Patent GB-570751-A (GB)
Title: Improvements in or relating to electrical strain gauges
IPC: G01L1/22
Filed: 1943-07-14
Abstract: 570,751. Resistance measurements; determining dimensions of materials.; temperature compensation; wire resistances ; combinations of resistances. AUGHTIE, F. July 14, 1943 No. 11443. [Class 37] [Also in Group XIX] A resistance strain gauge comprises two elements having different ratios of temperature coefficient to strain coefficient of resistance and so arranged that resistance changes in both elements are related in the same way to both the temperature and strain of the same area of surface of a specimen. If the temperature coefficient of expansion of the test specimen is known, it is then possible to evaluate both strain and temperature change. Each element may be carried on its own support, for example, as described in Specification 560,260, and the two supports be superimposed with an insulating layer, e.g. of thin paper, between them, the whole being bonded into an integral structure by means of a synthetic resin set by heat and pressure. Alternatively, the. corresponding portions of the two elements may be parallel and interspaced so that the separating layer is dispensed with. Each element may be carried on its own support and so arranged that the two can be brought opposite one another in such relative position that the portions on the facing surfaces are approximately parallel and interspaced. The elements can then be bonded together without an intervening layer. The two elements may be wound quasihelically as described in Specification 560,260, but of opposite hands. One element should have a very low temperature coefficient of resistance and the other a high coefficient. Copper-nickel alloy wire of about 0.001 inch diameter is suitable for the first element and tungsten wire of about 0.0005 inch diameter may be used for the second. The separate resistances of the two windings may bo measured first in the initial condition and then with the test object strained and at a temperature different from the initial temperature. From these measured resistances and the known temperature coefficients and strain sensitivities the initial and final strains of the test object can be evaluated by a series of approximations. Alternatively, a high fixed resistance may be connected in series with the element of high temperature coefficient and of such a value that the effective temperature coefficient or the effective strain sensitivity of the two is equal to that of the other element. In the bridge shown, a resistance comprising two parts R1, R2 is connected in series with the strain gauge element M of high temperature coefficient and is of the required value to give strain determination, the other element A forming the adjacent arm of the bridge. The other arms are formed by resistances R3, R4, R5 and slide wires R6, R7. A galvanometer G1 determines the balance point for strain determination, whilst a galvanometer G2, connected to the junction point of the resistances R1, R2 determines the balance point for temperature determination. Strain and temperature may thus be directly and separately determined in the presence of changes of both. Specification 565,378 also is referred to.