Post Go back to editing

# CN0359 Conductivity Board Constant Resistance Error

My lab and I are using the CN0359 conductivity board to evaluate the conductivity of solutions. When the conductivity probe (Innovative Sensor Technology) is wired to the Analog Board, the conductivity measurements are all across the board. We have run the following experiment in hopes of ensuring that the Analog Board was functioning properly, yet the data suggests there may be an error within the board itself. How can we ensure the board is functioning properly?

Experiments

The Analog Board is capable of accepting resistances between 0.1 Ohms and 10 MegaOhms. If the resistance is constant, the conductivity of the system should be a constant as well. (The first picture presents the experimental set-up.)

Experiment #1

Pins 1 and 4 (red wires) were attached to an oscilloscope to measure the voltage and obtain the waveform. Pins 2 and 3 (blue wires) are wired to a 1 kilo-ohm resistor. The resistor simulates the resistance that would be expected in solution and is well within the range that should be accepted by the board. Additionally, it is a constant and removes possible error from another source. This should lead to a constant conductivity. The Analog board is then plugged in. The settings are changed to obtain a 0.7 V differential at an oscillating frequency of 160 Hz. The waveform is indeed a square wave, yet it is not completely symmetric (Figure 2). Specifically, the oscilloscope shows a max voltage of 770 mV and a minimum voltage of -870 mV. When we look at the Analog Conductivity board, the conductivity measurements bounce across the board. (Values continuously change from the microS/cm range to the mS/cm range and then shows a sensor error before beginning to jump from the uS/cm to mS/cm range again.)

Experiment #2

Additionally, it is worth noting that when we switch the polarity (switch Pins 1 and 4, the analog board stops working as no voltage drop can be measured (Image 3). We would have expected switching the leads to have no effect as it is an AC current that is generated. This was not the case.

Experiment #3

Here we tested to see if the results were reproducible. We returned to the configuration described in Experiment #1. We unplugged the board, waited a few minutes and plugged the board back in. The waveform that we obtained was shifted completely off center and was not symmetrical by any means. (Forgot to take picture.) We once again unplugged the board, waited and plugged that board back into the power supply. The waveform returned to what we saw in experiment #1 and we obtained the same (in my opinion, not sensible) results as previously described.

If these tests were not sensible, what tests can we run to ensure the board is functioning properly? I believe these tests should have functioned, if so, how can we fix the problem?