Ah, that one. I know it, looked at it. Very scary as you use a regular power plug (so someone may plug it in for whatever reason).
Range and accuracy are low for this technique as it's a voltage divider. Best accuracy is when your liquid's resistance is about the same as the other resistor's value. Also it uses DC which is bad for EC measurements. You have to use AC (1-1000 kHz). If using DC you will find that over the first few seconds the EC you measure changes considerably, and you will see bubbles forming at your probe (a sign of electrolysis).
My main focus has been on building an EC probe by timing the discharge of a small capacitor, and using the same cap to produce an AC current through the liquid. Getting excellent results, now got a bunch of test probes running here, and hope to turn this in a product really soon. Getting quite ready for that. Accuracy is about 2% over two orders of magnitude (0.05-5 mS/cm - typical hydroponic range - can measure higher with lower accuracy as well).
A second technique I'm now employing is by using a 555 timer, I'm getting a range of 0.001-100 mS/cm with little effort, and first results also indicate 1-2% accuracy. Needs more thorough testing, especially on long term stability.
Very important is temperature compensation, as the EC increases by about 2% for each 1 degree C increase in temperature for most ionic solutions. pH is also temperature dependent but less strongly.