PPM Scales and Why EC Is More Transferable
Distinguish true mass concentration from meter-derived TDS or ppm displays and standardize nutrient records in direct units.
Educational reference · evidence, sources, and limits shown below
Distinguish true mass concentration from meter-derived TDS or ppm displays and standardize nutrient records in direct units.
Terms to know
- mg/L
- Mass of a specified substance per solution volume; approximately ppm by mass for dilute water solutions when the substance is defined.
- TDS
- Total dissolved solids measured gravimetrically or estimated from conductivity using an assumed factor.
- 500 scale
- Meter convention that estimates displayed ppm as EC in mS/cm multiplied by about 500.
- 700 scale
- Alternative meter convention that estimates displayed ppm as EC in mS/cm multiplied by about 700.
Core science
The word ppm is ambiguous in cultivation. A fertilizer formulation may specify 150 mg/L elemental N, which identifies both substance and mass concentration. A conductivity meter’s ppm display is usually an estimated TDS value calculated from EC with a built-in factor. It does not measure elemental N or actual fertilizer mass.
At 2.0 mS/cm, a 500-scale meter may display about 1000 ppm while a 700-scale meter displays about 1400 ppm for the same solution. Neither number changes the solution. Natural-water EC-to-TDS factors also vary with ion composition, so even a laboratory estimate needs a stated method.
EC in mS/cm is more transferable because it reports the direct conductivity measurement without hiding the conversion convention. Formulation should use actual elemental mg/L, mmol/L, or mass per volume; field monitoring should preserve EC plus temperature and method. If a ppm display is retained, record the factor and meter model.
Why this matters in cultivation
- Many apparent feed-chart disagreements are unit disagreements. Converting every target into elemental concentration and EC prevents 500/700-scale errors and oxide-versus-element errors.
- Product mass, guaranteed analysis, injector ratio, final volume, and source-water contribution are needed to calculate nutrient dose; a ppm pen cannot reconstruct them.
Measure and record
Displayed unit
EC, microS/cm, mS/cm, dS/m, TDS, ppm, mg/L, mmol/L, or mass per volume.
Conversion basis
500, 640, 700, meter-specific factor, gravimetric TDS, or no conversion.
Defined substance
Elemental N/P/K/Ca/Mg or total dissolved solids; never record ppm without the subject.
Calculation
Product analysis, oxide conversion, density, dilution, final volume, and independent check.
Instrument
Model, mode, factor setting, temperature reference, calibration, and raw EC retained.
Common misconceptions
Correction: A meter-derived ppm value depends on the selected EC conversion factor.
Correction: The display estimates total dissolved solids, not a specific nutrient.
Correction: EC measures conductivity; TDS is a mass concept measured or estimated by a method.
Evidence limits
EC remains composition-dependent and cannot identify ions. For regulatory, research, or formulation decisions, use defined analytes and validated laboratory or gravimetric methods rather than a generic meter conversion.
Related encyclopedia topics
- THC-ENC-134, THC-ENC-136-140, THC-ENC-327-328, and THC-GROW-042.
Source notes
- U.S. Geological Survey (2023). Salinity and total dissolved-solid determinations using PHREEQCI.
- Cox D.A. Soluble Salts and Electrical Conductivity for Greenhouse Crops. University of Massachusetts Extension.
- THC Cultivation SOP Source Materials Packet v1.0 (project source, May 2026).
- THC Cannabis Plant Science Source Packet v1.1 (project source, May 2026).
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.