study at. Infact the university does not allow me to use a bunsen burner in the campus due to health and safety so I have to do this at home.

    Please help

    Regards
    Dopey
     
  2. jcsd
  3. Jan 20, 2010 #2

    FredGarvin

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    A rule of thumb is that a tube bend radius should be around 3-4 times the tube diameter. For example, a Ø.50" tube would have a bend radius of 1.5-2.0". Since you are working with copper, you may try to get a bit smaller. This also depends on tube wall thickness. A thicker wall will usually bend nicer than a thin wall. So if you are having trouble, try increasing the wall thickness.
     
  4. Jan 20, 2010 #3
    Use a coil wrapped around the tube when you bend it. Most hardware stores sell these as tube bending kits (if you live in the US, check Harborfreight, they have one for $2). The idea is to prevent the tube from deforming radially...
    Another approach used (for instruments) is to fill the tube with something that you can melt out (it used to be lead but I guess you could also use tin or maybe even wax). Once the tube is filled (and cold), bend it, then remelt its contents.
    Let us know what works.
     
  5. Jan 20, 2010 #4
    fill it with sand, or cut it off at a 45 and rotate and solder it together

    dr
     
  6. Jan 20, 2010 #5

    FredGarvin

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    miter bends are a no no in instrumentation lines.
     
  7. Jan 20, 2010 #6
    Yep.
     
  8. Jan 21, 2010 #7
    why is that?
    is the "rule of thumb" the minimum that the bend can be?

    dr
     
  9. Jan 21, 2010 #8

    FredGarvin

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    You want as little flow distortion as possible. A miter will not allow flow to easily make the bend. Usually we aim for a 4D for the bend radius as a starting point. That usually keeps you free from tube bending issues like kinking and flattening as well.
     
  10. Jan 21, 2010 #9
    now another question
    apparently you are trying to maintain a laminar flow in the tube, but couldn't the "turbulant flow" model be used and potentially get the same or better numbers?
    In actuality there really is not "true flow" in the pitot measurement being the tube dead ends in a sensor of some sort.
    aren't there some accurate models now based upon turbulant flow that could predict and pressure differential?

    dr
     
    Last edited: Jan 21, 2010
  11. Jan 21, 2010 #10

    FredGarvin

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    It's not so much laminar flow, it's just a constant velocity profile or knowledge of the velocity direction where the probe is located. In weird bends you get all sorts of recirculation zones. You are simply trying to minimize anything like that. It's like what you see in the lower left hand corner of the image

    http://www.chengfluid.com/images/meter_runs_2.gif [Broken]
     
    Last edited by a moderator: May 4, 2017
  12. Jan 21, 2010 #11
    ok, I understand all that (by the way, very nice diagram, I saved it for future ref)

    what I was trying to figure out was:
    the desired measurement is a pressure measurement that you are taking.
    there is max velocity at the point of entry, and zero velocity at the pressure sensor at the end. so the pressure is a function of the velocity at the point of entry (force) vs area of the opening
    some small pressure differential could take place between the opening, and the sensor, but that should be predictable

    dr
     
  13. Jan 21, 2010 #12

    FredGarvin

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    It's really more important when you get into aspirated probes where there is a constant flow through them. If we're talking a plain old total probe then it doesn't really matter. You still want to avoid miters because they introduce a weak point in the tubing. You really don't want a probe breaking off and going downstream.

    Usually the main aspects of uncertainty in the measurements are from the design of the nose, the direction of flow relative to the nose (yaw sensitivity) and the speed of the flow. Once you get into the compressible realm (we usually use around M = .2) you need to worry about measuring pressure behind a bow shock on the nose. You can correct for that but you usually need static pressures as well.
     
  14. Jan 21, 2010 #13
    thanks Fred

    dr
     
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