The Measurement of the Thickness of Metal Walls from One Surface Only, by an Electrical Method

Abstract

There is a definite need in the engineering industry for an instrument which will determine accurately the thickness of a metal wall from one side only, and which is portable, robust, independent of external power supply, and easy to operate. The instrument described in the paper fulfils all these conditions. The method is essentially that of comparing the resistance of a metal wall under test with that of a similarly shaped wall of the same material, of which the thickness is known. Current, usually under 10 amp., is supplied by a 6- or 12-volt battery to two current contacts held against the wall and usually spaced 2 or 3 inches apart. The current in the circuit is adjusted by varying the rheostat or the number of cells of the small car-battery employed, until the very small potential drop indicated by the deflexion of a galvanometer connected to two potential contacts reaches a predetermined fixed value. From a calibration curve the thickness of the wall corresponding to the current registered by the ammeter is determined. The contacts are made on the plates whose thickness is to be measured by means of Evershed and Vignoles spring-controlled rotating points that were originally designed for a four-point method of working. The flexible leads from these, sometimes 100 but usually 30 feet long, are rubber-covered, and are connected to the circuit in the box by plugs and sockets. These are removed for transport and carried in a separate box. The instrument was originally designed for measuring the thickness of boiler tubes, and examples are given showing that its accuracy in this respect is within a few thousandths of an inch. It was found in practice that the instrument could be used to measure the thickness of mild steel plates up to 1·25 inches and of iron castings, up to 3 inches, with the same order of accuracy. It is also used for the measurement of engine cylinder walls to detect core shift. Examples of these are given in the paper. One notable case was that of the examination of a bank of superheater tubes in a boiler that had given much trouble due to the use of an unsuitable chain-grate stoker. Thinned tubes were at once detected and on being cut out the thicknesses indicated by the instrument were confirmed. Examples are given of the regular use of the instrument in determining the thickness of boiler and superheater tubes, ships' hull plates, tank wagons, and various iron castings that were accessible from one side only.