This must have been done millions of times. The trouble with this measurement is the DC voltage on the ohm scale introduces polarisation of the ions in solution. Even the probes go into solution (slowly).
Try it . . . you'll see what I mean.

If it doesn't work then couldn't I just short the chart a little and dump out the water every 2-3 days to be on the safe side (into one of the other plants in dirt)? PH can be measured from strips so that doesn't worry me much.

One guy told me that if you keep your ohm scale probes in FAST flowing water (ie: near the exit of a pump) the readings become a lot more stable.
Why don't you try that.

Ericjwi,

Check out this page. It's a DIY conductivity meter, pretty informative and a real interesting read.

http://blea.ch/wiki/index.php/PPM_Meter

Best of luck!

-Brian

Here is a link that might be useful: DIY EC/PPM/Conductivity/TDS