Two growers can measure the same bucket and read 700 ppm and 500 ppm. Neither meter is broken. Type any one number below and every other way of writing it fills in.
Convert
Temperature correction
Conductivity rises with temperature by roughly 2% per °C, so the same solution reads higher when warm. Meters quote everything at 25 °C. A meter with ATC (automatic temperature compensation) has already applied this correction. Without it, correct here.
Target strength by crop and stage
Starting points from the same tables the Fertigation Recipe Analysis / Calculator uses. Always a starting point, never a prescription: verify against your own water, cultivar, and conditions.
Why two meters disagree
Nothing in a cheap meter can weigh dissolved solids. What it measures is electrical conductivity: how well the water carries a current, which rises as you dissolve more salt in it. That is the only measurement being taken.
"ppm" is that conductivity multiplied by a constant the manufacturer picked, based on which reference salt they decided to pretend your solution is made of. Potassium chloride conducts better than sodium chloride, so the same EC implies a different mass depending on the assumption:
So 2.0 mS/cm is 1000 ppm, 1280 ppm, and 1400 ppm, all at once, all correct. Nothing about the water changed. This is why a recipe that says "feed at 800 ppm" is ambiguous and one that says "feed at 1.6 mS/cm" is not.
In practice: EC is the unmediated measurement. If a recipe requires ppm, find out your meter's scale (it's in the manual, and often printed near the display) and write it down next to every number you record. And note what EC cannot tell you at all: which salts are in there. A solution starved of calcium and drowning in sodium can read exactly the same EC as a perfectly balanced one.