Stinky summed it up nicely. I just grow in soil, and every once in a while will just use the chemical pH kits/charts to see where I stand. Just to reiterate, it's not difficult to maintain a healthy pH in quality soil. Never felt the need to get one of those expensive pens... although I'm sure I will when I get my DWC system going. Instead of getting the EC/PPM meters, you might think about spending the money on a decent digital thermometer/hygrometer to keep in your grow area. That's all I use, and at the moment, is all I need.