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How to measure flowrate using a differential pressure sensor

  1. Apr 25, 2016 #1
    I have to build a digital peak flow meter. I ve read that you can measure the volume metric flow rate using a differential pressure sensor. H ow do i go about doing so. The peak flow meter will be used to measure asthma.
  2. jcsd
  3. Apr 25, 2016 #2

    Simon Bridge

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    1st work out how the pressure sensor works.
    This should let you know the relationship between what it says and the flow rate - which will also depend on the geometry of the device.
    Do you know the relationship between fluid pressure and it's speed?
  4. Apr 25, 2016 #3
    Initially i thought of just measuring the pressure at one of the inlet ( using it as a gauge pressure sensor). Hence, i would get the pressure readings and then just graph them. But it does not tell me the volume metric flow, and that data probably isnt useful.

    Differential pressure sensor.
    http://au.mouser.com/ProductDetail/Honeywell/SSCSNBN005PDAC5/?qs=%2fha2pyFadujTzXc4EG5AIpywoMeCdsHuWImDF3mnLPxOkVmFo1jG4A%3d%3d [Broken]
    Last edited by a moderator: May 7, 2017
  5. Apr 25, 2016 #4

    Simon Bridge

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    If you know the gauge-pressure at a position in some flow, that can be useful for telling you the speed of the flow... provided you know the relationship between pressure and air speed. You usually also need other information like the geometry of whatever the air is flowing through.

    Do you know how pressure relates to air speed?
    Do you know how air-speed relates to volume flow?
  6. Apr 25, 2016 #5
    I believe flow rate = cross sectional area x fluid velocity

    Im not sure about the pressure vs velocity

    pressure = force/area
    force= mass x acceleration
    velocity = integral of acceleration

    so pressure= ( mass x velocity x time) / area
  7. Apr 25, 2016 #6

    Simon Bridge

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    Flow rate: well done.

    Pressure/velocity: nice reasoning, but that would be (loosely) the pressure exerted by a jet of air directed at a surface on that surface
    You need the pressure of the air going past a surface. fast air has low pressure.

    ... look up "bournoulli's equation".

    You could probably work out the pressure-speed relationship for your design empirically.
  8. Apr 27, 2016 #7
    Have you thought of using a Pitot tube or an orifice? How critical is the pressure drop caused my the measurement device? I would imagine if you are measuring pulmonary air flow, the lower delta p the very better.
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