# Pressure differential through orifice plate

• russguk
An orifice in a testing chamber is not a primary source of flow rate measurement. The orifice is used to create a pressure differential between the test sample and the atmosphere. The flow meter is used to measure the flow rate through the orifice.

#### russguk

Hi,
This is probably an easy answer but I can't get my head round it! If I have a test rig that measures pressure differential in a test chamber at a set flow rate, how do I calculate what the pressure differential should be if I put an orfice plate in with a certain size diameter hole in the test chamber? I've attached a basic diagram of how the rig works.

Whilst using this test rig I've been obtaining some questionable results and wanted to verify that everything was as it should be with the rig. Therefore my thought was to set the rig with a flow rate of, say, 20 litres/second with an empty test chamber (closed obviously) and zero the PD meter. Then insert an orifice plate with a restriction hole in it and adjust the flow rate back to 20 litres per second and see if the PD meter reads approx the calculated value.

It's actually not an easy question. There are some empirical relationships that work reasonably well for situations where your orifice plate is situated in a circular tube, but how well they translate to the situation you have drawn above is not simple to ascertain. Wikipedia has a small entry on it, and I am sure you can find some better sources in engineering journals and literature, but your best bet is probably to just run some tests.

russguk
You can find significant information related to your issue by googling "orifice plate measurement accuracy standards". Based upon you diagram, one issue you should investigate is the recommended minimum approach and discharge straight pipe lengths for accurate flow measurements with orifices.

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russguk
Get a copy of ASME PTC 19.5. The position of your plate in a divergent convergent nozzle looks dodgy, in general for flow measurement you're looking for 10 straight diameters upstream and five downstream, it looks to me that in the setup you have would produce turbulent flow.

russguk
OK, thank you for your help. Maybe it's probably best waiting until the meters are recalibrated and then make up a reference sample which we can use to verify the performance at periods inbetween.

Thanks once again.

if your test rig is only going to be used to measure flow rate through an orifice then
you should remove the expansion chamber from your test and also remove all of the turns
as these will affect the downstream pressure leading to the orifice and the upstream pressure
leading away from the orifice and remove the flow meter if it restricts any fluid flow.