voltage measurement over long distant

[navneet]

Hello Forum,

i have a very important question. I want to measure the voltage difference between two locations on a ground surface which are location 100 meters away. So i will be using long cable wires. The sensors are reference electrodes. Which connection mode will be best for this measurement? Single Ended (one of the references as ground) ,or Differential?

Any ideas will be greatly appreciated and kudo'ed 🙂

[DsX · Accepted solution]

Hi,

I am not familiar with reference electrodes, but I think I would go with a differential setup for your application due to cable lengths (I am guessing 2x50m or more?). Here is a tutorial regarding voltage measurements: http://zone.ni.com/devzone/cda/tut/p/id/7113 I hope you can get some ideas from it.

The problem is that any current flowing through long wires will cause a noticeable voltage drop across that cable resulting in inaccurate measurements. This is further complicated by ground loops that are created between different ground potentials.

Best regards

Matej

[navneet]

I was having the same views, just wanted second opnion. 

Unfortunately with DIFF connections, I lower down the number of channels and I was having a hard time deciding 🙂

[johnsold]

If the measurements are important, concentrate first on making them accurate, then on how much equipment and how expensive it will be.  Making multiple poor measurements is worthless.

Depending on the overall configuration, what you are trying to do may be quite difficult or merely hard.  Can you provide more details on the nature of the measurements and the environment in which they will be made?  What is the expected magnitude of the voltages to be measured? (DC or AC?) What is the strength of any electric or magnetic fields at the sensors, around the cables, and at the measuring equipment? How fast must the measurements be made? How many channels will you eventually want?

Lynn

[Henrik_Volkers]

When I hear electrodes I think of small signals with a high impedance.

Depending of how noisy your surrounding is (free field or a fabrication plant with switched high power converters, lot of EMI etc.) you should maybe think of a signal conditioner right at the electrodes.

If you choose a 4-20mA current loop you even can do it with two wires.There are many kinds of conditioners (like B5 modules) on the market for industrial use to convert nearly any signal to a 20mA current or a low impedance 10V signal.

[navneet]

Thanks Lynn and Henrik,

Agree with you Lynn. Nothing can compete against good data. I am tryting to measure the potential of the ground at one location with respect to a reference placed 100 meters away. This reference is merely a need so that all measurements can be relative to a single location. There will be many sources of errors as the wire from one of the electrode will be in a reel. hence the choice of DIFF to allow common mode rejection. These measurements normally vary between 20 - 70 mV max, but can be as high as 500mV in anomalous locations. The measurements can be done at a sampling rate between 500 Hz to 1kHz, although smaller rates can be utilized, I want to maintain it at 1kHz. 8 DIFF channels is not bad, as I can very well work with these. I do not want to amplify the signals but to measure as they are. I have used bias resistors on both arms of the AI to avoid any accumulation. Oh and these are DC measurements.

[johnsold]

One thing immediately comes to mind: Watch for an AC component which may be much larger than the DC signal you are measuring.  The power line frequency component tends to get into everything.  In addition to differential mode consider synchronizing the sampling rate with the power line frequency and acquiring blocks which contain an integer number of samples.  This will facilitate averaging to eliminate the AC interference.  Also make sure that the peak AC componenet magnitude is not so large as to overdrive any active circuitry like amplifiers or the A/D converter.

Lynn