What is the difference between the 500 and 700 PPM scales?

TDS meters do not measure parts per million directly; they measure electrical conductivity (EC in mS/cm) and multiply it by a conversion factor. The 500 scale (Hanna / NaCl calibration) uses EC * 500. The 700 scale (Truncheon / KCl / European scale) uses EC * 700. For example, 1.5 EC equals 750 PPM on the 500 scale, but 1050 PPM on the 700 scale.

Why is Electrical Conductivity (EC) the preferred scientific metric?

EC is an objective physical measurement of electrical current carried through dissolved mineral salts, expressed in milliSiemens per centimeter (mS/cm) or microSiemens (µS/cm). Unlike PPM, EC does not depend on proprietary meter conversion algorithms.

How do you dilute a hydroponic reservoir when EC is too high?

When plants drink water faster than nutrients, salt concentration spikes. Use the dilution formula: Water to Add = Current Volume * ((Current EC - Target EC) / (Target EC - Fresh Water EC)).

Why does nutrient temperature affect EC readings?

Electrical conductivity increases by approximately 2% for every 1°C increase in water temperature because ions move faster in warm solutions. High-quality EC meters use Automatic Temperature Compensation (ATC) to normalize readings back to 25°C (77°F).

HydroEC Pro - Hydroponics EC to PPM Converter & Dosing Calculator

🔄 Universal Nutrient Scale Converter

Electrical Conductivity (EC)

mS/cm (or dS/m) — Scientific Gold Standard

PPM 500 Scale (Hanna / US)

Factor: 0.50 (NaCl standard)

PPM 700 Scale (Truncheon / UK / AU)

Factor: 0.70 (KCl standard)

PPM 640 Scale (442 / Myron L)

Factor: 0.64 (Natural water salt mix)

Conductivity Factor (CF)

1 EC = 10 CF (New Zealand / Hydroponic meter)

🌱 Current Strength: 1.8 EC is optimal for mature fruiting tomatoes, peppers, or mid-bloom hydroponic crops.

💧 Reservoir Dilution & Top-Off

Reservoir Current Volume

Current Measured EC

Target Desired EC

Fresh Top-off Water Baseline EC

Target Final Tank Volume: 28.9 Gallons.
If tank overflow is an issue, drain 6.2 Gal first and replace with clean fresh water.

🌿 Recommended Hydroponic Nutrient Ranges by Crop

Crop	Optimal EC (mS/cm)	PPM 500 (Hanna)	PPM 700 (Truncheon)	Target pH Range
Lettuce & Salad Greens	0.8 – 1.2	400 – 600	560 – 840	5.5 – 6.0
Strawberries	1.0 – 1.4	500 – 700	700 – 980	5.5 – 6.2
Culinary Basil & Herbs	1.0 – 1.6	500 – 800	700 – 1,120	5.5 – 6.5
Spinach	1.2 – 1.8	600 – 900	840 – 1,260	5.8 – 6.5
Bell Peppers (Bloom)	1.8 – 2.4	900 – 1,200	1,260 – 1,680	5.8 – 6.3
Tomatoes (Fruiting Stage)	2.0 – 3.0	1,000 – 1,500	1,400 – 2,100	5.8 – 6.5
Cucumbers	1.8 – 2.5	900 – 1,250	1,260 – 1,750	5.5 – 6.0

Understanding Hydroponic Electrical Conductivity & TDS Scales

In soilless cultivation (NFT, Deep Water Culture, Aeroponics, Rockwool, Coco Coir), dissolved nutrient salts (Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and trace elements) dissociate into electrically charged ions ($NO_3^-$, $K^+$, $Ca^{2+}$). The higher the salt concentration in the reservoir, the more easily electricity flows through the water.

The Great PPM Scale Confusion (500 vs 640 vs 700)

Because no handheld probe can directly count physical molecules in solution, meter manufacturers translate the electrical signal (EC) into an estimated "Parts Per Million" (PPM) using different calibration standard salts:

500 Scale (0.5 Conversion): Calibrated using pure Sodium Chloride ($NaCl$). Used predominantly by American growers and Hanna Instruments.
700 Scale (0.7 Conversion): Calibrated using Potassium Chloride ($KCl$). Universal standard across the UK, Europe, Australia, and Bluelab Truncheon pens.
640 Scale (0.64 Conversion): The "442" curve developed by Myron L, representing a blend of $40\%\ Na_2SO_4$, $40\%\ NaHCO_3$, and $20\%\ NaCl$ to mirror natural fresh river water.

Always compare feeding charts and fellow growers' notes in EC (mS/cm) to eliminate conversion confusion entirely.

Frequently Asked Questions

What does it mean when reservoir EC climbs while water drops?

If reservoir water volume drops while EC rises, your plants are "transpiring" (drinking water) faster than they are absorbing mineral salts. The current nutrient mixture is too concentrated, which can lead to salt burn and nutrient lockout. Immediately dilute the reservoir with fresh RO water.

What does it mean when reservoir EC falls?

If EC drops, your plants are feeding aggressively and taking up nutrients faster than water. This is an indicator that you can safely step up your target nutrient strength on the next feeding.

How often should hydroponic nutrient reservoirs be changed completely?

Commercial CEA best practice is to perform a 100% reservoir drain and reset every 10 to 14 days. Continuous topping off creates an accumulation of unused mineral ions (like sulfates or sodium) that skew nutrient balance even when EC reads normal.