A holiday—pinhole, flaw, void, crack—is a discontinuity within a coating system where a small area of thin or missing coating exposes the substrate to the environment.
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Due to a variety of potential errors in the application of a new protective coating, flaws—or holidays—can form. They can be invisible to the naked eye and are near-impossible to detect without proper instrumentation.
Holiday detection is a necessary step in identifying these areas of thin or missing coating. Porosity detectors are often used in applications where corrosion is difficult to monitor, or in aggressive service environments such as buried pipelines where performance of the protective coating is critical.
Outlined by standards organizations like AMPP (NACE), ASTM, and ISO, a holiday detector uses a charged electrode passed over a coating. When a holiday is encountered, current flows from the electrode into the conductive substrate below. The current returns to the detector through a ground wire, completing the circuit and triggering an alarm, alerting the inspector to the defect.
Pinhole and holiday testing can be performed with a variety of equipment designed to pass an electrical charge over a paint or coating system to detect flaws and defects (pinholes and holidays). With a wide range of designs and form factors available, depending on the manufacturer and specifics of the job requiring holiday testing.
There are three main types of Pinhole/Holiday Detectors:
Low voltage Pinhole Detectors, such as the PosiTest LPD use a wet sponge to detect pinholes in coatings less than 500 microns (20 mils).
High voltage Pulse Holiday Detectors, such as the PosiTest HHD use pulse DC and either a spring or brush electrode to detect holidays in coatings greater than 200 µm (8 mils) thick. By using multiple pulses of high voltage every second, the unit is able to:
High voltage Continuous Holiday Detectors, such as the PosiTest HHD C use continuous DC and either a spring or brush electrode to detect holidays in coatings. They operate at the same voltage range as High voltage Pulse holiday detectors, and have the benefits of being light, compact, one-handed instruments. Unlike some continuous DC models, the PosiTest HHD C doesn't require a separate shoulder battery bag. The powerful Li-ion batteries fit neatly within the wand handle reducing weight and eliminating operator fatigue.
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The correct instrument will depend on the thickness of the coating being inspected and the specified standard. Both the PosiTest LPD and the PosiTest HHD series work on non-conductive coatings over a conductive substrate (such as steel, aluminum, other metals, and concrete).
The PosiTest LPD has a voltage output of 9–90 V and is suitable for coatings less than 500 microns (20 mils).
The PosiTest HHD uses pulsed DC, has a voltage output of 500–35,000 V and is suitable for coatings greater than 200 µm (8 mils).
The PosiTest HHD C, uses continuous DC, has a voltage output of 500—30,000 V, and is suitable for coatings greater than 200 µm (8 mils).
When the electrode of a Pinhole/Holiday Detector is placed onto the coating, it puts an electrical load on the instrument. The Holiday Tester must compensate for this load by regulating its power output, to ensure that the voltage at the electrode remains at the set voltage. Many older or less expensive Holiday Testers do not have this functionality, meaning that the test voltage at the electrode could differ significantly from the set voltage and lead to invalid test results.
The PosiTest HHD series of High voltage Holiday Detectors and the PosiTest LPD Low voltage Pinhole Detector are factory calibrated and include a Certificate of Calibration traceable to NIST. They feature a built-in voltmeter that constantly monitors the voltage at the electrode. Auto-regulating circuitry inside the porosity detector adjusts the power output to ensure the correct test voltage throughout the measurement process.
To determine the correct voltage setting of the pinhole/holiday detector, consult the coating manufacturer’s technical data sheet and the appropriate standard.
The PosiTest HHD includes a voltage calculator feature—choose from one of 11 international standards, and input the coating thickness as determined by the PosiTector Coating Thickness Gage to automatically calculate the required test voltage.
The PosiTest HHD C also includes a voltage calculator feature, using an equation from the upcoming revision of ASTM and NACE standards. Simply enter the coating thickness as determined by the PosiTector Coating Thickness Gage to automatically calculate the required test voltage.