Holiday detectors are tools used for a specialized form of non-destructive testing. Holiday testing is a non-destructive test method applied on protective coatings to detect unacceptable discontinuities such as pinholes and voids. The test involves checking an electric circuit to see if current flows to complete the circuit. This testing is used to find coating film discontinuities that are not easily visible. Holiday detectors are usually used for tests performed on tank interiors, chemical storage vessels and buried structures because of the importance of maintaining adequate coating protection in aggressive service environments. Holiday testing is also known as continuity testing.
Holiday detectors typically are designed to perform three flaw detection methods: UV pinhole detection, wet sponge technique, and high voltage technique. The UV pinhole detection methods utilizes an additive that when exposed to a flashlight reveals the pinhole. The wet sponge technique begins with a low voltage amount applied to a sponge. When the sponge moves over a coating flaw, liquid penetrates to the substrate and completes an electrical circuit, setting off the alarm. The high-voltage, or porosity, technique can be used to test coatings up to 1" (25mm) thick and is ideal for inspecting pipelines and other protective coatings.