m0useanswered grow question 3 years ago Often the literature will use microSiemens (uS/cm) and EC standing for electrical conductivity. These two units are the same.
http://www.maherjabado.com/ec-ppm-table.JPG
Just may need to round the decimal point over a bit to get a EC of 1.0,
Depending on the scale 500ppm or 640ppm 700ppm is all 1.0EC,
That's why EC is more accurate then PPM. PPM is not truly accurate because all TDS pens do is measure EC.
A true PPM count is done with a different machine.
If you make a solution and test its EC at 1.0 or 1000ms/cm, then switch it to ppm you will find your scale.
If your meter says 500ppm then you know that's your scale.
Can test with salt water and save nutrients.
Good Luck!