Conductivity measurements of demineralized water - Unexpected results. Looking for explanation

I'm trying to measure the conductivity change of demineralized water over a few days. But I get results I can't explain.


Setup
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This is the circuit and cell I'm using:


[![enter image description here](https://i.stack.imgur.com/f58HE.jpg)](https://i.stack.imgur.com/f58HE.jpg)


Where $R\_x$ is the conductivity cell, R is $1797 \Omega$.  

The right picture shows the cell ($R\_x$). The metal plates shown (barely visible) are made of stainless steel.


Evaluation
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On ch 1 and ch 2 I measure the voltage by consecutively taking a sample of $10^4$ points over 4 ms on each channel then I calculate the RMS.
Using ohms law I calculate the resistance of the cell:


$R\_x = R \frac{Ch\_1-Ch\_2}{Ch\_2}$ 


The applied current is a sine curve of 0.5V with varying frequency between 1, 5, 10, 50 and 100 kHz. It is turned on for about 2-3 seconds during the reading of the two channels. 


For each frequency I make 10 measurements and take the mean of those.  

These measurements are then repeated every 30 min. Shown in the graphs below. 


(Sample 1-3: I changed the water and repeated the measurement to see if it's caused by some residue in the cell from previous measurements, although I cleaned it thoroughly)


[![enter image description here](https://i.stack.imgur.com/oASzX.png)](https://i.stack.imgur.com/oASzX.png)


Why do the curves look like this? 
If it's caused by polarization of the ions in the solution wouldn't the resistance increase with time?  

I would be happy if someone could point me in the right direction finding a theory explaining this.


