Coatings applied can have irregularities due to different circumstances. Surface uncleanness, paint differences and dust or air bubbles enclosed can cause pinholes, thin spots, cracks etc. To prevent an early breakdown of the coating system causing corrosion underneath, tests has to be made to find the weak spots and action can be taken before problems occur over time. For these kind of tests High Voltage Holiday Detectors provide an accurate way for locating faults in non-conductive coatings and linings and detection of pinholes, flaws, inclusions, thin spots and bubbles in a coating.
High voltage testing of coatings
A High Voltage Holiday Detector is specifically designed to revolutionize high voltage DC testing of coatings, making it safer, easier and more reliable than previously possible. The gauge can only be used to find flaws in coatings whose substrate is made from a conductive material (metal, concrete etc.). The units have variable sensitivity and are fitted with audible and visual fault alarms.
The high voltage technique can be used to test coatings up to 36 mm thick. This method is ideal for inspecting paint on pipelines, tank bottoms and other protective coatings. Coatings on concrete can also be tested using this method.
The instrument has a lot of unique features. A current limiting to avoid coating damage and a safety hand grip without sensitive electronics. Extended ribbing on the handle provides an effective barrier between the high voltage and the user. Accurate sensitivity adjustment allows use on metalized or slightly damp coatings.
As most instruments are portable there is no need to swap power supplies or change probes to change voltage. The Detectors have clip-on battery packs that can be charged separately. Applied voltage and battery condition are continuously shown on the Liquid Crystal Display.
To perform tests without damaging the coating Test Voltages need to be limited and set according to regulations according to NACE.
a. The Nace RP-0490-2001 calculates the voltage in the formula: Voltage in kV = square root of the layer thickness in microns * 125. For example, when a coating thickness of 400µm: Root 400 = 20 * 125 = 2500V (2.5 kV). NB. The rule is a directive and cannot be seen as an absolute value.
b. The Nace RP-0274-98 calculates the voltage in the formula: Voltage in kV = square root of the layer thickness in microns * 250. For example, when a coating thickness of 3200Î¼m: Root 3200 = 56.5 * 250 = 14140V (14.1 kV). NB. The rule is a directive and cannot be seen as an absolute value.
Low voltage testing of coatings
In those cases where high voltage testing is not desirable or possible due to thin coating thicknesses up to 500 micron, another test system should be used. For these a Low Voltage Pinhole Detector is used with selectable voltages of 9, 67.5 and 90 volts. Depending on the coating thickness under test use 9 volt for coatings up to 300 micron, 67.5 volt for coatings up to 400 micron and 90 volt for coatings until 500 micron. The system consists of a battery operated instrument with earth lead and a wet sponge as electrode. By moving the slightly wet sponge over the test area moisture will pass the pinhole due to capillary absorption making contact with the base substrate causing an alarm to indicate a pinhole.
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