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I need to read the value of a 4-20 mA sensor, however I need to not interrupt the existing equipment that is already running.

I have a plc that monitors a 4-20 mA pressure sensor. I need to build a 3rd party device, that can get the reading of the pressure sensor, but not prevent the PLC from working.

Is it possible to have two masters / readers on a 4-20 mA current loop?

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    \$\begingroup\$ "A" for ampere. ("V" for volt, etc.) Fixed. Capitals matter - particularly in engineering units. \$\endgroup\$ – Transistor Jul 24 '17 at 19:02
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You can place a second receiver on an industrial 4-20 mA loop. The key to success is to ensure that the added receiver presents a minimal voltage drop across its current loop terminals.

For example, a 47 ohm resistor usually is not a problem as this represents only a ~1 volt drop. If your secondary monitoring device shares no grounds with the remainder of the system, you can then directly feed this into your ADC. If grounds are shared then a differential amp or an opto isolator is required.

Alternatively, if the PLC has communications capabilities, you may find that querying the PLC through OPC UA or similar, will net the value of the current loop.

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Monitoring a 4-20 mA signal with a second device.

Most 4-20 mA devices are internally a 0 - 5 V or 0 - 10 V analog input with, respectively, a 250 Ω or 500 Ω resistor to convert the 20 mA signal to a voltage.

If your second device can be configured as voltage input (e.g., by leaving out the current input link) then it can monitor the other input without loading it significantly.

Both devices will have to share the same DC common so take care.

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