Question 1

What is the difference between 500, 640, and 700 scale PPM?

Accepted Answer

PPM cannot be measured directly by handheld meters; it is calculated from electrical conductivity (EC). The 500 scale (Hanna / Milwaukee / US standard) assumes 1.0 mS/cm = 500 ppm, based on a Potassium Chloride (KCl) solution. The 700 scale (BlueLab / Truncheon / Australian standard) assumes 1.0 mS/cm = 700 ppm, based on Sodium Chloride (NaCl). The 640 scale (Eutech / European standard) represents mixed laboratory salt 442 standards. If your feeding chart calls for 1000 ppm on a 700 scale (1.43 EC) but you use a 500 scale meter, you will unknowingly deliver 2.0 EC and risk burning your root zone.

Question 2

How do I convert EC to PPM for hydroponics?

Accepted Answer

Multiply the EC reading in mS/cm by your meter's conversion factor: for the 500 scale, multiply EC by 500 (e.g., 1.6 EC × 500 = 800 ppm); for the 700 scale, multiply EC by 700 (e.g., 1.6 EC × 700 = 1120 ppm); for the 640 scale, multiply EC by 640 (e.g., 1.6 EC × 640 = 1024 ppm). If your reading is in microSiemens (µS/cm), first divide by 1000 to obtain mS/cm.

Question 3

Does water temperature affect EC and PPM meter readings?

Accepted Answer

Yes. Ion mobility increases as water warms, which elevates electrical conductivity by approximately 2% for every 1°C (1.8°F) increase in temperature. A nutrient bath at 18°C (64°F) will appear artificially weak on an uncompensated meter compared to the same solution at 25°C (77°F). Always ensure Automatic Temperature Compensation (ATC) is enabled or adjust your reading using the standard 25°C baseline formula.

Question 4

How do I lower EC in a hydroponic reservoir without dumping nutrients?

Accepted Answer

Add pure Reverse Osmosis (RO) or low-EC freshwater directly to your reservoir. The dilution equation is: Water to Add = Current Volume × (Current EC - Target EC) / (Target EC - Freshwater EC). For example, to reduce a 25-gallon tank from 2.4 EC down to 1.6 EC using RO water (0.0 EC), add 12.5 gallons of RO water. This preserves your balanced nutrient ratio while instantly reducing salt stress.

Question 5

Should I subtract tap water EC before adding hydroponic nutrients?

Accepted Answer

Yes. Municipal tap water contains dissolved carbonates, silicates, chlorine, and sodium that register on EC meters (often 0.2 to 0.6 EC) but provide negligible plant nutrition. If your target is 1.6 EC and your tap water is 0.4 EC, feeding until the meter reads 1.6 EC gives you only 1.2 EC of actual bioavailable fertilizer. For accurate feeding, add your target fertilizer EC on top of your source water baseline, or subtract the source baseline from your final reading.

Question 6

What is an ideal EC level for vegetative vs flowering plants?

Accepted Answer

For most fruiting and flowering crops like cannabis or tomatoes, optimal vegetative growth requires an EC of 1.2 to 1.6 mS/cm (600–800 ppm on the 500 scale; 840–1120 ppm on the 700 scale). During peak bloom or fruit enlargement, nutrient demand rises to 1.8–2.4 mS/cm (900–1200 ppm on 500; 1260–1680 ppm on 700). Leafy greens (lettuce, spinach) prefer cooler, weaker solutions between 0.8 and 1.2 mS/cm to prevent tip burn and bitterness.

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