How to Use and Read a PPM Meter Using a TDS meter will help you tell how much ferts you truly have in your water. Alot of Fert company's are vauge about mixing and everyone's water is different for instance I found that my ferts were not truly full strenth. I also use the TDS to help me identify unmarked containers because I know were on the TDS scale each fert is. First you need to identify what kind of pen you have, a good start if your brand is not listed is to look at the way it is displayed and match it up with the chart. I mixed up some full strength ferts and since I know that you should not exceed 0.8 EC on soil I was able to determine which convertion to use. Soil should not Exceed 0.8 EC Hydro Should not Exceed 2.0 I tryed to keep it simple and I wanted to give back. Please help if I made any mistakes or could have worded something better.