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The 500 scale (NaCl equivalent) is used by many American meters. 2.0 mS/cm equals 1000 ppm on the 500 scale.
PPM500 = EC (mS/cm) × 500The 700 scale (442 standard) is used by Hanna and some European meters. Same EC reads 40% higher on 700 vs 500 scale.
PPM700 = EC (mS/cm) × 700Reverse conversion: divide ppm by your meter's scale factor. Always know which scale your meter uses before comparing to published ppm targets.
EC = PPM ÷ Scale factorUpdated: July 2026
Online guide says 1400 ppm (700 scale) — your 500-scale meter should target 1000 ppm (2.0 mS/cm EC).
Lab EC meter reads 1.8 mS/cm; handheld TDS reads 1260 ppm — confirms 700 scale meter (1.8 × 700 = 1260).
Product label lists EC targets (2.2 mS/cm bloom) — convert to 1100 ppm (500 scale) for your Bluelab TDS pen.
Nutrient meters display either EC (electrical conductivity) or TDS (ppm), and scales vary by manufacturer. Enter your reading to convert between EC in mS/cm and ppm on both the 500 (NaCl) and 700 (442) scales used by popular meters.