A holiday detector, or porosity, continuity, holiday tester, is a non-destructive inspection instrument used for detecting discontinuities in a coating system, including holiday, pinholes, cracks and voids. Holiday detectors are commonly used to evaluate the rate of premature coating failures on a metallic surface by sensing the presence of coating defects and flaws (holidays) and the porosity of the coating to ensure structural integrity and assist with corrosion prevention efforts.