Device and process for measuring the diameter of a pipe

A device is described for measuring the equivalent diameter of a pipe, particularly of a small diameter borehole, characterised in that it includes a long probe body that moves along the pipe by such means as a suspension cable and fitted with a gamma radiation detector, arms fitted to the probe body, distributed angularly around it and of which a part carrying a gamma source tends to move elastically away from the probe body to enter into contact with the pipe wall, and means for holding these arms against the probe body, that can be deactivated from the end of the displacement facilities of the probe body at a distance from it