Water Properties

Water Conductivity & Dielectric Properties 

 Water's electrical properties—conductivity and dielectric constant—directly affect WFC performance in VIC circuits. Understanding these properties helps predict circuit behavior and optimize design. 

 Dielectric Constant of Water 

 Water has an exceptionally high dielectric constant due to its polar molecular structure: 

 Relative Permittivity (ε r ): 

 

 

 Pure water at 20°C: 

 ε r ≈ 80 

 Pure water at 25°C: 

 ε r ≈ 78.5 

 Pure water at 100°C: 

 ε r ≈ 55 

 

 

 Temperature Dependence: 

 ε r (T) ≈ 87.74 - 0.40 × T(°C) 

 Why Water's ε r is High 

 Water molecules are polar (have positive and negative ends). In an electric field, they align with the field, effectively multiplying the field's ability to store charge. This is why water-based capacitors have such high capacitance per unit volume. 

 Comparison with Other Materials 

 Material 

 ε r 

 Relative Capacitance 

 Vacuum/Air 

 1 

 1× (reference) 

 PTFE (Teflon) 

 2.1 

 2.1× 

 Glass 

 4-10 

 4-10× 

 Ceramic 

 10-1000 

 10-1000× 

 Water 

 80 

 80× 

 Water Conductivity 

 Conductivity measures how easily current flows through water: 

 Conductivity (σ) Units: 

 

 

 Siemens per meter (S/m) 

 Microsiemens per centimeter (µS/cm) - most common 

 Millisiemens per centimeter (mS/cm) 

 

 

 1 S/m = 10,000 µS/cm = 10 mS/cm 

 Resistivity (ρ = 1/σ): 

 ρ (Ω·cm) = 1,000,000 / σ (µS/cm) 

 Conductivity of Different Waters 

 Water Type 

 σ (µS/cm) 

 ρ (Ω·cm) 

 Source 

 Ultra-pure (Type I) 

 0.055 

 18,000,000 

 Lab grade 

 Deionized 

 0.1-5 

 200,000-10,000,000 

 DI systems 

 Distilled 

 1-10 

 100,000-1,000,000 

 Distillation 

 Rain water 

 5-30 

 33,000-200,000 

 Natural 

 Tap water (typical) 

 200-800 

 1,250-5,000 

 Municipal 

 Well water 

 300-1500 

 670-3,300 

 Ground water 

 Sea water 

 50,000 

 20 

 Ocean 

 0.1M NaOH 

 ~20,000 

 ~50 

 Electrolyte 

 Calculating Solution Resistance 

 For Parallel Plates: 

 R sol = ρ × d / A = d / (σ × A) 

 Example: 

 

 Tap water: σ = 500 µS/cm = 0.05 S/m 

 Electrode area: 100 cm² = 0.01 m² 

 Gap: 2 mm = 0.002 m 

 R sol = 0.002 / (0.05 × 0.01) = 4 Ω 

 

 Effect on Q Factor 

 Solution resistance directly impacts circuit Q: 

 Q total = 2πfL / (R choke + R sol + R other ) 

 Example Impact: 

 

 

 Water Type 

 R sol 

 Q (if R choke =5Ω) 

 Distilled (σ=5 µS/cm) 

 ~400 Ω 

 Q ≈ 1.5 

 Tap (σ=500 µS/cm) 

 ~4 Ω 

 Q ≈ 70 

 Electrolyte (σ=20000 µS/cm) 

 ~0.1 Ω 

 Q ≈ 125 

 

 

 Insight: Very pure water has high Q losses! For VIC resonance, moderate conductivity may be optimal. 

 Frequency Dependence 

 Both ε r and σ vary with frequency: 

 Frequency 

 ε r Effect 

 σ Effect 

 DC - 1 MHz 

 Constant (~80) 

 Constant (DC value) 

 1 MHz - 1 GHz 

 Begins to decrease 

 May increase 

 >1 GHz 

 Decreases significantly 

 High dielectric loss 

 For VIC frequencies (1-100 kHz), these effects are negligible. 

 Temperature Effects Summary 

 ε r : Decreases ~0.4% per °C (capacitance drops as water heats) 

 σ: Increases ~2% per °C (resistance drops as water heats) 

 Net effect: Resonant frequency increases slightly with temperature 

 Measuring Water Properties 

 Conductivity Meters: 

 

 

 TDS meters (approximate, assume NaCl) 

 True conductivity meters (more accurate) 

 Laboratory grade (calibrated, temperature compensated) 

 

 

 DIY Measurement: 

 

 Use known electrode geometry cell 

 Measure AC resistance at 1 kHz (to avoid polarization) 

 Calculate σ from geometry and resistance 

 

 VIC Matrix Calculator: Enter water conductivity in the Water Profile section. The calculator computes solution resistance and shows its impact on circuit Q. Temperature compensation is also available. 

 Next: Calculating WFC Capacitance →