The issue of small-signal rejection in flowmeters
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Among the options for flow meters is small signal rejection; I’ve always wondered what the purpose of this feature is and under what circumstances it is necessary to use it on a flow meter.
For orifice flow meters, due to the square root operation, even small changes result in large output values (a 0.01 mA signal becomes 0.1 mA after taking the square root). As a result, measurement errors are significant at low signal levels. Additionally, the range ratio of orifice flow meters is small (usually 3:1), which also leads to large errors at low signal levels. To ensure accurate measurements, low signal levels that cause large errors are filtered out.
Generally, small-signal rejection is applied to meters or important flow meters; false flows generated by signals detected by the sensors can cause errors in measurement. Some are based on the percentage removed (such as 0.3%, 0.5%), while others are calculated using a numerical value – no measurement is taken below that specific value.
Actually, cutting off this small signal from the mass flow meter is a problem; for continuous low-flow conditions, such cutting will result in significant cumulative errors
It is generally cut off at three percent
Won’t the cumulative error be smaller if you don’t perform small-value signal removal? It should be understood that the use of small-value signal rejection is intended to address the large measurement errors that occur at low flow rates in flow meters; if this rejection is not applied, the accumulated error at low flow rates becomes artificially low. To achieve a small error, setting an appropriate cutoff point for the small signal is the key.
You’re right about that; I didn’t express it clearly before. The low-flow cutting setting needs to be reviewed and adjusted after installation, based on the actual operating conditions; otherwise, when the feeding time is long and the flow rate is low, the feeding error becomes extremely large.
If the flow rate remains low over a long period, it indicates that the flow meter selected is too large; relying solely on the function to filter out low signal values is not sufficient.
There is another scenario in which there is a slight leakage in the valve pipeline, especially internal leakage; it becomes even more difficult to detect when the flow meter signal is disabled…