A nuclear level measurement system basically includes following components:
- A source of gamma radiations
- A continuous detector and
- A microprocessor
In general, the gamma source emitting radiations is located external to the process vessel. These radiations passes through the vessel walls and material gets accumulated towards the detector. The detector is installed on the other side of the vessel. In case, the vessel contains no contents, the transmitted gamma rays arrive at the detector. As soon as the level of the contents increases in the vessel, there is a decrease in the amount of gamma rays getting to the detector.
Thus, this gamma energy drops in an inverse proportion to the process level. “A computer processes the detector signal and transmits the process variable as 4-20 mA analog or digital fieldbus signal. Multiple gamma sensors, mounted outside the tank, help see through buildup on tank walls for a true level reading.”
Major characteristics of gamma radiations which make them useful over other technologies for level measurement applications are listed below:
- They can detect both solid as well as liquid levels.
- They are resistant to obstructions in the process vessel.
- They offer an extensive range of operating temperature.
- Their chemical properties are not very much crucial.
- Their performance does not get affected by factors like surface turbulence or changes in flow etc.
- They can measure level in applications involving mist, foams and intense vapors too.
On the other hand, the readings of other level measurement techniques like radar, ultrasonic etc. give errors when exposed to foams or variations in gas density. Actually, in gamma level sensor designs, compensation can be provided for these errors by attaching a second point detector to the microprocessor.