Process control devices for non-contact measurement and indication of level, density, concentration, thickness, media interface etc. which are not subject to the supervision by the State Nuclear Supervision Authority, Sanitary Epidemiological Service and Ministry of Home Affairs and do not enlarge the radiation background; do not require special radiation shield; do not pollute the environment; do not require specially trained personnel; do not require specially equipped and certified premises; do not create problems when dismantling the equipment and disposal of radioactive materials. They are very convenient tool at drilling mud density measurement at the well inlet and outlet, at oil-well cement density measurement, at pulp density measurement and density control for drilling mud and cement slurry at the well inlet.