Patent GB-858345-A (GB)
Title: Method and apparatus for the measurement of the thickness of metallic and other conducting coatings on backings of conducting materials
Inventor: BAILEY GORDON LENNOX JAMES
IPC: G01B7/06
Filed: 1956-04-06
Abstract: 858,345. Electric tests. BRITISH NON- FERROUS METALS RESEACH ASSOCIATION. April 8, 1957 [April 6, 1956], No. 10585/56. Class 37 . In a method of estimating the thickness of one or more conducting coatings on a conducting backing in which heat is applied to the body under test by one or more probes and the thermoelectric potential between the probe and the backing measured to determine the individual coating thickness 1, the probe is provided with a plated layer the temperature, composition and a thickness of which may be so chosen as to give the thickness required. A combination with further probes may be used to give a comparison set of readings or two different temperatures may be used. Fig. 1B shows a probe having a massive copper part acting as a heat reservoir and a hemspherical end 2 in contact with the top layer B of two layers, B, C on a base (5). In one example discussed for determining the thickness of chromium and nickel on an iron layer a single probe is used but measurements are made at two different temperatures. A chromium tip is used for the probe. As shown in Fig. 3, the nickel-iron and chromium-nickel potentials move in opposite directions as the temperature varies. Thus the potentials derived with the probe at two different temperatures are different functions of the thickness of the coatings B and C and the thickness of each may be derived from the two observations. Where it is required to determine the thickness of nickel and copper coatings on steel and on zinc base alloys it is stated that a chromium tipped probe gives a direct measurement of the thickness of the nickel coating irrespective of the thickness of the copper coating within certain ranges of both coatings. Is is stated that changes in the chro- .mium-nickel potential compensate for changes in the intermediate potential introduced by changes in copper thickness so that the total potential observed is a direct measure of nickel thickness. Examples using different thickness of chromium layers are given. Where the copper coating is required in the case of nickel and copper coatings on a zinc base a nickel plated probe gives a measure of nickel plus copper and a zinc steel base of nickel plus copper. By combining these readings with the nickel thickness determined by the chromium plated probe the thickness of the copper coating can be calculated. Probe construction (Fig. 4). The hot probe consists of an outer. cylindrical sleeve 3 of electrical and thermal insulating material for example Keramot closed at its lower end by a conical wall 3a through which protrudes the tip of a copper probe 1. The probe carries a heater winder 4 and is supported by a guide block 5 sliding in a brass bush 6. The guide block is attached to a tubular stem 7 which carries a compression spring 9 enabling a constant pressure to be exerted on the probe tip when it is pressed on to the test piece. Contacts in a potential measuring circuit may be closed when the probe is lifted off the conical seating by pressure on the work. Specification 768,448 is referred to.