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The thickness of a vapor barrier layer embedded in a co-extruded plastic fuel tank is measured using ultrasound. An interference method using ultrasonic tone-bursts employs a frequency progression wherein a frequency producing a minimum in the reflection echo from the barrier layer identifies the th

The thickness of a vapor barrier layer embedded in a co-extruded plastic fuel tank is measured using ultrasound. An interference method using ultrasonic tone-bursts employs a frequency progression wherein a frequency producing a minimum in the reflection echo from the barrier layer identifies the thickness of the barrier layer. The method is useful in polyethylene fuel tanks wherein an ultrasonic attenuation which is highly dependent upon frequency limits the resolution of barrier layer thickness.

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Ultrasonic apparatus for measuring thickness of a first layer made of a first material having first and second edges, said first edge contacting an adjacent layer made of a different material in a multilayer structure, wherein said first layer has a thickness substantially less than a thickness of s

Ultrasonic apparatus for measuring thickness of a first layer made of a first material having first and second edges, said first edge contacting an adjacent layer made of a different material in a multilayer structure, wherein said first layer has a thickness substantially less than a thickness of said adjacent layer, and wherein said first layer thickness is measured using ultrasound passing through said adjacent layer, said apparatus comprising: an ultrasonic transducer for transmitting vibrations to and receiving vibrations from said structure; a signal source coupled to said ultrasonic transducer and producing a series of electrical transmission pulses causing said ultrasonic transducer to vibrate, each of said electrical transmission pulses having a respective frequency; a receiver coupled to said ultrasonic transducer for receiving electrical echo signals produced in response to vibrations returning to said ultrasonic transducer from said structure after each respective electrical transmission pulse of said series of pulses, each of said electrical echo signals including a respective interference echo signal from said first layer; and a controller coupled to said signal source and said receiver, said controller controlling said signal source to cause each pulse of said series of pulses to have a pulse duration less than the time for a pulse to traverse through said structure to said first layer and to cause said series of pulses to provide a predetermined frequency progression, said controller selecting said interference echo signal resulting from each electrical transmission pulse from other electrical echo signals also resulting from each electrical transmission pulse, and said controller determining said thickness of said first layer in response to a respective frequency of said electrical transmission pulses that produces an interference echo signal having a minimum magnitude as compared to all other interference echo signals.