Abstract: What is claimed is an inspection device for in-tube monitoring of main pipelines by the method of ultrasonic wall thickness metering realized in a device traveling inside the pipeline and performing measurements, acquisition of measurement data and their interpretation. The device comprises a probing pulse generator, an ultrasonic transducer, an amplifier, a comparator with an analog input, a digital timer, a processor and a data storage module, and a controlled reference voltage source connected in series. The output of said reference voltage source is connected to the reference voltage input of the comparator, said reference voltage source being capable of setting at least two different voltages at its output.