Patent GB-1225682-A (GB)
IPC: G01G9/00
Filed: 1968-03-05
Abstract: 1,225,682. Radiation measuring apparatus; voltage measurement. INDUSTRIAL NUCLEONICS CORP. March 5, 1968 [March 10, 1967], No.10748/68. Headings G1A and G1U. Apparatus for measuring a characteristic of a material includes a radiation source, applying radiation to the material, and a detector, receiving radiation from the material dependent on said characteristic, the detector output signal being summed with a nulling bias signal of opposite polarity to obtain a difference signal which is used to adjust mechanically the nulling bias to reduce the difference to zero, and an indication of the characteristic being obtained by use of a summation of the nulling bias (or a signal dependent on it) and the difference signal (or a signal dependent on this), the indication being on a device having a faster response rate than the mechanical adjustment device for the nulling bias. Thus in the embodiment of Fig. 1, showing a nucleonic transmission gauge 16, 18, measuring e. g. thickness or density as dependent on weight per unit area, the detector output voltage e d  is opposed at junction 40 by the nulling bias e b . Any difference signal is amplified by amplifier 24, which may be of the electro-meter type, to produce a voltage e o  feeding servo-amplifier 28. Servomotor 30 adjusts the potentiometer 32 providing the nulling bias, until the voltage e o  is zero. The voltage e o  is also fed back to be effectively summed with voltage e b  in opposing e d . In this way, a fast response signal is able to be picked up at 32 (or 40) which provides a much faster measurement on variance computer 54, frequency analyzer or absolute value indicator 52, than is provided by mechanical indicator 46 ganged with the tapping 32. In a modification, Fig. 2 (not shown), an indicator is connected to a potentiometer connected to that 34 of Fig. 1. In a further embodiment, Fig. 3 the nulling bias is provided by a potentiometric bridge network 78. For indication purposes, a further potentiometric bridge network 64 is provided, which is adjusted in correspondence with bridge 78 by means of link 60. The voltage it develops is e b2  which is summed with the amplifier 24 output e o  by means of operational amplifier 100, to feed an indicator or thickness control mechanism. R 1  and R 2 , the input resistors, may be equal or in an appropriate ratio to equalize the input voltages to amplifier 102. The span of the system is set by potentiometer 94 in accordance with the meter range. However, this may introduce errors, so that the circuit is modified, as in Fig. 4 (not shown), by having a variable R 1  (or R 2 ) ganged to potentiometer 94. Also this system has provision for standardization in accordance with changes due to e.g. source decay, build-up of dirt thereon &c., in the form of variable resistors 86<SP>1</SP> and 72<SP>1</SP> in the bridge networks 78, 64, ganged to R f  in the operational amplifier. In a further modification, Fig. 5 (not shown), the amplifier output e o  applied to the operational amplifier is adjustable by a voltage divider network, or a potentiometer. Alternatives to the illustrated arrangements include the provision of sets of differently valued resistors R 1  and 106 of which individual ones are switched into circuit. A backscatter gauge having the same detector output measurement circuits is also envisaged.