Soil Conductivity Measurement

[Shazlan]

Hi all,

I am trying to measure soil conductivity using Wenner array method. The attached picture is referred. I have a board that generates an AC voltage. The AC voltage is supplied into the soil/earth using two coulters (a round plate, partly buried in soil and attached behind a farm tractor). In between these two coulters, I have another two coulters to measure the potential difference between these two points. With regards to the measurement, I will need to measure current output of the AC voltage and the potential difference. I am using USB-6215. I have put a known resistor in series along one leg of the AC voltage. The idea is to measure the voltage across the resistor (and hence, I can get current flow) since USB-6215 cannot measure current directly.

Now, my question is, I think, a simple one. I got confused on the connection that I need to use. I know I need to use differential measurements to measure both, the current and potential difference. However, referring to NI's Field Wiring guidelines for AI (http://zone.ni.com/devzone/cda/tut/p/id/3344), I am not sure whether my case is considered as grounded or floating signal source.

Advices are much appreciated!

[KateB · Accepted solution]

Hi Lan 78,

Your signal is grounded signal.

Therefore, I recommend differential connection.

However, there are couple of things you need to be aware:

1. Make sure that voltage across resistor doesn't exceed 10V.

2. 6215 is bus isolated, not channel to channel isolated. Therefore, if you measure a large voltage different on multiple channels, it probably break down the device.

3. Bus isolated of 6125 is 30Vrms. Make sure that it doesn't exceed this requirement.

Regards, Kate

[Shazlan]

Hi KateB,

Thanks for your reply! Yes, I will be using differential measurement. Also, the supplied AC voltage is about 10V but the current is low, about 10-20mA. Hence, if I put a 100 Ohm resistor in series, my theoretical reading should be around 2V. At the same time, the voltage reading of my other AI will be in the mV. Anyway, I need a few clarifications, though - just for my own understanding.

When you said that the bus isolated limit for 6125 is 30Vrms, does that means that the summation of the voltages read at all channels has to be lower than 30Vrms? I am not very clear the significance of this bus isolation. Also, does this means that, while the differential voltage readings in all channels are very well below 10V, this still does not guarantee that the board won't break down due to overvoltage... does it?

Many thanks.

[KateB]

Hi Lan 78,

It's not summation of the voltage, it's a voltage between channel and the system bus of 6215 as shown in "USB-6215/6216/6218 Maximum Working Voltage" image in http://sine.ni.com/ds/app/doc/p/id/ds-9/lang/en.

This voltage different causes from the difference between source ground and bus system ground voltage.

 However, it sounds like your voltage is not that high.

Regards, kate

[Shazlan]

Thanks Kate. It's not very clear to me right now but I'll get the local NI engineer to explain this to me. It's for my reference when choosing the DAQ module in the future 🙂