Instrumentation Tubing

Discussion in 'Instruments / Avionics / Electrical System' started by GESchwarz, Aug 31, 2019.

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  1. Aug 31, 2019 #1

    GESchwarz

    GESchwarz

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    Is there a minimum tubing ID for pitot and AoA systems? The length of the run for these tubes is about 16 feet.
     
  2. Aug 31, 2019 #2

    Victor Bravo

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    Shouldn't be in theory, however I would also make the tubes 3/8 ID if I could. 16 ft. is pretty far, and adding line losses or whatever small hysteresis comes form viscosity and friciton could potentially skew the readings a bit.
     
  3. Aug 31, 2019 #3

    wsimpso1

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  4. Aug 31, 2019 #4

    Toobuilder

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  5. Aug 31, 2019 #5

    BJC

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    The only question that I have about the small tubing is how well it will drain any water, especially the static line. It may be a long time before you have any water issues out there.


    BJC
     
  6. Aug 31, 2019 #6

    Toobuilder

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    Agreed. Thats my one area of concern too.
     
  7. Sep 2, 2019 #7

    C Michael Hoover

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    You are not measuring flow, but only measuring relative pressures. The tubing size should make a difference of exactly "zero". If either pitot or static sources are subject to water ingress, then a drain needs to be considered.
     
  8. Sep 2, 2019 #8

    pictsidhe

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    There will only be flow when pressures change. Small tubing may introduce some lag, but I doubt that it would be noticeable. Water could be problem in any line. Perhaps the annual could include blowing the lines out?
    I'll be using the smallest, lightest stuff I can find.
     
  9. Sep 2, 2019 #9

    TFF

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    T fitting at a low point with 4-6” line pointed down capped off is how most do a sump for pitot water, pretty standard.
     
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  10. Sep 2, 2019 #10

    Pops

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    Or find one of the plastic bottles Cessna used on the C-150/172's.
     
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  11. Sep 2, 2019 #11

    pictsidhe

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    A bottle on thin line may make lag noticeable.
     
  12. Sep 2, 2019 #12

    BJC

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    My concern about small (1/8” OD) static and pitot lines, stated above, is their poor capacity to drain water (condensate or rain) due to the tendency for capillary action.

    Unobstructed small tubing will have less lag when driving an EFIS.


    BJC
     
  13. Sep 2, 2019 #13

    Pops

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    Apparently Cessna has no problems. Cessna uses 1/4" dia tubing.
     
  14. Sep 2, 2019 #14

    TFF

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    Mid level Regional planes use the same stuff. I will say Collins has these neat quick disconnect jars on the air data computers, but the lines to them are the same plastic.
     
  15. Sep 3, 2019 #15

    pictsidhe

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    What is the science behind less lag? I would model it as a Helmholtz resonator, where lag would go down with larger tubing.
     
  16. Sep 3, 2019 #16

    BJC

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    The smaller diameter tubing driving an EFIS, as opposed to a bellows or bourdon tube, will have a smaller volume of air to compress or expand with a change in static or dynamic pressure. But even with steam gauges, lag is not significant.


    BJC
     
  17. Sep 3, 2019 #17

    pictsidhe

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    I had a look at weights for nylon tubing

    1/4 0.050: 1.39 lb/100ft
    1/4 0.030: 0.95lb/100ft
    3/16 0.025: 0.58lb/100ft
    5/32 0.025: 0.47lb/100ft
    1/8 0.025: 0.35lb/100ft
    1/8 0.015: 0.25lb/100ft

    We use literally miles of the stuff at work. Air and lube lines on injection moulding presses. It does not seem to fatigue, the push fit fittings are extremely reliable, though not perfect. Replacement is usually because an irresistable force (up to 1400ton press) has had a wrench or some other lump of metal left in a way, which catches a line when the press closes. Well, the line is tough good, but not that tough... Sometimes the lines are not tied up well, and with an operation or two per minute, will wear through rubbing on a way. That still takes a really long time, though. I've just persuaded the boss to buy me some 3/16 line to fix the abraded lube lines on a press with an orange monochrome screen. It's one of the older ones...
    The push fit fittings have an irritating habit of staying in the bottom of my pockets in my small change when I empty tools out before I go home. I just had a look, the plastic ones have a grey-ish body and orange push bits. Marked SMC and S'pore
     
    Last edited: Sep 3, 2019
  18. Sep 3, 2019 #18

    pictsidhe

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    The volume is less, but the area for the gas to flow down is less, too. If it is just a blank end tube, lag should be the same. If there is a cavity volume V at the gauge, it will vary with the sqrt(V*A/L). Unless the entrance hole is significantly smaller than the tubing?
     
  19. Sep 3, 2019 #19

    Topaz

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    Interesting side-note of some relevance: Dr. Bowers in one of his talks at the ESA Workshop this year showed a custom pressure-measuring system, with 96 channels (and therefore, 96 individual tubes), that his students designed and built for some of the Prandtl-D work before he retired from NASA. The tubing on all of this system, leading from the pressure tap ports on the wing to the sensors in the unit, was 1/8" I.D. at most, based on comparison to some other items in the photo. Very probably smaller.

    Seems, for simply measuring pressure, small tubes don't make much difference. Keeping them clean and clear of water is another matter, and not of concern for a research aircraft such as Dr. Bowers was describing.
     
  20. Sep 3, 2019 #20

    pictsidhe

    pictsidhe

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    Small tubes make zero difference to measuring pressure. I beleive that Dr Bowers was testing stuff in a desert? Condensation isn't a big concern there...
     

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