ppm500 (the 500 scale, NaCl scale) is the most common ppm scale on handheld TDS meters. The meter measures EC and multiplies by 0.5, a factor that approximates the relationship between conductivity and dissolved mass for a sodium chloride solution. It is the usual default on meters sold in North America.
In practice
A reservoir at 1.6 mS/cm reads 800 ppm on a 500-scale meter. The same reservoir reads 1,120 ppm on a 700-scale meter. Neither is wrong; they are different conversions of the same EC. Trouble starts when growers copy a chart written for one scale with a meter set to the other, and over- or under-feed by 40%.
If your meter's manual says "NaCl" or "0.5", you are on ppm500. When in doubt, switch to EC mode and work in mS/cm.
Units and formula
- ppm500 = EC (mS/cm) × 500
- EC (mS/cm) = ppm500 ÷ 500
- ppm700 = ppm500 × 1.4
Don't confuse with
- ppm700: the 442 scale, which reads 40% higher for the same solution.
- ppm of an element in a recipe: a true concentration, not a meter scale.
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.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.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.ppm700 is the TDS scale that converts EC to ppm with a factor of 0.7 (the 442 natural-water standard), so 1.0 mS/cm reads 700 ppm.Ppm of an element means milligrams of that element per litre of solution (mg/L), the unit nutrient recipes use; it is not the ppm shown on an EC meter.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.
