Sec. 1-1] 



MECHANICAL INPUT TRANSDUCERS 



13 



Fig. (1-1)10. Magnetic thickness 

 gauge of the permeability-variation 

 type. 



The range of the system is about 0.001 to 0.750 in., the accuracy 

 about ^5 per cent or +0.1 mil. 1 



permeability-variation system. The system described in Fig. 

 (1-1)10 consists of a permanent magnet A brought in contact with 

 the ferromagnetic test object through laminated pole pieces B. The 

 flux density in the magnetic circuit increases with the thickness 

 of the test object. The magnetic permeability of ferromagnetic 

 materials changes, in general, with the flux density. The perme- 

 ability is measured by a small a-c . 

 field, superimposed on the field 

 caused by the permanent magnet, 

 and produced by a current in the 

 primary coil P. The a-c field induces 

 a voltage in the secondary coil#; the 

 voltage is proportional to the perme- 

 ability of the test object and, hence, 

 is a function of its thickness. 



The method has been used for the measurement of thickness from 

 0.01 to 0.25 in.; the accuracy varies for different constructions from 

 0.0005 to 0.003 in. 2 



b. Eddy-current Systems. A test probe consists usually of a coil 

 wound on an insulating carrier, as shown in Fig. (1-1)11. The coil is 

 supplied with alternating current, and the test probe is placed in 



contact with the metallic specimen 



to be examined. The magnetic 



a-c field causes eddy currents in 



the specimen which, in turn, cause 



a magnetic field of a direction 



opposite to that produced by the 



coil. The result is a reduction of 



the inductance of the coil, which 



can be measured. Alternatively, 



the test probe can contain two 



coils, as shown in the eddy-current displacement transducer (1-22). 3 



This method requires access to one side of the sample only. The 



system is suitable for thickness measurements of nonmagnetic and 



magnetic metals as well as for nonconducting samples that are 



1 G. Keinath, The Measurement of Thickness, Natl. Bur. Standards Circ. 585, 

 sec. 4.2, 1958. 



2 Ibid., sec. 4.3. 



3 Application as thickness gauge by W. W. Wood, AIEE Meeting, Los 

 Angeles, Calif., June, 1954. 



-Metal coating 



"Base metol 



Fig. (1-1)11. Eddy-current thickness 

 gauge. 



