I use a bluelab truncheun e.c meter it automatically calculates into ppm's for you but there is a simple equation to figure it out your self. Knowing your ppm's will help you know how much you are feeding your plants, in hydro you can find this quite useful and be able to tell weteher or not your plants are drinking more water or nutes. In hydro if you start your water with say a ppm of 800 and 6 hours later your ppm's are 900 or 1000 you know that your plants are drinking more water, and if in 6hours it says 700ppms you know they are eating more than they are drinking. Once you use one it all becomes aparent on how useful it can be. In my experience i would get a ph meter first though.