Level Measurement: The Very Last Resort

How do you make the measurement that can’t be made?

By Walt Boyes

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Gamma Transmission Measurement

Figure S1

 

Everything between the source and the detector attenuates the radiation beam. For example, one inch of steel reduces the intensity of the beam by 50%, or one “half-value,” whether the instrument is a point level gauge or a continuous level gauge. Either one  is calibrated by determining the level of radiation when the vessel is empty. This level, automatically corrected for source decay, is not likely to change unless the geometry of the vessel changes. This becomes the zero level measurement.

As the level in the vessel rises, the intensity of the beam is further attenuated by the amount of level present in the beam. In the case of a point level measurement, the beam is very narrow and horizontal, so that it provides a single point trip when the rising or falling level reaches the centerline of the gamma beam (Figure S2).

 

Point Level Operation

Flexible Detection

Neutron Backscatter Gauge

Figure S2

Figure S3

Figure S4

Point level detection uses a focused beam of radiation directed at a small detector.
Curved fiber-optics scintillator detectors make calibrating measurement easier in round tanks
These gauges are used for very short spans and point level measurement.

 

In a continuous level application, the gamma beam has a triangular conic section that produces a nonlinear output that must be linearized in the electronics. The linearized “percent attenuation” is inversely proportional to “percent level.” At zero level, the intensity of the radiation received at the detector is maximum, while at maximum level, the intensity of the radiation is minimum.

Some suppliers produce detectors that are flexible (Figure S3), and can be molded to conform to vessel shape in the case of round or other oddly shaped vessel geometries. This method requires less linearization than the straight detector tube and makes installation in tight spaces easier.

Some suppliers use source holders that are designed to hold multiple sources in a strip that is as long as the detector on the other side of the vessel. This is another way to reduce the amount of calibration error due to linearization, and minimize the size of the gamma source.

 The other method of measuring level with nuclear gauges is neutron backscatter (See Figure S4). Because of its cost, this method is primarily used for measuring very short spans and point level measurements. In this method, a neutron source is beamed into the vessel (typical application, level in petroleum cokers). The interaction of the neutrons and the hydrogen atoms present in the vessel cause some neutrons at vastly lowered energy state (thermal neutrons) to be scattered. The ones that are scattered back (hence the name neutron backscatter) to the detector are counted. As the level rises, the amount of hydrogen rises, and the number of backscattered thermal neutrons rises, producing a level measurement.

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