First everything is measured as a Siemen (unit) in mS by your meter. This number is then calculated depending on how you read your meter as: EC, PPM 500, or PPM 700 using the formulas below:
mS (Microsiemen) = EC (Electrical Conductivity) so 1.0 mS = 1.0 EC (It's that straightforward.) Calculator[2]
1.0 EC = 1.0 mS
1.0EC x 500 = 500ppm (500 NaCl scale) or 500ppm / 500 = 1.0EC
1.0EC x 700 = 700ppm (700 442™ scale) or 700ppm / 700 = 1.0EC
The second example below uses 2.4 mS as the base value instead of 1.0 mS as above:
2.4 EC = 2.4 mS
2.4EC x 500 = 1200ppm (500 NaCl scale) or 1200ppm / 500 = 2.4EC
2.4EC x 700 = 1680ppm (700 442™ scale) or 1680ppm / 700 = 2.4EC
How your meter would calculate a solution of 1200 PPM using the PPM 500 NaCI scale:
(Note: most meters skip the step of converting µS to mS/EC and just read mS/EC)
- Reads electrical conductivity as 2,400 µS
- Divides 2,400 by 1,000 to get 2.4 EC/mS
- Multiplies 2.4 EC/mS by 500 (NaCI scale) to get 1200 PPM
What if I only know my PPM & EC, what is my scale?
Switch your meter to PPM, drop in solution, let stabilize and record PPM reading.
Switch your meter to EC, record EC reading.
PPM / EC = PPM Scale
1680 / 2.4 = 700 PPM
Here is a chart:
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EC = Electrical Conductivity
TDS = Total Dissolved Solids
PPM = Parts Per Million
µS (or µS/cm) = micro-Siemens (one millionth of a siemen.)
mS (or mS/cm) = milli-Siemens (one thousandth of a siemen.)
NaCl = Sodium Chloride
KCl = Potassium Chloride
442 = 442 Natural Water (The "442" is an abbreviation for 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride.)
