Sec. 1-1] 



MFJ 11. 1 .\7f '. 1 /, IS PUT THAXSDl ( <EI!S 



11 



1-12. Inductive Thickness Gauges 



All inductive thickness gauges are based on the direct or indirect 

 measurement of a magnetic flux. The thickness variation of the test 

 object causes a measurable variation of the flux density. Four 

 different cases can be distinguished: 



1 . The test object is of a ferromagnetic material, Fig. ( I - 1 ) la. An 

 increase of the thickness t of the test object causes an increase of the 



Test object varies 

 in thickness 



Reluctance-variation 

 gauges 



Gauge 



I 



y////////////////m 



(a) 



Test object causes a variation 

 of distance between gauge 

 head ond base 



Eddy -current 

 gauges 



Gauge 



! c : . ■,\\\\\\\V 



[c] 



£ 



zz 



Fig. (1-1)7. Synopsis of inductive thickness gauges. 



flux density. Magnetic flux meters for this application are described 

 under Reluctance-variation Gauges (1-1 2a). 



2. The test object is of a material with a magnetic permeability in 

 the vicinity of 1 (nonferrous metal or insulator), but is, or can be, 

 deposited on a ferromagnetic base, Fig. (1-1)7&. Also in this case 

 reluctance-variation gauges of the type described under 1-1 2a can be 

 used, provided the ferromagnetic base is so thick that the magnetic 

 reluctance in the circuit is primarily determined by the distance t 

 between the gauge head and the base plate, i.e., by the thickness of 

 the test object. 



3. The test object is of a conductive material having a magnetic 

 permeability of 1. In Fig. ( 1 -1)1 c it is nonferrous metal. This prob- 

 lem calls for the use of an eddy-current gauge, as described under 

 1-126. 



4. The test object is nonconductive (e.g., paint) but is, or can be, 

 deposited upon a conductive base, Fig. (1-1)1 d. This problem can 

 also be solved with the eddy-current gauge as described under 1 - 1 2b. 



