Extracting RMS value

Q: The plant I support has two existing speed controlled simplex diaphragm pump installations (manual stroke adjustment) w/ Coriolis mass meter configured so Flow adjusts stroke speed. Flow has a pulsing frequency of ~ 1sec. Because these are simplex pumps, pulsing affects flow more than 5%, in spite of the pulsation dampener. The second flow is ratio controlled from the first flow. I would like to take advantage of the constant volume characteristic of the metering pump to compare the flow generated by the pump to the flow measured by the cmm, as a plausibility check of the health of the cmm and pump. In a sense an on-line prover. This seems straight forward: stroke speed *volume* cmm density where volume is the calibrated volume @ a stroke setting. However the computed value is in effect a smoothed value while the meter is instantaneous. I imagine I need an RMS value from the Micro Motion. First Any advice on how to extract or generate the value? Secondly, I am trying to determine whether ratio control with the computed values or the RMS values will reduce downstream variability induced by the feed control & pump system. What computation would I make with vendor data, to show which is likely to work better? (If there is a way, where would I find the method?)

A:

Base the plausibility check on the totalized mass flow computed within the coriolis meter.  Chose some time interval - one minute, five minutes, or whatever you prefer.  The total mass through the coriolis meter is the change in the totalized mass flow over this interval (the longer the time interval, the less the difference is affected by the pulsing frequency). Compare this to the total mass flow computed from the pump stroke data.  You could also compute a "meter factor" to force the flow computed from the pump stroke data to agree with the coriolis meter.

Base the instantaneous ratio on the mass flow computed from the pump stroke data.  But if you have a batch process and the ratio of the total amount fed is most important, you could use the totalized mass flow from the coriolis meter.

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