TDS scales 500, 640 and 700 explained
Why hydroponic PPM meters read differently, what the 500, 640 and 700 factors mean, and how to keep EC and PPM readings consistent.
PPM starts life as an EC measurement
A total dissolved solids meter does not weigh the salts in your reservoir. It passes a small current through the water, measures the electrical conductivity, then multiplies that reading by a fixed number to display a PPM value. The conductivity is the real measurement, and PPM is a convenience figure derived from it. That is why a plain EC reading in millisiemens per centimeter is the most portable way to describe solution strength.
Where 500, 640 and 700 come from
Each scale is the assumed PPM produced by an EC of 1.0 mS/cm. On the 500 scale, EC 1.0 is called 500 ppm, a convention used by Hanna and many United States meters. On the 640 scale, favored by some Eutech instruments, EC 1.0 is 640 ppm. On the 700 scale, used by the Bluelab Truncheon and much of the European market, EC 1.0 is 700 ppm. The scales exist because different reference salts and calibration standards were adopted by different makers, and none is more correct than the others.
The same solution, three different numbers
Take a reservoir that measures EC 2.0 mS/cm. On the 500 scale it reads 1000 ppm, on the 640 scale it reads 1280 ppm, and on the 700 scale it reads 1400 ppm. Nothing about the water changed, only the multiplier. This is the single biggest cause of confusion when growers swap recipes online, because a target quoted only in PPM is meaningless until you know which scale it assumes.
How to avoid mixing scales
Pick one scale, set your meter to it, and record every target in EC alongside PPM so the reading is never ambiguous. When you copy a feeding schedule from another grower, convert their PPM to EC first, then to your own scale, using this calculator. If your meter cannot switch scales, note which one it is fixed to and translate incoming numbers rather than trusting them at face value.