Do mass flow meters have excessive errors at low flow rates?
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Since there is a ready-made mass flow meter with a range of 80 t/h and a DN80 interface, during installation a DN50 pipe was used for the connection from the pump outlet to the reactor’s main pipeline. However, straight DN80 pipes were installed before and after the flow meter as required by the specifications. The amount of material fed in is only a few hundred kilograms; normally, the instantaneous flow rate is controlled to around 15 t/h through valves, and the measurement remains fairly accurate. But most recently, due to a blockage in the pump inlet pipeline, the instantaneous flow rate was only 2.5–3 t/h. Yet, with the same cumulative reading from the flow meter, an actual amount of material fed in was several dozen kilograms higher – I’m not sure why Do mass flow meters have excessive errors at low flow rates?
The most direct approach is to consult the flow meter manufacturer, as they are best aware of the conditions of their own instruments. Our customers always come to us directly when they have problems; if telephone guidance isn’t sufficient, we send technicians to the site to take a look
At lower flow rates, the measurement accuracy of mass flow meters decreases significantly, and the measurement errors become larger.
All measuring instruments have their appropriate measurement range; when the value exceeds this range, the accuracy is naturally affected
The flow rate is too low; the DN50 meter may give accurate readings, while the reading from the DN80 meter might be higher than the actual flow rate, resulting in inaccurate measurements. Errors only occur when both meters are used for measurement. Try increasing the flow rate and observe again with both meters to see the difference.
The flow rate that can be measured accurately by a mass flow meter is subject to certain range limitations, and its accuracy is generally specified as the precision achieved at these normal flow rates. The accuracy of any flowmeter is not fixed; it is conditional. Too high or too low flow rates can result in poor accuracy for mass flow meters; especially at very low flow rates, the effect on accuracy is significant because the Coriolis force is not pronounced. For process metering, why use mass flow meters with such high precision? It seems the company is well-funded and doing well financially. Additionally, if feeding is to be controlled, and the amount of material fed varies significantly, this situation should be taken into account during design by using two separate feeding pipelines or measuring devices to measure the material individually. Anyway, there’s money, right?
Low flow rates affect accuracy, while high flow rates affect pressure loss; check the accuracy curve provided by the manufacturer.
Mass flow meters have a wide measurement range; as long as the pipe is full and there is no two-phase flow, they should be able to measure low flow rates.
The author’s suspicion is correct; mass flow meters exhibit large errors when measuring low flow rates. To achieve the precision specified by the manufacturer, the flow rate needs to be approximately 1 m/s or higher ； From 1 m/s to 0.1 m/s, the error can generally be kept within 5 times the nominal error, provided that the zero point is stable ； Below 0.1 m/s, it’s very poor – it can basically not be used as a flow meter. Therefore, if the flow rate is low, manufacturers will recommend using products with a smaller diameter in order to increase the flow rate; this is the manufacturer’s approach. As a user, one can choose other types of flow meters.