Definition
A wet sponge leak detector is a portable electronic instrument used to detect discontinuous defects such as pinholes, cracks, porosity on the surface of non-conductive coatings or overlays. It is widely used in quality control in the fields of anti-corrosion coating and insulation layer by contacting the surface to be tested through a wet sponge probe that applies low voltage to the coating surface and identifying defect locations according to the change of current circuit.
Principle
Wet sponge leak detectors work based on the principle of conductivity difference. The internal circuitry of the instrument includes a low-voltage power supply (usually 9-90V DC or pulsed DC), a current detection module, and an alarm unit. When the wet sponge probe touches the full coating surface, the coating acts as an insulator to block the current loop; If there is a penetrating defect in the coating, the electrolyte in the sponge contacts the substrate (usually a conductive metal) through the defect, forming a closed circuit, and the instrument detects a small current and triggers an audible and visual alarm. The current size can be roughly described by the following relationship:
I = V / (Rc + Rs + Rp)
where I is the loop current, V is the applied voltage, and Rcis the coating resistor, Rsis the resistor of the substrate, RpIt is the contact resistance of the probe. R when the defect existscsignificantly lowered, resulting in an increase in current.
Measurement method
Before measuring, the surface to be measured should be cleaned to ensure that there is no dust or oil stains. Select the appropriate voltage according to the coating thickness (lower voltage for thin coatings, higher voltage for thick coatings), immerse the sponge probe in a special electrolyte and squeeze evenly until wet but not dripping. The probe moves smoothly on the coating surface at a speed of about 0.3 m/s, and the movement trajectory should be partially overlapping. When an alarm is detected, slow down to pinpoint the defect edge and mark it with a marker pen. After the test, the residual electrolyte on the tested surface should be cleaned with clean water.
Influencing factors
The surface humidity of the coating affects the conductivity, and excessive humidity may produce false alarms. Ambient temperature affects electrolyte conductivity and instrument sensitivity. The thickness of the coating should be matched with the detection voltage, and too high a voltage may break down the intact coating. The substrate material affects the conductivity, and the non-metallic substrate needs to be used with the conductive base coating. The consistency of probe pressure and movement speed has a direct impact on defect detection rate. The electrolyte concentration should be formulated according to the standard, and concentration deviations may change the detection sensitivity.
Applications
Used in shipbuilding to detect the integrity of anti-corrosion coatings. It is used in the automotive industry to verify the quality of the coating of the electrophoretic coating. In pipeline engineering, check the pinhole defects after the construction of the anti-corrosion layer. Tank liner coating is accepted to ensure no penetration points. Aerospace inspection of insulating coatings for non-metallic components. Verification of insulating coatings on electronic product shells. Integrity inspection of fire protection coatings for building steel structures. Defect localization of solder mask on printed circuit boards.
Selection reference
Choose an output voltage adjustable model based on the coating thickness range, with common models covering 5-90V DC output. Considering the environmental explosion-proof requirements, the hazardous area needs to choose the intrinsically safe type. The continuous working time and battery capacity should match the requirements of on-site testing. The probe design should be suitable for the surface shape, and flexible sponge probes can be used when there are many curved surfaces. The instrument should have sensitivity adjustment to accommodate different conductive substrates. It complies with international standards such as ISO 2746, ASTM D5162, DIN 55670, etc. Weight and ergonomics affect long-term operating comfort. Additional functions such as automatic voltage calibration, data logging, etc. can be configured on demand.