An ultrasonic method of gauging

A method is described of measuring wall thickness accurately although only one surface may be accessible. A variable frequency oscillator is employed which sets up standing waves between the transducer and the reflecting surface. At the resonant frequency and its harmonics the oscillator anode current increases and measurement of the frequency interval between these peaks enables the thickness to be calculated. The paper describes:¿(1) A portable instrument in which the frequency, modulation is by a mechanically-rotated capacitor, and audible or meter indication is given; (2) A larger instrument employing electrically variable inductor tuning and direct reading c.r.t. presentation. Consideration is given to the design of transducers, choice of couplants, applications and limitations, and to accuracy and range.