The issue of cumulative indication in electromagnetic flowmeters
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Recently, we encountered a problem: the cumulative reading of the electromagnetic flowmeter installed in our new unit did not match the cumulative value displayed on the DCS system operator station for the same time period, and the difference between them was quite large. We are not sure what the cause is. Who can help analyze it and see what possibilities there are? Thanks!
Are the range on-site and the range of the cumulative values in the DCS the same? Our company encountered such a situation once: the range was changed in the on-site design, the instantaneous flow range in the DCS was adjusted, but the cumulative value range remained unchanged.
Is the on-site range setting consistent with that of the DCS? Inconsistencies can lead to discrepancies. Has anyone on site modified the settings, resulting in the cumulative value being reset and thus causing a mismatch with the DCS?
We have encountered this situation as well; I believe it is due to differences in the cumulative precision of the two methods. In DCS, accumulation is usually done over one scanning cycle, with the unit being milliseconds, and even minor deviations can lead to significant errors over time； The cumulative accuracy shown on-site is likely to be expressed in seconds. As we all know, the instantaneous flow rate on-site is constantly changing; as a result, there will definitely be some discrepancies in the values taken as samples for cumulative calculations. Over countless milliseconds, these discrepancies accumulate and lead to differences between the two cumulative values ； So all our current important cumulative values are based on the on-site forms as a reference
The DCS scanning cycle is in milliseconds, but when performing accumulation calculations, it is done on a basis of 3600 seconds; it is estimated that this value is not very different from that of DCS. You need to test the 4–20mA signals, and make sure the measurement range is set correctly. Additionally, check whether the reaction speed setting for the flow meter is too high
Is the on-site range setting consistent with that of the DCS? Inconsistencies can lead to discrepancies. Has anyone on site modified the settings, resulting in the cumulative value being reset and thus causing a mismatch with the DCS? I agree with the above points! ! ! !
Is it a problem with the 4–20mA signal, or has someone at the site reset it...
Although DCS calculates accumulations on a basis of 3600 seconds, during signal acquisition the instantaneous flow rate changes, and thus the instantaneous values vary at different sampling times; these minor differences will inevitably lead to errors after being accumulated over countless milliseconds
Can the sampling period in DCS be changed to seconds?