Patent GB-1067459-A (GB)
Title: Improvements in dielectric gauges
IPC: G01N27/22
Filed: 1963-10-14
Abstract: 1,067,459. Measuring moisture content. INDUSTRIAL NUCLEONICS CORPORATION. Oct.14, 1963 [0ct. 12, 1962], No.40508/63. Heading G1N. Relates to a dielectric gauge apparatus, for measuring for example the moisture content of inhomogenous materials, and includes arrangements to counteract errors in measurement due to the non-uniform distribution of moisture in a material and the fact that the mass/unit area is not the same throughout the material. The application to paper making machines is suggested. A preferred embodiment includes part of the arrangement shown in Fig. 2 together with Fig. 9 which is a modification of the final stages of Fig. 2. Electrodes of a spray field capacitor probe 22 (Fig. 2) are positioned adjacent a travelling paper sheet 14 and are connected to a bridge circuit 24 energized by two oscillators 26, 28 which provide alternating voltage at two different frequencies f 1 , f 2 . The amplified bridge output signal contains components indicative of the unbalance conditions of the bridge at each frequency f 1 , f 2 . , These components are separated by filters 34, 36 and detected at 38, 40 to provide individual signals S H , S L  defined by equations - where p is the filling factor and is proportional to mass/unit area and q is the percentage moisture content of the paper. The signals are fed to a ratio computer 100 (Fig. 9) which solves the equations 1, 2 to obtain the result f(q) = SH SL which eliminates variable p and provides an analogue signal which is a function of the moisture content and independent of mass. The arrangement so far described gives erroneous readings of q because of moisture stratification in the paper which must be corrected, and the true moisture content is given by false moisture content reading . - This equation is solved by Fig. 9 arrangement. In Fig. 9 a mechanical output connection 104 from motor M of the ratio computer 100 moves in accordance with a function of f(q') independently of mass variations and drives an arm 106a of a non-linear potentiometer 106. The potentiometer 106 is calibrated and arranged so that its resistance varies along its length in accordance with the combined variations of functions and f(q) and is energized from a voltage obtained from a variable tap 110a of a potentiometer 110 which has a signal voltage S H  applied across it. In order that the voltage from tap 110a will be proportional to SH, in spite P of continuous variations of weight of the paper sheet 14, a continuous mass indication is obtained from a gauging head 112 comprising e. g. a conventional beta radiation source and detector unit, whereby mass/unit area is continuously registered on recorder 114 which also drives the tap 110a. Assuming that the recorder varies linearly with mass/unit area and therefore with filling factor p the pot 110 comprises a conventional inverse function potentiometer to provide at tap 110a a voltage proportional to SH and the P voltage obtained from the movable tap 106a will be directly proportional to the true moisture content q whether the moisture content is homogenous or not. The output may be indicated by a linear meter, recorder or control device for the paper making machine, represented by 118.