This article focuses on the metrological support of the invented national instrument for measuring the amount of radon in air, water, and soil (radon tester). Metrological maintenance establishes and applies scientific, technical, organizational, legal, and regulatory rules necessary to ensure the uniformity of measurements and the required accuracy. Products (services) and technological processes, equipment of enterprises and testing laboratories, methods and means of measurement, testing and inspection, normative documents, and systems of metrological assurance of scientific, production, and social and environmental activities are considered objects of professional activity.