FWIW, conductivity meters do not work in the way this DIY meter works. The exact size of the electrode surface will greatly impact the measure conductivity in a DIY meter, and they do not apply a DC voltage. In fact, they use an AC voltage to avoid effects at the electrodes, and the best meters use 4 electrodes rather than 2.
I discuss how these work in these two articles:
What is TDS?
http://www.reefkeeping.com/issues/20...ture/index.php
Using Conductivity to Measure Salinity
http://www.aquariumfish.com/aquarium....aspx?aid=1804
from the first one:
Can I Just Use a Multimeter to Measure Conductivity?"
No. Several factors make it impossible to accurately measure conductivity with a standard multimeter. The size and shape of the electrodes are significant, but more important is what happens at those electrodes. If a DC current is applied to seawater, numerous reactions take place when the ions hit the electrodes. Some ions will plate out on the electrodes, some may bubble off as gases, and the electrodes themselves may dissolve. These and other effects all serve to change the nature of the solution at the electrode, impacting the measured conductivity.
So how do conductivity probes get around this problem? They use an AC current rather than DC. Using fields that oscillate very rapidly, there is no overall movement of ions toward one electrode or the other. The ions move one way for a tiny fraction of a second, and then back the other direction for the second half of the cycle. Overall, the solution and electrodes stay unchanged and the conductivity is accurately measured. Modern conductivity meters use complex AC waveforms to minimize additional complications such as capacitance, which can interfere with simple conductivity measurements.
In practice, commercial conductivity probes have either two or four electrodes, with the four-electrode version being more resistant to fouling and other effects that can cause degradation of the measurement. The electrodes are made of nonreactive materials such as epoxy/graphite, glass/platinum or stainless steel. The choice depends primarily on the nature of the solution to be tested, but nearly any commercial unit will be suitable for tap water.