Abstract: A method of detecting the level of molten metal existing within a mold comprises the steps of disposing a transmission coil and a receiving coil opposite to each other with the mold interposed therebetween, applying an AC voltage to the transmission coil for producing an alternating magnetic flux such that at least a part of the magnetic flux passes through the mold and the molten metal, if any, and reaches the receiving coil, and determining the level of the molten metal on the basis of at least one of a voltage value and a phase of an AC signal induced in the receiving coil by the alternating magnetic flux, and an apparatus for carrying out the above method.

Abstract: First, an equation expressing the relationships among the equivalent of the subsurface residual stress at a given point of material strip before entering a spiral-pipe forming machine, the equivalent of the subsurface residual stress at the corresponding point of formed pipe, the dimensions of the material strip, the outside diameter of the pipe, and the normalized magnitude of ring opening in the pipe is established using their measured values. Then, the equivalents of the subsurface residual stresses are measured while the pipe is being manufactured, and the normalized magnitude of the ring opening in the pipe is indirectly determined from the above equation using the measured equivalents.