Patent GB-1483251-A (GB)
Title: Wide range radiation gage for determining a material property
IPC: G01N23/16
Filed: 1974-08-12
Abstract: 1483251 Radiation thickness gauges BETHLEHEM STEEL CORP 12 Aug 1974 [20 Aug 1973] 35390/74 Heading G1A Apparatus having means 14 for converting radiation 13 into light which varies in response to a material property P, includes a controllable photodetector 17 and a comparator 21 with an integrator 22 in a feedback loop 15, and means for using a signal derived from the feedback loop to measure the material property P. Hot steel plate P up to three inches thick exposed to a source 10 e.g. Cs 137 emitting γ-rays at 667 KeV, attenuates the radiation exponentially with plate thickness. Emergent rays 13, converted to light by a non-phosphorescent doped plastics scintillator cube 14, excites a photo-multiplier 17. The elements 10, 14 and 17, and a shutter and calibration standards unit 11, are housed in a C- shaped housing (not shown) arranged to move relative to the plate P. Photo-multiplier current, amplified at 18, is compared with a constant reference supplied via a resistor 23, the difference being amplified in an integrating comparator 21, 22 and fed to a controllable voltage source 27 supplying the photo-multiplier dynodes. The effect of thus maintaining constant the photo-multiplier current is that the required dynode voltage is roughly an inverse logarithmic function of the light intensity and therefore constitutes a measuring signal as an approximately linear function of the plate thickness. Furthermore the stabilisation of the photomultiplier working current has the effect of stabilizing the slope and curvature of the calibration curve. The dynode voltage necessary to balance the loop in the absence of a plate P is offset by a manual comparator 29, and by automatic zero-adjusting means 81 which stores the offset voltage sampled in the absence of a plate P when gated by a material detector 82, to correct for zero shift due to dirt on the scintillator and decay in the source and photomultiplier. Linearity errors are corrected by a lineariser 31 which combines the raw measuring signal on a line 30 with a linearising signal generated by a log amplifier 32 as a predetermined function of the raw signal. A compensator 48 associated with a data source 58 permits correction of the linearised thickness signal on a line 47, for errors due to material grade and temperature, both of which affect the density and therefore the absorption characteristic of the plate P, the thermal expansion further affecting accuracy since the thickness may be measured at temperatures varying over a range 1100-2200F. An optical pyrometer 61 supplies a temperaturedependent signal to the compensator 48; the data source 58 carries data about the material composition and thermal expansion coefficient, which may be displayed to allow manual adjustment of the comparator 48 or used automatically. The compensated measurement signal, amplified at 79, is applied to a numerical display or recorder 88 which may also be controlled to supply a thickness reference to a D/A converter 92. A selector switch 93 may be used to apply the converter 92 output to the amplifier 79 whose output, now representing thickness deviation from a design centre, is displayed on a meter 96. The apparatus may be calibrated by a shutter and calibration standards unit 11 controllable to insert standard plates into the radiation path 12.