4-20 ma flow transmitter with siemens plc

[Sabarish]

Hi ,when i check the 4-20 ma output from flowtransmitter with multimeter ,it can shows 4 ma , but when i check with process meter it only shows 2ma instead of 4ma , and in siemens plc 1200 analog input also shows the same as processmeter as 2ma ,what will be the issue ? for all the values from 4-20 ma it follows the same ,in 20ma plc can read lesser than 10 ma .Is it because of the load resistor issue on the flowtransmitter?

 

[danw]

All dc currents are read as a voltage by any instrument by feeding the dc loop current through a precision dropping resistor and the reading the voltage drop across the resistor.

The power supply powering the transmitter determines how much loop resistance the transmitter can drive 20mA through. For 4-wire devices, the maximum loop resistance is either stated as a value in the device's spec sheet, like 500 Ohms or 700 Ohms or 1000 Ohms. For 2-wire devices, the maximum loop resistance is read from a 'Load Line' chart/graph where the loop resistance is a function of the loop power supply voltage.

DVM's typically use a low value precision resistor to read current. But a process meter will typically use a much higher value dropping resistor.

Depending on what else in the circuit, the input resistance of a process meter might well overload the ability of a transmitter and its power supply to supply the expected current.

But it isn't clear whether there is anything other than the transmitter and the DVM or the process meter.

If the transmitter is a 2-wire transmitter then make sure that its loop power supply is functioning properly.