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Correction Of Transfer Function

  1. May 24, 2015 #1
    Hello everyone;
    My question is regarding to velocity effect on transducer diaphragm. When fluid flow is acting to differential pressure transducer's diaphragm, the diaphragm is osculating(vibrating) and there will be fluid flow which is transverse(opposite flow,back flow) to normal fluid flow inside transfer line. This creates fluctuations on pressure measurement. However I do believe this effect on velocity, I wonder whether it is negligible. If It is, can you please give me briefly information about it. Thank you.
     
  2. jcsd
  3. May 24, 2015 #2

    jim hardy

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    Good to see you're thinking about this early rather than late.

    Better DP sensors have a damping adjustment that rolls off frequency response , so as to suppress that "process noise" .
    Thats why you often see different model numbers on DP sensors intended for flow versus level measurement.
    Long sense lines will also provide natural damping because the mass of of the fluid in the lines must be moved in order to deflect the transducer's diaphragm. So inertia in the sense line and compliance of the sensor form a natural low pass filter.
    But with a highly compliant transducer like a bellows driven dial indicator
    IM000592.JPG
    you can wind up with a Helmholtz resonance slow enough to interact with control systems....

    If you're measuring with a fast computer , make provision to smooth out the "process noise" before handing it to any control algorithm.
    You'd be well advised to take a spectrum of your signal to decide what's appropriate filtering, and if you're lucky that might be 'none' .

    Lastly - any motion of the dp sensor itself that's perpendicular to the plane of its diaphragm will show up as signal.
    F=(Δp X area+MA) and the transducer is only aware of the sum.
    So pay attention your mounting stanchions. I had to relocate lots of transmitters off structures that were not sufficiently rigid to have mechanical natural frequency above transmitter's frequency response.

    Search on buffeting in flow measurement and you'll find scholarly articles aplenty.
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770022156.pdf

    And observe your signals real time with a recorder or frequency analyzer because " One experiment is worth a thousand expert opinions."

    old jim
     
  4. May 24, 2015 #3
    Dear Mr. Jim,

    Thank you for your response.
    It's quiet scientific topic because I am just doing my thesis with DSA3307 differential pressure device which can measure 500 hz/channel. My professor just wants to know generally, what is the effect of this vibration into pressure transmission line's velocity profile? What is the influence of backflow? He wants me to find a relationship between voltage and pressure as well. Actually, This is called as unsteady pressure measurement and correlation. In literature, I didn't see anything about the effect of backflow in transfer function. He just told me that this is negligible but proof it. I checked a lot of sources but couldn't find anything.
    http://www.acoustics.asn.au/conference_proceedings/ICSVS-1997/pdf/scan/sv970424.pdf
    Here an example.
    dsa3307_02b.jpg
     
  5. May 24, 2015 #4

    jim hardy

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    One has to assume a driving pressure(noise)
    then apply that function to the mechanical system of the transmitting sense line, as in that paper you linked
    and see what arrives at the sensor

    so :
    are the sense lines filled with compressible or incompressible fluid ?
    What is its mass?
    What is displacement vs pressure of the sensor at end of that sense line?
    Now you have a spring-mass-damper system that'll lend itself to analysis just as in harmonic motion physics homework problems(see conclusions paragraph).

    that seems to be the thrust of that paper.
    You can design your sense lines to avoid resonance in your frequency range of interest, or just demonstrate there are none..
     
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