This may be a dumb question but if im going to run Jack's 321 which ppm scale do I use 500(tds)or 700. I know there is a ac function as well. I tried to look on the packaging all I saw was ec.
Use whatever scale you want just don't go between 500 and 700 scale.
I use 500 just because I know .1ec = 50ppm on the 500 scale. 700 is different and I don't know the conversion.