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Operating principle of Nuclear Level Sensors

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Nuclear based level measurement sensors can be used for point as well as continuous level measurement applications.  The concept of nuclear level sensors is based on the fact that certain types of nuclear radiation easily penetrate the walls of industrial vessels, but is attenuated by traveling through the bulk of material stored within those vessels. By placing a radioactive source on one side of the vessel and measuring the radiation making it through to the other side of the vessel, an approximate indication of level within that vessel may be obtained. Other types of nuclear radiation are scattered by process material in vessels, which means the level of process material may be sensed by sending radiation into the vessel through one wall and measuring back scattered radiation returning through the same wall.

In a typical industrial nuclear level measurement sensor shown below
, the nuclear level sensor comprises a shielded radioisotope source attached to one side of a vessel or pipe and a detector placed on the opposite side:
Nuclear Level Measurement Sensor
Gamma rays are emitted from the source and are focused to travel through the tank wall, the medium in the tank and the far tank wall through to the detector. Apart from Gamma ray radiation, the other common source of nuclear radiation is neutrons. Neutron radiation can also penetrate metal very effectively, but is strongly attenuated and scattered by any substance containing hydrogen (e.g. water, hydrocarbons, and many other industrial fluids), which makes it almost ideal for detecting the presence of a great many process fluids.

Gamma rays and neutrons based nuclear sensors are the most common in industrial level measurement applications, with gamma rays used in through-vessel applications and neutrons typically used in backscatter applications.

A typical nuclear radiation source consists of radioactive samples contained in a shielded box. The sample itself is a small piece of radioactive substance encased in a double-wall stainless steel cladding. The specific type and quantity of radioactive source material depends on the nature and intensity of radiation required for the application. The smallest source capable of performing the measurement task is the best option for any given application.

Common source types for gamma-ray applications, which is the dominant nuclear radiation source for nuclear level devices, are Cesium-137 and Cobalt-60. The nuclei of these isotopes are unstable, decaying over time to become different elements. Cobalt-60 has a relatively short half-life of 5.3 years, whereas Cesium-137 has a much longer half-life of 30 years. This means nuclear-based level sensors using Cesium will be more stable over time (i.e. less calibration drift) than sensors using Cobalt. The trade-off is that Cobalt emits more powerful gamma rays than Cesium, which makes it better suited to applications where the radiation must penetrate thick process vessels or travel long distances (across wide process vessels).
  1. They can be used for both point and continuous level measurements in liquids and solids as well as interface.
  2. They are non-contacting
  3. They are unaffected by high temperatures, pressures, corrosive materials, abrasive materials, viscous materials, and agitation or clogging
Application Limitations
  1. Nuclear level measurement devices are very expensive
  2. Density changes can create measurement errors
  3. Material build up on vessel walls can affect measurement results
  4. Licensing from relevant authorities is required to use them
  5. Regular leak checks as well as high degree of health and safety checks and source handling and disposal are critical requirements. 

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