Radiometric measurement — at home in extreme process conditions

Radiometric measurement, also known as nuclear- or gamma-based measurement, is an extremely reliable and safe, non-invasive solution that is suitable for level limit detection, continuous level measurement, and density and interface layer measurement.

Due to its non-contact detection principles, it is especially suitable for applications with extreme process conditions involving abrasive, corrosive or toxic mediums, high temperatures or pressures. In short, radiometric measurement can go where other technologies can’t.

The principal of radiometric measurement is in itself quite simple — it employs a radioactive source which emits gamma radiation that passes through the walls of the pipe or vessel containing the medium being monitored. A detector is mounted on the opposite side of the container which senses the radiation that is not absorbed by the medium and is directly related to the parameter being measured.

Applying the basic principles of radiometric measurement and using just a few components, several types of measurement are possible including:

• level limit detection,
• continuous level measurement,
• interface layer measurement,
• density or concentration measurement,
• temperature-compensated density or concentration measurement,
• mass flow measurement.

Specialised software is used to simplify the selection of source intensity and material ensuring effective measurement while keeping the level of radiation used to an absolute minimum to optimise operator safety. Critical system safety parameters and installation criterion are automatically calculated and graphically represented.

The radioactive source is enclosed in a lead-lined steel source container that is designed to direct the radiation emission only in one direction, and screening it off in all other directions. Thus radiation exposure to operators is maintained well below levels considered safe by Australian regulators and the International Atomic Energy Agency (IAEA).

Due to their safety, accuracy and reliability in the most adverse conditions, radiometric measurement systems are used in many applications throughout the world. In Australia, they have been successfully applied in the mining, metal processing, chemical, petrochemical, paper, oil and power generation industries.

During commissioning, a background calibration eliminates the effects of constant ambient radiation. However, all radiometric measurement systems become unreliable when subjected to fluctuating ambient radiation caused by gammagraphy (non-destructive material testing), a dense population of radiometric measurement points or process radioactive slurries such as uranium. Because of its variable nature, it is impossible to compensate for fluctuating ambient radiation.

This problem has been solved through the development of a gamma modulator by Endress+Hauser. The device is fitted to the system’s radiation source. It contains a slotted shaft that rotates and alternatively screens off the gamma beam or allows it to pass. Using a frequency filter, the system’s detector is able to separate the useful signal from the interference radiation allowing accurate measurement and improved system reliability in spite of fluctuating ambient radiation.

Now radiometric measurement can be used to solve the most challenging measurement problems in extreme process applications even in installations with sporadic ambient interference radiation.

*Simon Rigling of Endress+Hauser Australia is Product Manager – Gamma.