Why are the cumulative DCS flow data and the cumulative mass flow meter data different?
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As is well known, mass flow meters are currently the most accurate flow measurement devices available. However, in practical applications, we find that the flow accumulation value indicated by the mass flow meter does not match the flow accumulation value transmitted to the DCS in analog form. Could anyone explain the exact reason for this discrepancy and suggest ways to ensure that the two sets of data are consistent?
It’s unlikely to be exactly the same, as the cumulative value of a mass flow meter goes through fewer stages. The measurement data from the mass flow meter must undergo digital-to-analog conversion to become a 4–20mA signal, which is then transmitted to the safety barrier and further to the DCS analog input card. There, it is converted back to a digital signal before being accumulated; there are too many intermediate steps in this process.
I’m not sure how the accumulation of data from your mass flow meter on the DCS side is done – is it through pulses sent by the flow meter to the DCS, or via 4-20mA signals? When pulses are fed into the DCS, it depends on whether the DCS scanning period is long; some pulses may not be counted. When analog signals are fed into the DCS, the signal may experience some distortion due to intermediate processing steps. Furthermore, the real-number calculation precision inside the flow meter differs from that of the DCS processor, which can also lead to discrepancies in the results obtained on both sides. As long as the gap isn’t too large, it should be acceptable.
If the on-site flow meter transmits data to the DCS system via communication, the data on both sides should be consistent. If it is transmitted via a 4–20mA signal, there will be errors in signal conversion, card accuracy, and the accumulation algorithm; as a result, the accumulated value will not match the actual value on site. If pulse counting is used, the accuracy will be better than that of current signals, but it still will not match the field data.
Look at the DCS sampling period; it’s the period used for calculation. Personally, I think using PROFIBUS communication is better
This post was last edited by massflowmeter on 2016-6-21 09:22. Sorry, I deleted it myself.