A tool for evaluating the internal surfaces of tubular is provided, wherein one or more arms extend outwardly from the tool into contact with an inner wall of the tubular, and changes in the dimensions or condition of the inner wall result in changes in the position of the ends of the arms relative to the tool. This motion is converted, through an electromagnetic transducer, into an electrical signal, the accuracy of which deteriorates as the temperature of the tool changes. Calibration transducers are provided on the tool to provide correction factors to convert the electromagnetic transducer output into an accurate reading of the distance of the end of the arms from the tool thereby increasing the accuracy of the assessment of the inner wall of the tubular.