E.C. this P.P.M. that What to use?????????

All right . I need some advise/help. I have a hanna growcheck that shows temp,ph,ppm of tds and ec. Ive been reading a little about how E.C. is the universal reading and the P.P.M. of meters vary with manufacturer. With that being said is there anyone on here that can explain in terms a pipe fitter can understand how to use the E.C. as opposed to P.P.M. I guess what I'm asking is why should I use the E.C. reading as apposed to the P.P.M. reading?Or am I just being overly cautious and don't need to be?Any help will be greatly appreciated.:thumbsup:

There are two different formulas to convert ppm into EC, depending on manufacturer, check this out Difference between ppm and EC