GB873629A - Improvements in and relating to measuring systems - Google Patents

Abstract

873,629. Radiation detector for thickness or density measurement. INDUSTRIAL NUCLEONICS CORPORATION. Nov. 22, 1957 [Dec. 18, 1956], No. 36546/57. Class 40(3) A system for measuring a variable characteristic, e.g. the thickness or density of material, includes a high impedence between the radiation detector and an electronic discharge tube, an adjustable voltage connected in opposition to that produced across the high impedance and a compensating variable voltage network which cancels, signals arising from variations in the input impedance of the discharge tube or compensates for unbalance due to grid current variations, e.g. when a tube is replaced. The current in radiation detector 10 develops a voltage across impedance 15 which is balanced in its application to the grid 51 of tube 24 by a variable potentiometer voltage at 17. The output of tube 24 stabilized by resistor-capacitor 29, 30 is fed to the grid of a tube 25, the anode of which feeds a voltage divider 38, 39 to give positive feedback to the screen grid of tube 24. The cathode load resistance of tube 25 comprises voltage dividers 31, 32 the mid-point 33 of which is connected through a meter 34 and variable resistance 36 to ground. The opposing voltage network 16 comprises three distinct elements, (a) a potentiometer 21 of low resistance and switches 42, 43 for effecting compensation with change of grid current, i.e. when the tube 24 is replaced, (b) a potential source 18 and highprecision calibration potentiometer 17 for bring- ing meter reading 34 to mid-point for standard thickness or measurement and (c) a standardization potentiometer 19 and fixed resistor 20 for correcting source type drifts, e.g. due to decoy of the radio-isotope source 11. With no material at 12 and the potentiometer 17 at its bottom end, the calibration potentiometer 19 is adjusted to give zero deflection at rotor 34. The potentiometer 17 is then calibrated for different thicknesses of material necessary for zero deflection. Potentiometer 27 has previously been adjusted to give zero reading with switch 22 closed. Closure of switch 22 is also used to check periodically on the drift of the amplifier. When switch 22 is open and a positive signal from the detector impressed on grid 51, the plate voltage of tube 24 falls and less current in cathode follower 25 results in point 33 becoming more negative. This potential cancels the positive signal on grid 51, allowing the latter to assume the voltage to maintain the necessary feed back. The potential registered on the meter 34 is a measure of the thickness variation. The meter can be replaced by a device automatically controlling the thickness of material 12. Specification 873,630 is referred to.