an apparatus for measuring the thickness of a tubular plastic layer during the process of its fabrication , which includes an ultrasonic transducer which generates a signal dependent on the thickness of the shape being measured while the shape is at a non - uniform condition of temperature , and circuitry for generating at least one signal from known process parameters , and an arithmetic unit for processing the signals derived as described above to generate an error signal which compensates for the influence of temperature on the test .