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4 to 20 ma wiring detail

GeodeNZ May 22, 2020 08:56 AM

I'm setting up a CR1000 with ten 4-20 millimap transmitters/sensors. I'll be measuring on single ended channels. I have the precision 100 ohm shunt resistors. My question is in regard to grounding the resistors. In the past I used the adjacent analog ground terminal which seemed to do okay. The CR1000 manual suggests terminating the resistor to a ground point, not AG. This makes sense. But then I will have 10 return wires needing to connect to a ground connection on the logger wiring panel?  I'm wondering the best way to get these ten conductors to a suitable ground connection. Possibly I'm over thinking it ;)


JDavis May 22, 2020 04:42 PM

The reason for using the power grounds (G) instead of analog grounds is to avoid inducing offsets on analog ground with the current flowing. 

You just need to physically get those wires connected to the G ground. You can combine wires with things like spring nuts fairly easily. If you search online for "Wago spring nuts" you will find several options.

aps May 26, 2020 01:51 AM

One thing to be careful of is that if you make single ended measurements and use different gorund connections (especially with up to 200 mA of ground current flowing from ten sensors), there may be small differences caused by the differences in the two ground potentials.   For the best measurements you should measure differentially across the 100R resistor.

GeodeNZ May 26, 2020 11:01 AM

Thank you both for the suggestions. I'll measure as many as I can differentially, but will be running out of channels in all liklihood and will need to do a few single ended. My environment will be noisy, I have my voltage instructions configured with the 60Hz rejection option. Would it help to take a number of measurements and average the result? Speed is not critical here, my main scan will be in the 1 to 2 second range.


JDavis May 26, 2020 02:24 PM

Mark your VoltSE measurements to measure the offset each time. The offsets caused by the 4-20 mA signals will be bigger than AC noise.

GeodeNZ May 27, 2020 04:40 PM

Thanks, will do!

zms Aug 12, 2020 10:53 AM

This thread is helpful with some dosing pumps that I need to control.  However, it is my understanding that on my CR6 (SN 13699) that I can implement a currentSE instruction rather than a voltSE instruction with CURS100 terminals.  Is this correct?  The instruction is not present in PC400 V4.5.  

Also, I will need to both read current settings from the pump, but also issue them.  Can I send amperages via these commands, if I'm understanding correctly? 

thank you

JDavis Aug 13, 2020 02:41 PM

If the CurrentSE instruction does not appear in your CRBasic Editor, download the current CR6 OS. Run that .exe file on your computer, and it will update your CRBasic Editor instruction list.

To output a 4-20mA signal, you would need a SDM-CVO4.

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