Isotope Gauges: An Analysis of Some Errors in Hot Metal Thickness Measurement

Abstract

Isotope gauges are particularly suitable as thickness measuring instruments for hot metal above 5 cm. thick and their application for this purpose is continuing to increase. The paper discusses some of the factors affecting the accuracy of these gauges and, in particular attempts a theoretical evaluation of the errors due to steel composition, temperature distribution and statistical variation. These errors are evaluated for three carbon steels. Errors in thickness measurement due to the different compositions of these steels are evaluated for the two isotope sources most commonly used for gauging purposes namely, Americium 241 and Caesium 137. These isotope sources are representative of those used for the entire practical range of thickness measurement. The paper considers the effect of temperature distribution across the depth of metal on the mass attenuating the γ-rays and the thickness measurements so evaluated are compared with those determined assuming a homogenous stock temperature. The principle of constant absorption is described and also some of its limitations where a wide range of measurements is required. Finally an artifice is discussed whereby some of the limitations may be overcome.