This board has a continuous and on demand mode. Send it an "E" command to terminate continuous. Send it a "C" command to get a value periodically say every 10 seconds. Calibration has to be performed with a 7 and 4 or 7 and 10 standard. Do the 7 first as this fixes the intercept then do 4 as this fixes the slope.
You may also want to consider using analog ports rather than serial when you have more than one sensor (pH and EC). These are some options.
http://webpages.charter.net/tdsmeter/
You could add a temp sensor on yet a another analog port but the accuracy required for hydro use may not justify it unless you will be controlling envrionmental equipment.