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Orifice Plate Pressure Ratio


In ISO 5167 standard, in the equation for calculating expansion factor, it considers a parameter called P2/P1 which it calls as "Pressure Ratio" for orifices. The description is given as the ratio of absolute static pressure at downstream tap to absolute static pressure at upstream tap. 

Can someone help me please with how to calculate this pressure ratio for orifice (P2/P1)?

Thanks in advance.



3 Replies

  • Hi Shaiq,
    The pressure ratio you refer to is the ratio of upstream to downstream line pressure before and after an orifice plate. In most gas applications, the user would measure the upstream line pressure (P1) and the differential pressure signal created by the orifice plate (DP). So the user does not have a direct measurement of the downstream line pressure (P2). But this is can be calculated easily since we know that DP = P1 - P2. Just make sure to convert both pressure units so they match.

    Be aware that the expansion factor only applies in gas applications. If you are measuring liquid flowrate, then the expansion does not apply and can be set to 1.

    Would be glad to help more if you need more clarity.

  • In reply to Steve Ifft:

    Steve is right on, here.

    The only follow up I have to offer is the caution that you need to make sure that your upstream pressure transducer is measuring absolute pressure, not gauge, particularly in lower pressure gas flow applications. If you only have gauge pressure transducers, you can either assume a constant atmospheric pressure offset (14.7 psi, or using the average absolute pressure for your location, if known) that you add to the gauge pressure. More accurately, you can have an additional barometric pressure measurement that you can use to correct any gauge measurement to absolute. For higher line pressure applications, the error created by not correcting for changes in atmospheric pressure may be negligible.
  • In reply to Jeffrey Mach:

    If your site is not close to sea level, you should use an average atmospheric pressure suitable for the location (14.7 psi is suitable for sea level).