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mfkuss

Need help with compute function in RSLogix 5000

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Does anyone know of a formula I can use to convert differential pressure in "H2O to flow in SCFM. I have created a table of various flows vs. pressures, but don't know of a formula. I am using a differential pressure transmitter reading from a pitot tube. The transmitter can be setup to transmit the 4-20 signal as linear (pressure 0 - 10"H2O) and also has a Sqrt output. I am not sure how to get the data converted to a flow. If I try the standard y=mx+b, it only is accurate at one point, not full range. Any help would be appreciated.

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Flow is proportional to the square root of the differential pressure. The orifice plate or whatever should have some calibration data stamped on it.

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The key is that you must know what the 0-10" represents. I have several venturi meters, all different (0-15 MGD, 5-50 MGD, etc). Each of the transmitters is ranged to match the mechanical device (the respective venturi). Once you know the range, it is very simple to bring that 4-20 signal to the analog input module of your PLC. Then configure that particular channel of the A/I module to match the range of the transmitter. Viola, the actual flow value will be displayed as the raw analog. Am I on the right track, or was this not what you were asking? Bill

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It follows a square root curve. Also the absolute pressure and a host of other factors are involved in the conversion for accuracy reasons. Look on the web site for Dwyer which has most of the correction factors on it.

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You will need to take into consideration a density calculation as different fluids have different density's and also possible a temperature. because if the H20 is above 100DegC then it may be steam rather than water.

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Since you've already gone to the trouble of generating a table of flow versus pressure, you could also create your own formula by creating a curve-fit function.

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