I use milwaukee digital ph pen...ebay 40 bucks...digital tds pen from Reverse-Osmosis site for 25 bucks, but, watch out, and make sure that you get a pen that can read over 1000ppm! I got a great deal, but when I juiced them mid/late bloom.it just shut off...you see they sell a 0-1000 tds pen, because they just want you to see that your water quality is better, say 003ro water-200 or so tapwater...So, make sure you get a pen that can read up to 1500-2000ppm's at least...hope this helps