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Pressure Measurement

  1. Jun 29, 2015 #1
    What is the effect of air bubble in transmission line when measuring differential pressure by the using an electronic transducer? Purging is a must before measurement, however if we still have some bubble inside the transmission line or in front of transducer diaphragm, how it may effect to my measured values?
  2. jcsd
  3. Jul 3, 2015 #2
    This is probably a question best addressed in the manual for the pressure sensor or by the application engineers at the company that made it.

    You are asking (in essence), "How much is it gonna hurt if I do not follow instructions?" That depends a lot on the specifics of the device which you have not provided, and I would tend to be skeptical of answers that do not have knowledge and experience with that specific transducer.
  4. Jul 5, 2015 #3

    jim hardy

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    Three effects, two for sure and one maybe......

    1. Process noise
    To transmit pressure from one end of the tube to the other takes very little time so long as the contents of the tube are incompressible.
    Short term fluctuations in system pressure should arrive at both sides of your dp sensor simultaneously so they'll cancel out.
    An air bubble in one line delays transmission of pressure to that side, so system pressure fluctuations appear briefly as differential pressure . The sensor obediently reports derivative of system pressure in addition to whatever you're trying to measure, usually liquid level or flow.

    2. Offset...
    Draw the system and write Kirchoff's Pressure Law around it.
    A bubble in a horizontal run does not add or subtract any head
    but a bubble in a vertical segment of the tube adds error to your dp measurement in amount g*hbubble*(ρliquid - ρbubble )

    3. System interaction..
    The mass of fluid in the sense line and the compressibility of the bubble have a natural frequency just like any spring-mass system does.
    Heaven help you if it's anywhere near that of a control system using the signal.

    Get the air out.

    old jim
    Last edited: Jul 5, 2015
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