Capacitance-Based Online Moisture Monitoring of CUI Insulation: Principle Detection

A capacitive sensor (or flexible conformal strip) placed beneath the insulation generates an electric field; when moisture intrudes, the electric field changes the total capacitance.

Continuous Tracking

The change is directly converted into the volumetric water content, or a binary dry/wet state, at the insulation-to-pipe interface.

Insulation (top section): a C∥R parallel network in series with a capacitor C

This is the equivalent model of the insulation material. In the dry state, the insulation behaves approximately as a pure capacitor (dielectric constant ε of only 1.2~2.5). Once moisture intrudes, water (ε ≈ 80) substantially raises the value of the parallel capacitor C, while the conductivity brought by the water lowers the leakage resistance R.

Paint coating (bottom section): a series-parallel combination of multiple C and R

This represents the capacitance and leakage impedance of the anti-corrosion coating itself. When the coating is dense, its impedance is high and it protects the metal; when it blisters / ingresses water / deteriorates, the impedance-spectrum characteristics change noticeably.

Entire monitoring chain: capacitance → εᵣ → water content θ

• Edge-field detection: The sensor (or flexible conformal strip) is attached beneath the insulation; the emitted electric field penetrates the insulation, and moisture intrusion changes the total capacitance.
• Measure capacitance → calculate εᵣ: From the total-capacitance change measured via the equivalent circuit above, the effective dielectric constant εᵣ at the interface is derived.
• εᵣ → volumetric water content θ: Converted using the Topp empirical formula: θ = −0.053 + 0.0292·εᵣ − 5.5×10⁻⁴·εᵣ² + 4.3×10⁻⁶·εᵣ³. When dry, εᵣ ≈ 2~3 and θ is close to 0; after water ingress, εᵣ rises and θ increases rapidly, indicating the interface is wet and has entered the CUI risk zone.