ec meter

i have a tds4 by trans instruments and i dont know if it measures 1.0 as 500ppm or as 700ppm, does anyone know. or is there a way of finding out. because if my reading is showing 1.5 then it could be either 750ppm or 1050ppm. i asked in my local hydro store and they dont know either but are trying to find out for me. is there any liquid which has a ppm that anyone knows then i could use that to find out.

I'm looking at thier site and don't see a TDS-4. I see a TDS-2. If it's a TDS model it's suppose to read in TDS- PPM. Where did you get it and it did not come with any calibration solution ? You can get 750 and 1000 PPM solutions. Here is a link to trans Trans Instruments. Hope this helps