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| Advantages
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Typical moisture gauge applications
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Principle of Measurement The nuclear method of moisture measurement is based on the principle that fast neutrons are slowed down by scattering hydrogen nuclei, but that they are hardly slowed down by scattering materials of higher atomic numbers. A cloud of slow neutrons is created around a source of fast neutrons, and its concentration essentially depends only on the hydrogen content in the volume or the volume moisture. If a fast neutron source is combined with a detector for slow neutrons, the result is a moisture probe which operates without contacting the measured medium.
Temperature, pressure, pH value, grain size and the material's chemical bonds do not affect neutron deceleration. On the other hand, it must be noted that the chemically bound hydrogen content or that of the constituent water is also detected in addition to the hydrogen content of the free water or moisture. The fluctuations in constituent water content limit the attainable accuracy of moisture measurement. The same applies to fluctuations in the bound hydrogen content. However, it follows from the relationship of the molecular weight of hydrogen to water that a fluctuation in hydrogen content by 1 % simulates a fluctuation in the moisture display of 9 %. The display is approximately proportional to the moisture in percent by volume. The moisture in weight percent is given by the quotient of volume moisture and material density. The neutron display may be calibrated directly in "weight percent moisture" if the apparent density is sufficiently constant.
Configurations
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Bunker probe version The probe is installed in the outlet section of a bunker or a continuous flow tank so that the built-in neutron source is sufficiently surrounded by material to create an effective measurement volume. The probe is mounted in a dip pipe socket, closed at the bottom end, which passes through the tank or vessel wall at an angle of 30-45 to the horizontal or, alternatively, in a protective tube passing through both walls. The protective tube does not interfere with measurement since it is permeable to neutrons. It must be noted, however, that the protective tube wall reduces the indicated value by approximately 5% per mm steel as a result of neutron backscatter. If the temperature in the tank or vessel can reach temperatures in excess of 50 C, air must be blown through the protective tube to cool the probe. In this case, the diameter of the protective tube must be increased and the probe must be held in the centre of the pipe by spacer blocks. |
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Surface probe version on tanks and vessels Bunker probes (penetration
probes) are generally used for moisture measurement of bulk goods in tanks or bunkers.
However, if there is a risk of interference with material flow by the internal structures,
it is possible to perform measurement using a surface probe mounted on the vessel wall. In
this configuration, however, the neutron flow at the detector is approximately 5 times
less than in the case of the bunker probe for the same moisture, and more powerful
radiation sources are therefore required. In addition, the bunker or tank wall attenuates
the indicated value by around 5% per mm steel as a result of neutron backscatter. At he measuring point, the wall should therefore not be too thick on the one hand and, on the other, not be easily worn away, so that errors due to changes in thickness are avoided. |
The LB 350 compact evaluation unit
Application Notes
for more information, email
SE Asia and Australasia: industrial@berthold.com.au
North America: usinfo@berthold.com.au
Europe: euroinfo@berthold.com.au
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