You use a ec or tds meter...ppm is parts per million.. you use it too tell how much of your nuriants are in the water after the plant has been drinking the water and eating the nutriants... ppm will go down if your plant eats more nutriants than it drinks water, and it will go up if it drinks more water than it eats...this helps you see if you are feeding your plants the right amount of nutriants..

the site oldsanclem listed will answer all your questions... the whole site is helpful..but this section on FAQ will give you a good answer on everything you want to know and it is nicely arranged in catagories and will most likely give you a better understanding than me

Sharp Rubber Duck (.NET) - Cannabis cultivation: Overgrow's GrowFAQ
cnhazty Reviewed by cnhazty on . Air/PPM questions I am waiting for my light and a few other things to come in and i will start my first grow...I have 2-18 gallon rubbermaids set up holding 4 plants a peice... My plan is to do a grow journal and wait too see the sex before desiding how to split the females between the rubbermaids.... I have some concerns on my grow... I have everything for growing other than a ec meter.. how needed is this? would i be able to survive if i was let my tap water sit out about 24 hours before adding nutriants Rating: 5