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Can a Air bench flow detect small changes? http://www.flowbenchtech.com/forum/viewtopic.php?f=3&t=1342 
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Author:  86rocco [ Sat Jul 08, 2017 2:52 pm ] 
Post subject:  Re: Can a Air bench flow detect small changes? 
damunk wrote: The calculator download from here tells you to only change the following : Orifice diameter 2.10 inches Pressure difference 16.0 WC in inches Discharge coefficient 0.62 1) So you input orifice diamter which in my case is 2.5 2) Pressure difference: Is this the test pressure: which I should always keep to 16.If drop increase vacuum power. If lower higher than 16 then lower vacuum power? 3) discharge coefficient: leave that as it is? So the psi measurement from the manometer I put into this calculator? Pressure difference here is NOT the test pressure, Test pressure is the pressure drop across whatever you are testing, the pressure difference here is the pressure drop across the orifice, on a bench with fluid manometers, this is what would typically be measured using an inclined manometer. And the test pressure is typically measured with a vertical manometer The discharge coefficient is a physical property of your particular orifice, it's a measure of how efficiently air can move through it. The 0.62 in the spreadsheet is a pretty good guess for a sharp edged orifice but the only way to get the true number is experimentally by flowing a test piece with known flow characteristics, like one of Bruce's plates and then adjusting the discharge coefficient until the flow numbers match. If you do not calibrate the discharge coefficient of your plates, the flow numbers you get may not be exactly correct however numbers will be consistent so comparisons made between different tests using the same orifice will still be valid. The spreadsheet is set up for pressure measured in inches of water not psi. to convert psi to inches of water column, 1 psi = 27.679 904 843 inches of water 
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