Chow_13answered grow question 4 years ago From what I can see with the vivosun meter is µs/cm or TDS.
It would be best to use the µs/cm Which means you would divide by 1000 to get EC. so a reading of 400 would be .4 EC.
TDS calculation is different depending on the manufacture. most will go on a 500 scale or 700 scale. but there is also a 550 and a 640. This calculation to get EC would be to divide by the scale that the meter uses. so If the meter read 350 and the meter was on a 700 scale you would take 350/700 = .5 EC
Also, are you running hydroponics with the roots directly in water? If so your PH should be between 5.5 and 6.5.( 6.5-7.0 is for soil )