ppm700 (the 700 scale) converts EC to ppm by multiplying by about 0.7. It is based on the "442" natural-water reference, a mixture of 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride meant to mimic the ions in fresh water. It is common on meters sold in Europe and Australia and in some nutrient charts.
In practice
A solution at 1.6 mS/cm reads 1,120 ppm on a 700-scale meter and 800 ppm on a 500-scale meter. If a feeding chart says "1,000 ppm in flower" and does not say which scale, it could mean 1.43 mS/cm (700 scale) or 2.0 mS/cm (500 scale). That is a big gap at the plant.
Check your meter's manual or settings for "442" or "0.7". When sharing numbers, give EC in mS/cm, or at least label ppm as ppm500 or ppm700.
Units and formula
- ppm700 = EC (mS/cm) × 700
- EC (mS/cm) = ppm700 ÷ 700
- ppm500 = ppm700 ÷ 1.4
Don't confuse with
- ppm500: the NaCl scale; 40% lower readings for the same solution.
- ppm640: a factor used in some agronomy references (640 ppm per dS/m).
EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Sodium (Na) is not essential for cannabis; it enters through water, softeners and some inputs, and at high levels causes salt stress and crowds out potassium and calcium.Sulfate (SO4 2-) is the form in which plants take up sulfur, delivered mainly by magnesium sulfate and potassium sulfate in mineral feeds.As a plant nutrient, chlorine is used as the chloride ion (Cl-) in tiny amounts for photosynthesis and osmotic balance; excess from water or salts is the usual problem.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.A feed chart is a manufacturer's or grower's schedule of nutrient doses by week or stage; treat it as a ceiling to test against, not a rule.ppm500 is the TDS scale that converts EC to ppm with a factor of 0.5 (based on sodium chloride), so 1.0 mS/cm reads 500 ppm.Input EC is the electrical conductivity of the nutrient solution going into the pot or system, in mS/cm, including the base EC of the source water.TDS (total dissolved solids) is the mass of dissolved material in water; grow meters estimate it from EC with a conversion factor and display it as ppm.The conversion factor is the multiplier a TDS meter applies to EC to report ppm, commonly 0.5, 0.64 or 0.7; it is why ppm readings differ between meters.
