What Does Holiday Detector, Low Voltage Wet Sponge Type Mean?
Holiday detector, low voltage wet sponge type is a non-destructive test instrument that uses 5 to 90 volts direct current (DC) to find minute bare spots and other discontinuities collectively known as holidays in thin, non-conductive coatings painted over conductive substrates. This type of holiday detector is designed for use on coatings less than 20 mils (510 micrometers) in dry film thickness.
Coating discontinuities caused by improper handling or normal wear and tear are not considered holidays.
Low voltage wet type holiday detectors are appropriate for thin coatings that may be damaged if tested with a high voltage detector.
Industry experts recommend attaching a wet cellulose sponge to the inspection electrode during testing.
Corrosionpedia Explains Holiday Detector, Low Voltage Wet Sponge Type
A wet cellulose sponge is attached to the inspection electrode of a holiday detector low voltage wet sponge type instrument. During the test, as the inspection electrode moves over the coated surface, moisture from the wet sponge penetrates any discontinuities in the coating and makes a conductive path to the substrate. Then, depending on the design, the holiday detector signals that a discontinuity was found either through an alarm or a flashing light indicator (usually red).
A discovered holiday is marked for repair before further testing.
The wet sponge must be moist to the touch but not so saturated that the wetting agent runs from the sponge. Plain tap water is used when testing coatings up to 10 mils thick. A non-sudsing type wetting agent is added if the coating is between 11 mils and 20 mils thick.
Some of the industry references on the procedure and criteria for holiday detection are the following: ASTM G62-A, ASTM D5162, ASTM G6, ASTM D 4787, AS3894.2, BS 7793-2, BS EN ISO 8289 A (Wet Sponge Testing), NACE RPO274-98, NACE RP 0188. However the user should still refer to the manufacturer’s instructions for the specific limitations of the specific model.
A recommended practice is to perform holiday testing after the application of the next-to-last coat of paint or before the final curing of the coating material so that any applied repair material can successfully bond to the previous layer.