from the Signal Transmission department...
How to check 4-20 mA signal
Engineering and workplace issues. topic
Posted by Sanath Sahu on 23 November, 2008 - 8:28 pm
I want to check the correctness of 4-20mA signal in my measurement loop. When I connect the multimeter (in series) in the negative line it shows 16 mA. Whereas on connecting the multimeter (in series) in the positive line it shows 0 mA. The transmitter reading varies as per the process requirement.

Please help me as I am confused.

sanatsahu at gmail. com

Posted by Roy Matson on 24 November, 2008 - 2:54 pm
This often occurs in a panel where there are multiple loops connected to 1 I/O module. There must be an alternative path for your positive line (some wire in parallel). You need to make certain that the positive and negative current to/from the transmitter in the field is in the same cable pair otherwise your signal is likely to be noisy. Measure the positive and negative wire at the field device, there both wires must be carrying the same current. If not, post a more detailed description of your circuit.

Hope this helps.


Posted by zahid ali khan on 25 November, 2008 - 12:09 am
Dear Sanat Sahu,

I think your instrument is not in working. It is internally short with ground. In 4-20mA loop positive and negative current should be same.

Zahid Ali Khan

Posted by yantsi on 25 November, 2008 - 12:15 am
What do you mean by connect to the positive line and negative line? You just do the series connection, that is ok.

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