We use radiometric measurement when other methods won’t work because of conditions like abrasion, build-up, corrosion, high temperature or high pressure. We usually use gamma gauges because they provide accurate and reliable data with non-invasive, plug-and-play technology.
What is radiation?
Radiation is energy flowing through space at the speed of light in the form of particles or rays (electromagnetic waves). Some materials have the ability to emit radiation in the form of alpha and beta particles. If they have unstable atoms, they can emit gamma rays too.
Nature exposes us to several types of low-intensity radiation, also known as background radiation. This comes from the sun, outer space, natural materials, even our bodies, and foods. However, we also encounter several types of artificially produced radiation, like those from X rays, nuclear power plants, and certain industrial processes.
Gamma rays and gamma gauges
Gamma rays are electromagnetic waves emitted by certain materials, similar to X rays and ultraviolet rays but from different sources and a different band in the electromagnetic spectrum.
Gamma gauges measure level, density, and interface measurement because they can penetrate heavy materials like steel, aluminum, and concrete.
The working principle of radiometric level measurement
We use two types of units to measure radioactivity, or the strength of radiation sources:
- Curie (Ci) = emissions similar to 1 gram of radium 226 = 37 billion disintegrations per second
- Becquerel (Bq) = 1 disintegration per second → 1 mCi = 37 MBq
The sources used for level measurement usually have radioactivity from 100 µCi/3.7 MBq to 500 mCi/18.5 GBq. To measure effects on exposed organic structures, we use the following:
- Roentgen equivalent man (rem) = rad energy adjusted for biological effect
- Sievert (Sv) = 100 rem
Radiometric level measurement
Radiometric measurement often shows up in level measurement and generally looks like the image below. This graphic shows that radiometry allows for non-invasive measurement. Nothing in the system comes in contact with the product or the process atmosphere.
First of all, the most important highlight in this drawing is that radiometric level measurement is the only completely non-invasive technique, since we can see, no component of the system is in contact neither with the product nor with the process atmosphere.
How do gamma rays and gamma gauges fit in?
Gamma rays flowing through a vessel will become partially attenuated by the product inside. A source inside a shielded container can emit a beam of gamma rays above an operating angle, usually up to 40 degrees. Then a gamma-ray detector with an integrated transmitter, mounted on the opposite side of the vessel, can measure and send the data to the control system.
The operator must know the strength of the gamma source so that when the product is at the top of the measuring range, the detector transmits fewer pulses to measure the higher levels. Conversely, when the product is at the bottom of the range, the detector transmits more pulses to measure the lower levels.
We can calculate gamma source activity with software available from multiple vendors. We will need to know the vessel’s internal diameter and wall/liner thickness and density, as well as the product density and detector you have.