Measuring volume flow rate

  1. Hi
    I have a question about measuring volume flow rate with a laminar flow element.
    I read the pressure difference across the element (mmH2o) and I get volume flow rate (cfm) from a calibration law. If that calibration law was obtained at standard conditions, would I have to correct it for different conditions? Thanks!
  2. jcsd
  3. FredGarvin

    FredGarvin 5,084
    Science Advisor

    Not that I don't believe you, but I find it hard to believe that the unit was calibrated at standard conditions. The calibration was probably referenced to standard conditions. You might want to check the documentation with your unit. The ones we have have correction factors for calculating the actual flow for non standard conditions.
  4. calb law is not at standard coditions ;-) I meant calibrated at certain conditions and then used at others.
  5. It depends. Usually if you calibrate a measurement device its for a range of operating states or measurements. Otherwise there wouldn't be much of a point in calibrating it would there? What kind of change in conditions are you referring too? Pressure, temperature, less gravity, what? Are you talking about a wind tunnel or flow venturi or something?
  6. FredGarvin

    FredGarvin 5,084
    Science Advisor

    A laminar flow element is a type of air flow meter. They are sometimes used as calibration masters.
  7. russ_watters

    Staff: Mentor

    The biggest thing that typically affects airflow measurements is the density of the air. But what determines if you need to make a correction is how big the error is. If it is less than, say, 5%, then it is probably inside the error margin and repeatability of the device and system.
  8. FredGarvin

    FredGarvin 5,084
    Science Advisor

    That's one of the things about these particular instruments. They are often used as calibration standards. The 5% number is huge. I have one on my desk right now that measures up to 150 scfm. It's not much but it does require corrections even for small variations if you need that level of accuracy. These guys are well beyond pitot tubes.
  9. thanks for replying.
    Should I correct the air volume flow rate for the inlet pressure ratio? I mean, if the calib. law is
    Q [cfm] = (calib. coeff.) * Delta_h [mmH2O]
    should it be Q [cfm] = (calib. coeff.) * Delta_h [mmH2O]* P_upstream/P_atm ?
  10. The equations for many properties of air as a function of temperature can be found in ASTM C-680
  11. Also, be very careful with "Standard Conditions" my "Standard" for air properties is 60 F but with a slight change of 8 degrees I almost lost my whole heat exchanger design once.
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