CN0359 Conductivity Board Constant Resistance Error

Before we even plug the sensor in, we are looking for an experiment to test and ensure the Analog board is working properly. If this experiment was not valid, can you suggest one?

If not, I can plug in the sensor, yet we were trying to simplify everything and leave the sensor completely out of the analog board validation. (The we got the same results when the sensor was plugged in measuring standard known concentrations of KCl salt.)  

Hi ND_Deuterium,

Assuming your scope is a standard benchtop unit (and not a small, battery-powered, floating unit with very low self-capacitance), I suspect the problem is that you're shorting nodes to ground that really only should be connected to the sensor's excitation terminals (or your test resistor.)

In experiment 1, the excitation return is shorted to ground. This node is actually a virtual ground and should not be loaded.

In experiment 2, The excitation drive is shorted to ground, so you would expect no signal.

And another thing to confirm - is your power adapter ground tied to Earth ground? Even it it is "floating" when measured with an ohmmeter, the capacitance to Earth ground can be high (though the adapter's noise reduction Y-capacitor.)

Aside from that - the fixed resistor test is exactly what you want to do, but pins 1 and 2 must be shorted together, and pins 3 and 4 must be shorted together.

So try shorting 1-2 and 3-4, and disconnect the scope entirely and see what happens.

It's okay to probe those signals, but scope ground should only ever be connected to CN0359 circuit ground (pin 5.)

-Mark

Sorry for the extremely late reply to this message. 

1. I went ahead and performed the fixed resistor test that you previously suggested. I shorted pins 1 and 2 together and then shorted pins 3 and 4 together. I used a 1000 ohm resistor and upon turning the conductivity probe on, I was able to obtain a stable reading. Fantastic. I did plug in the scope to probe the signals and everything looked good. 

2.  We are now trying to have the conductivity board probe a standard solution that is being continuously stirred. We are using an LFS 1107 conductivity sensor from Innovative Sensor Technology (IST). We know that the LFS 1107 probe is working properly. As instructed by IST the probe was isolate with a non-conductive epoxy. A 200Hz square wave AC current was driven with a function generator. The voltage drop across the electrodes was measured with an oscilloscope. With the known cell constant, we were able to measure the CORRECT conductivity of the calibration solution. Numerous calibration solutions were tested, the voltage drop varied between 700mV and 60mV (depending on the molarity of the calibration solution). The LFS prove held constant at all calibration solutions and provided the correct reading. 

We then plugged the four wires of the probe into the Analog conductivity board. We are not able to get a steady signal. The board settings were indeed changed to provide a 200 Hz current with a voltage drop held at 700mV. The measurement is not constant, no matter the concentration of the calibration solution. The conductivity board measurement jumps from uS/cm to mS/cm to kS/cm all while in the same standard solution. We attempted to change the desired voltage drop to numerous settings yet the results were consistent, the probe continue to jump around and cannot provide a steady measurement.

How can this be addressed and what is likely going on? Please let me know if the description of these experiments is unclear or if it would be helpful to have more information on the equipment we are using. 

So it looks like you may be encountering an issue that recently came up with another customer. As with any conductivity measurement device, there will be a small, nonzero DC component to the excitation signal, and a finite bias current on the sense leads. This is normally not a problem with larger conductivity probes, but appears to cause issues with small, planar probes, like yours:

https://www.ist-ag.com/sites/default/files/dclfs1107_e.pdf 

We don't have a schedule on when updated hardware will be released, but the fix is pretty straightforward - capacitively couple the excitation and sense leads, ensuring zero DC content. (Noting that "zero" is an impossibly small number - there will always be some leakage through any capacitor.)

Here is the original discussion:

https://ez.analog.com/circuits_from_the_lab/f/q-a/120649/cn0359-conductivity-measurement-problem/360220#360220

And another more recent issue:

https://ez.analog.com/circuits_from_the_lab/f/q-a/119491/cn-0359-testing 

-Mark