Patent GB-1395819-A (GB)
Title: Gauge for determining the percentage by weight of moisture contained in a bulk material transported on a moving conveyor
IPC: G01N23/12
Filed: 1972-05-11
Abstract: 1395819 Measuring moisture content OHMART CORP 11 May 1972 [11 May 1971 #2] 22254/72 Heading G1A The percentage by weight of moisture of a bulk substance transported on a conveyor belt is determined by apparatus including a fast neutron source and a γ-source irradiating the belt and substance  detectors for slow neutrons and γ-rays emanating from the belt and substance, and a signal processing circuit, wherein the detected slow neutrons and γ-rays emanate from the same area of belt and bulk substance which area is less than the entire cross-sectional area of the bulk substance. As shown, Figs. 1 and 2, a lower arm 24 contains a combined γ and neutron source 28 and an upper area 16 contains three slow neutron detectors 22 and a γ-ray detector 20. The source 28 comprises a steel rod having separate slots containing 7 and neutron emitting radio-active material mounted in a collimator comprising an aluminium "window" having a sector shaped crosssection with the remainder of the cross-section comprising lead screens surrounding the γ-emitting material and water extended polyester potting material surrounding the neutron emitting material. Two different arrangements of the slots and radio-active materials are desbribed to provide sources suitable for use with bulk materials of high and low moisture contents (e.g. wood chips and sinter mix respectively). A suitable γ-emitter is Cs137 and suitable neutron emitters are Am-Be, Pu 238-Li and Am241-Li. The radio-active source is arranged so that it extends across one third of the width of the belt and since the neutrons are scattered to a greater extent than the γ-rays the neutron emitting material is placed nearer to the centre than the γ-emitting material so that they irradiate the same area of bulk material. The source 28 is mounted so as to be rotatable between an operative and an inoperative position by means of a handle 44. The neutron detectors 22 comprise proportional counter tubes filled with BF 3  and over them is arranged a neutron reflector 69 of methyl methacrylate which reflects fast neutrons back to the belt and slow neutrons back to the detectors to increase the sensitivity and to provide a detection characteristic which does not vary with the relative position of the belt and contents between the source and detectors. The γ-detector 20 is mounted above the neutron reflector and comprises an ionization chamber containing Ar or Xe. In the signal processing circuit, Fig. 8, the outputs of the three neutron detectors (22-1, 2, 3) are amplified (85) and discriminated (86) to reject small pulses due to the γ-radiation. The remaining pulses are counted (88) for a selected time and the total stored (90). The stored count is converted to an analogue signal (94) and a signal corresponding to the average signal produced from the belt when empty is substracted (97) and the output is linearized (100) to produce a signal proportional to the weight of hydrogen in the sample. A predetermined proportion of the total sample weight signal from the γ-detector channel is subtracted (104) to compensate for the hydrogen content of the dry bulk material so that the signal at 108 is proportional to the weight of hydrogen in the water contained in the sample and hence to the weight of water. The output of γ-detector 20 is amplified (122) a signal corresponding to the average output due to the belt when empty is subtracted (124) and the output is linearized (126) to produce a signal which is integrated (129) over the same period as the corresponding neutron count and the result stored (136) to produce the signal proportional to the total weight of the moist sample. The output from the neutron detector channel is divided by that from the γ-detector channel to produce a signal proportional to the percentage by weight of moisture in the sample. This signal is adjusted for maximum and minimum expected values (114) and applied to a record 118. The moisture content signal is also subtracted from unity in op amp 140 and multiplied by a signal proportional to the total weight of the material on the belt to give the bone dry weight of the material. In alternative embodiments the area of material observed may be restricted by using collimating plates between the detectors and the belt Fig. 10 (not shown) or by using detectors of the required restricted length. In both cases the whole width of the belt is irradiated by the sources. The detectors may also be arranged in a backscatter mode instead of in the transmissive mode.