Definition
A DC spark leak detector is a non-destructive testing equipment used to detect defects such as pinholes and cracks in non-conductive coatings or linings. It localizes the defect point by applying high-voltage direct current to create an insulating state in the intact area of the coating and generate an EDM phenomenon at the defect. This instrument is widely used in the quality assessment of anti-corrosion layers on surfaces such as pipelines, storage tanks, and steel structures.
Principle
DC spark leak detectors work on the principle of high voltage discharge. The high-voltage generator inside the instrument converts low-voltage DC into thousands of volts of DC-high voltage and applies it to the surface of the coating under test through a probe. When the probe is fully coated, there is only a faint leakage current in the loop due to the insulation of the coating. When the probe passes through the coating defect, the high voltage breaks through the air gap at the defect, generating instantaneous spark discharge and forming a current flow path, and the instrument alarms through sound and light signals. Its basic discharge conditions can be expressed as:
V ≥ d × E
where V is the applied voltage, d is the thickness of the air gap at the defect, and E is the dielectric strength of the air.
Measurement method
Before measurement, it is necessary to select the appropriate detection voltage according to the coating thickness and material properties, and refer to relevant standards (such as ISO 2746, GB/T 7993). During measurement, the ground wire should be well connected to the metal of the substrate to be measured, and the probe scans the surface under test at a constant speed of about 0.2 m/s to maintain stable contact between the probe and the coating surface. When spark spots are found, the defect location should be marked and recorded. For workpieces of different shapes, ring probes or flat plate probes can be used to adapt to the inspection needs.
Influencing factors
The setting of the detection voltage directly affects the detection sensitivity, too high voltage may lead to the breakdown of the intact coating, and too low voltage may miss the detection. Ambient humidity affects the dielectric strength of the air, and the discharge voltage may decrease in high humidity environments. Contaminants such as moisture and salt on the surface of the coating can form conductive pathways and interfere with the test results. Uneven probe movement speed can cause a delay in signal response. Differences in the conductivity of the matrix material can also affect the quality of the ground loop.
Applications
This instrument is mainly used for pipeline anti-corrosion layer detection in the petrochemical industry, including buried pipelines and overhead pipelines. In shipbuilding, it is used for hull coating integrity checks. It is used in construction engineering for defect locating of steel structure fireproof coatings and waterproof membranes. The automotive industry can be used to inspect insulating coatings on painted car bodies. In addition, it is also used in storage tank linings, concrete rebar protection layers, transmission line insulation layers, and other fields.
Selection reference
The maximum output voltage range should be considered when selecting, and the general coating requires a voltage of 3000-4000 volts per millimeter thickness, and the upper voltage limit can be determined according to the detection thickness. The instrument should have a continuously adjustable voltage function to accommodate different materials. The alarm style should also have sound and light indications, and a model with vibration prompt can be selected in noisy environments. Depending on the testing environment, you can choose explosion-proof or portable design. The probe type should match the shape of the workpiece, flat brush head should be used for flat workpieces, and ring spring electrodes should be used for pipeline inspection. The instrument should have a self-test function and safety grounding protection.