Patent GB-781068-A (GB)
Title: Improvements in or relating to thickness measuring gauges using radio active material
IPC: G01N23/16
Filed: 1954-01-13
Abstract: 781,068. Radioactive thickness measuring gauges. EKCO ELECTRONICS, Ltd., THOMPSON, W. E., and TAYLOR, L. E. Jan. 13, 1955 [Jan. 13, 1954], No. 951/54. Class 40 (3). In a thickness measuring gauge comprising a radioactive isotope source of # radiation and an ionization chamber exposed to radiation from the source alter passage through the material. the chamber output is compared with a standard voltage to give a difference signal which is applied to an amplifier associated with a utilization circuit and means is provided for jointly varying the standard voltage and amplifier sensitivity in such a way as to compensate for the effect of contamination of the chamber and decay or contamination of the isotope source. In the Fig. 1 embodiment the output of chamber 1 is compared with a voltage from potentiometer 25, the difference voltage being measured at 3 after amplification at 2. To standardize the device potentiometer 25 is short-circuited by switch S5 and the material removed from between the isotope source and chamber. In the absence of decay meter 3 reads zero and any deviation from this is corrected by adjusting potentiometer 8 which is coupled to potentiometer 6 controlling the degree of negative feed-back in the amplifier. Thus the reduction in the difference voltage applied to amplifier 2 is compensated by an increase in amplification. The curve relating chamber output plotted to a log base against mass per unit area is a straight line above a certain minimum mass per unit area and a permanent absorber is used between source and chamber to ensure working in this region. An arrangement suitable for controlling or measuring thickness of a coating applied to a moving sheet is shown in Fig. 2. Two assemblies of source and chamber are used, associated with the uncoated and coated sheet respectively. The distance of the source of the first assembly 12 from its chamber is variable by servomotor 23 and is adjusted to give zero output to meter 14 for correct coating thickness. Variations of thickness give a meter reading or the meter current may control the coating process to give a uniform thickness. Operation is as follows. Normally the coated and uncoated sheets are measured by devices 11, 12, respectively, and the difference voltage balanced out by a voltage from potentiometer 26 to give zero input to amplifier 13. To standardize the apparatus both measuring devices are withdrawn from the material and potentiometer 26 short-circuited. To balance for differential contamination in the two devices switches S1, S2, S4 are moved to the positions shown so that unbalance operates motor 23 to restore balance. Switches S3 and S7 are then opened, S4 closed and S2 switched to its alternative position. Unbalance voltage appearing at 16 operates motor 24 to adjust potentiometers 18 and 17 which are ganged together. The sheet is then introduced to the devices 11 and 12, switches S3, S4, S7 are closed, S1 moved to its upper contact and S6 opened. Effects of contamination may be periodically reduced by replacing the permanent absorber by a thinner one with appropriate adjustment of the standard voltage and amplifier sensitivity.