need good link explaining npk, ppm, and ec

looking for a good link explaining npk, ppm and ec. any one got anything? i understand the basics...

looks like ec is used in other countries and ppm is used here but i guess there are different conversion ratios? should i be working in ec or ppm?

i don't know how to post a link but there is a thread in this forum named tds,ppm,and ec that might help you. I'm trying to learn how to control my nutes and I got a pen from amazon that measures EC, TDS(ppm), and temp for about $50. IMO EC is better to work with. hope this helped PJ