MAR-APR 2017

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42 INTECH MARCH/APRIL 2017 WWW.ISA.ORG Back to basics: Radiometric level and density measurement R adiometric or gamma level/density instru- ments are most often used in applications where other measuring techniques fail due to extreme temperatures or pressures, toxic media, complex geometries of vessels or pipes with dif- ficult installation requirements, high viscosities, changing fluid behaviors, or abrasive or corrosive properties of the process media. Because a radiometric measuring system is a noninvasive measuring technique (i.e., the emitter and detector are mounted external to the process), the behavior of a medium inside a vessel can be precisely observed with equipment fitted out- side the vessel (figure 1). A very simple installation is shown in figure 1. Because there is an agitator inside the ves- sel, in stalling a level measuring device inside the tank, such as an ultrasonic or guided radar instrument, may not be suitable. Depending on the conditions inside the vessel, the medium may vaporize, or the rotating agitator might cause a vortex at the surface of the fluid. These conditions may interfere with other types of level measuring devices, which are installed inside the vessel walls. With radiometric measurement, it is possible to detect all medium conditions. Benefits of using radiometric include: l noncontact and noninvasive measuring l guaranteed process safety due to being out- side the process vessel l precise and repeatable measurement for lev- el, density, and interface applications l safe and easy to install l reliable measuring equipment Radioactivity basics Radioactivity can be roughly classified into three types, each emitted by the decay of the radioactive isotope: l alpha radiation: particle radiation in the form of a helium nucleon (alpha particle) l beta radiation: elemental particle radiation in the form of electrons and/or positrons (beta particle) l gamma radiation: high-energy electromag- netic waves similar to radio waves and light Figure 1. In a radiometric level detection or density measure- ment system, an external source emits radiation that passes through the vessel and is measured by an external detector. By Gene Henry

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