A conversion factor, in this context, is the number used to estimate total dissolved solids (ppm or mg/L) from measured electrical conductivity. True TDS depends on which ions are present, so the factor is an approximation. Meters use fixed values: 0.5 (NaCl, the 500 scale), 0.7 (the 442 natural-water scale) or 0.64 (a common agronomy figure).
In practice
The factor is the reason two growers with identical reservoirs post different ppm numbers. To compare, convert both back to EC: divide ppm by the factor and then by 1,000 to get mS/cm.
Example: 900 ppm on a 0.5 meter is 1.8 mS/cm; 900 ppm on a 0.7 meter is 1.29 mS/cm. Following a 700-scale chart on a 500-scale meter therefore feeds about 40% stronger than intended.
Some meters let you set the factor. Choose one, label it, and keep it for the life of your logs.
- ppm = EC (µS/cm) × factor
- EC (µS/cm) = ppm ÷ factor
- Common factors: 0.5, 0.64, 0.7
Don't confuse with
- CF (conductivity factor): an EC unit (EC × 10), not a ppm multiplier.
- Temperature compensation coefficient: the about 2% per °C correction for EC.
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.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.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.µS/cm (microsiemens per centimetre) is a unit of EC one thousandth of mS/cm, used mostly for low-conductivity water such as RO and rainwater.CF (conductivity factor) is an EC scale used mainly in Australia, New Zealand and the UK, where 1 CF equals 0.1 mS/cm (100 µS/cm).ATC (automatic temperature compensation) is a meter feature that corrects pH or EC readings for sample temperature, usually referencing EC to 25 °C.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.
