This standard applies to the manufacture and operation of electrical measuring systems and instruments utilizing radioactive sources (radiometric gauges) which are designed for measuring other quantities (e.g. level, density, thickness, flux rate). It does not apply to portable gauges and to gauges operated with X-ray tuebes, but it can be analogously applicable to the latter. The purpose of this standard is to specify constructional requirements for the design of instruments, a guideline for installation and an unique classification code.