Definition
A low-voltage EDM leak detector is a non-destructive testing equipment used to detect defects such as pinholes and cracks in non-conductive coatings or linings. It creates an electric field between the coating surface and the substrate by applying a relatively low DC or pulse voltage, which produces an EDM phenomenon when encountering a discontinuity point in the coating, thereby locating the defect location. This instrument is mainly used for integrity testing of anti-corrosion coatings, insulation layers, etc. on the surface of metal substrates to ensure the continuity and protective effect of their covers.
Principle
The low-voltage EDM leak detector works on the principle of high-voltage discharge. The instrument's internal circuitry converts the low voltage into a DC pulsed high voltage of thousands of volts, which is applied to the coating surface through the probe. The probe is usually constructed of conductive brushes or metal electrodes, and when the probe moves over the surface of the coating, if the coating is intact and insulated, the current cannot pass through; If the coating is defective, the high voltage will break through the air or weak layer at the defect, generating an instantaneous spark between the substrate and the probe, and the instrument will emit an audible and visual alarm signal. The discharge voltage U can be adjusted according to the coating thickness d and material properties, and the common empirical formula is U = k × d, where k is the coefficient related to the coating material, which needs to be determined with reference to relevant standards or experiments.
Measurement method
Before measurement, the appropriate detection voltage should be set according to the coating type, thickness and testing standards (e.g. ISO 2746, GB/T 7993). First, clean the coating surface to ensure that there are no moisture, dust, and other impurities that can affect the detection. Reliably connect the instrument's ground wire to the metal substrate, turn on the power and calibrate the voltage. The probe scans the surface of the coating evenly at a speed of about 0.2 m/s to keep the probe in stable contact with the coating. When a defect is detected, the instrument triggers an alarm that marks the defect location. After the inspection is completed, the number, location and size of the defects are recorded, and the coating quality is evaluated according to the standard. Note that explosion-proof equipment should be used in hazardous environments (such as flammable and explosive places).
Influencing factors
Coating thickness is a key factor affecting the detection voltage setting, and too thick may lead to missed detection, and too thin can easily cause false alarms. Coating humidity or surface contaminants can alter conductivity and interfere with the accuracy of spark discharge. The contact area between the substrate shape and the probe will affect the distribution of the electric field, and flexible probes should be selected for complex surfaces. Ambient temperature and humidity can change the air breakdown voltage and need to be detected or compensated for under standard ambient conditions. Uneven speed and pressure of the operator moving the probe may lead to missed detections and require stable operation. The instrument's own voltage stability and probe wear can also affect detection sensitivity and require regular calibration and maintenance.
Applications
This instrument is widely used in the petrochemical industry to inspect pipelines, storage tanks, and anti-corrosion coatings to ensure their corrosion resistance. In shipbuilding, it is used for integrity checks of hull coatings and cabin insulation. In the automotive industry, it is possible to detect defects in the electrophoretic coating or paint layer of the body. The equipment is also commonly used to test the lining coatings of metal containers and the anti-corrosion layer of brewing tanks in the food and beverage industry. In addition, in construction projects, low-voltage EDM leak detection technology will also be used for quality inspection of steel structure fireproof coatings and underground pipeline protective layers. These applications are based on their characteristics of fast localization and non-destructive testing of coating defects.
Key points of selection
The detection voltage range should be considered when selecting, and the thickness requirements of the coating to be tested should be covered, usually the voltage adjustable range is between 0.5-30 kV. Choose ordinary or explosion-proof type according to the testing environment, and explosion-proof design must be used in flammable places. The probe type should match the shape of the object to be measured, and the flat surface should be brushed probe, and the curved surface or weld should be used with a spring electrode probe. The instrument should have an audible and visual alarm function, and some models can integrate automatic voltage adjustment and data logging functions. Refer to international standards (such as ISO, ASTM) or domestic standards (such as GB, JB) for instrument performance requirements to ensure its calibration accuracy and reliability. In addition, device portability, battery life, and user-friendly design of the user interface are also important considerations in practical use.