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I am wanting to read a single 4-20mA signal from a water meter on two separate data loggers. I need help on the wiring in order to split the signal to the two loggers. The first logger is a Campbell Scientific CR800 data logger https://www.campbellsci.com/cr800 which provides power to the loop. The second is homemade a non powered 4-20mA register connected to a Raspberry pi.

4-20 mA loop piHat connected to a Raspberry pi

From the water meter which is an Octive Master Meter is outfitted with a 4-20mA output device, there is a + , - , and a ground wire coming from the meter.

Any and all help on this topic will be appreciated and welcomed. I hope this is enough information, please let me know if you need any additional information. Thanks.

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You can simply put current meters in series.

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    \$\begingroup\$ This is only true if all but one of the current meters is unpowered. In this case it would work providing if the OP is correct and only the CR800 is powering the loop. The other issue to be considered is that many loop meters are very sensitive to any one of them being turned off. I've seen considerable changes in readings without a well designed meter front end (leakage to supply or ground). \$\endgroup\$ – Jack Creasey Jan 16 '18 at 23:13
  • \$\begingroup\$ That's very true! But the alternative would be to convert the current to a voltage (resistor), and buffer that voltage, and convert it back to two currents with a voltage controlled current source each. That solution would then have different component realizations per measurement "end". \$\endgroup\$ – Marcus Müller Jan 16 '18 at 23:16
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    \$\begingroup\$ Very true indeed .....but you need to read the voltage over the resistor with a differential amp since there will be offsets from ground. Even here you must take care that any protection on the differential input (typically diodes) doe snot affect the other meters when turned off. \$\endgroup\$ – Jack Creasey Jan 16 '18 at 23:18
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    \$\begingroup\$ gut feeling, and general commentary: Digital data is easy to copy losslessly. Analog phenomena are not. So. One data converter. \$\endgroup\$ – Marcus Müller Jan 16 '18 at 23:19
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A 4-20 mA remote device is powered by the interface it reports into; you can make multiple 4-20 mA outputs that respond to a single remote device, but that would couple the power sources (possibly cross-connect the grounds) and isn't compatible with the full range of schemes used in dataloggers' interfaces. It's possible with isolation amplifiers, or perhaps with matched optoisolators, to make floating 4-20mA slave outputs operated by a single input, like:

schematic

simulate this circuit – Schematic created using CircuitLab

Easiest, however, is to make a conversion from 4-20 mA to 0.4-2.0VDC, and do datalogging with the voltage signal (easily buffered to drive any number of voltage-input devices).

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