Thickness standard sample and method of calibrating gage
BACKGROUND OF THE INVENTION 
     This invention relates to a thickness standard sample applicable to calibration of a thickness gauge using fluorescent X-ray or other radiant rays. 
     Prior thickness standard samples are formed with thin films electroplated or coated on base plates. These films are, for instance, 300 grams/m 2  of zinc in thickness and have thickness fluctuation of ±50 to 80 grams/m 2  in range in a sample of 100 mm×200 mm surface area. 
     Because of this uneveness of the film thickness, they are not suitable for accurate calibration of industrial gauges such as X-ray thickness gauges. 
     BRIEF SUMMARY OF THE INVENTION 
     It is a principal object of this invention to provide a precise thickness standard sample having an even foil on a base plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawing is a perspective view showing a thickness standard sample according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the invention will be described hereinafter with reference to the attached drawing, in which numeral 1 designates a foil with even thickness of, for instance, 10 to 50 μm and made of either metal (e.g. zinc or aluminium) or nonmetal, and numeral 2 designates a base plate which is made of either metal or nonmetal and as shown in the drawing is of greater thickness than the foil. Zinc foil is preferably selected for gauging the thickness of galvanized zinc on steel sheet and a steel base plate is advantageous for the same case. The reason for this selection is that, for this gauging, fluorescent X-rays from zinc are selected and detected, leaving fluorescent X-rays from the base steel, but prefect separation of these X-rays is impossible and the zinc X-ray zone includes some steel X-rays. 
     Aluminium foil is preferable for gauging the thickness of tin-plate sheet, because tin foil as thin as plate tin, approximately 50 μm thick, could not be obtained and because fluorescent X-rays from the base steel somewhat absorbed by tin in passing through the plated tin film are detected for thickness gauging, i.e. the absorption method is applied for gauging of tin-plate, and the thicker aluminium foil, which is easily obtainable, absorbs somewhat equivalent X-rays to plated tin. 
     Foil 1 has an even composition and is fixed to the base plate 2 by means of adhesion or the like. As illustrated in the drawing the base plate is rectangular and is of larger area than said foil so as to have a marginal portion not covered by said foil. Uneveness of the adhesion film does not give any influence on the accuracy of practical gauging. Precise parallelism of the foil upper surface to the base plate bottom surface is not required since a ±3 μm range of surface position shift in the normal direction thereof is allowed for usual thickness gauging. The foil may extend fully over the base plate without any bad effect.