Firstly, the electromagnetic flowmeter must be full to measure the accurate flow. The upstream of the flow regulating valve is installed to avoid the error caused by the flowmeter of the electromagnetic flowmeter when the flow regulating valve is not full. Here to clarify a concept, the control valve is not the valve, the valve should be added before and after the flow meter and control valve. Generally speaking, if there are both flow meters and control valves in the pipeline, basically the flowmeter is used to measure the flow, and then the control valve is used to control the flow. There is no control valve in the front, the flow meter is in the back, there is. The advantage of this is that in many cases, the flow meter is sensitive to pressure changes, the control valve is added to the front, the pressure drop is mostly on the valve, and the subsequent flow measurement is stable. But the problem is that the measurement is lagging behind the control, that is to say, the flow measured by the flowmeter is out of control, and it flows to the back, which is unfavorable in control. Therefore, in most cases, the control valve should be placed behind the sensor.