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We are often asked for the relation between flow rate and analog output (DC 4-20 mA) of flowmeter. This is an easy explanation of such analog output.
At analog output type flowmeters the flow rate value is converted to electric current and transmitted.
The output current is designed so that 4 mA corresponds to 0% of full scale range of the flowmeter and 20 mA corresponds to 100% of full scale range of the flowmeter irrespective of absolute flow rate. The change of current output is linear to the flow rate change.
For example, when the scale range of flowmeter is designed as 0-30 m3/h, the output
current will be;
|Percentage of full scale||Actual flow rate||Current output figure|
|10%||3 m3/h||5.6 mA|
|50%||15 m3/h||12 mA|
|100%||30 m3/h||20 mA|
This is indicated by the following graph.
When the load resistance of the circuit is within the limited figure, the output current which is proportional to the flow rate is obtained irrespective of the load resistance.
the power supply line (normally, DC 24 V) and signal current line are common. For indication,
a current meter, on which 100% is graduated at 20 mA point and 0% at 4 mA, is connected
serial in such signal line.
Wiring diagram of AM-1520 type variable area flowmeter is shown for reference.