Which is the ppm scale measurments everyone is referencing to online naci kci or 442? Which one I should be using
I've used mostly the 442(700) for long time. That doesn't make it any better to you because I do though.
What ever one you get, understand what that conversion factor is, so when you post about ppm #'s on forums, you can also tell them at what conversion factor that ppm# is based off of
Or look at a chart and convert it to EC, so when you post online you can use the universal EC instead
Or get a meter that does EC, or, some are switchable between .5, .7, and EC.