This part of IEC 61788 specifies a test method for the twist pitch measurement of Nb-Ti/Cu and Nb-Sn/Cu composite superconductors by an untwisting method.

The test method is applicable to Nb-Ti/Cu and Nb-Sn/Cu composite superconducting wires with monolithic structures, which have either a round cross section with a diameter ranging from 0,2 mm to 2 mm or a rectangular cross section that is equivalent in area to the round cross-sectional wires. These wires possess a filament diameter ranging from 6 µm to 200 µm, a twist pitch between 5 mm and 50 mm, and a matrix of copper or copper alloy. This document uses nitric acid to remove the matrix (copper or copper alloy), so the surface of the composite superconducting wire can be plated with a material that is dissolvable by nitric acid.

Though uncertainty can increase, the method can apply to Nb-Ti/Cu or Nb-Sn/Cu composite superconducting wires when the parameters of cross-sectional area, filament diameter and twist pitch are out of the limit.

The test method specified in this document is expected to apply to other types of composite superconducting wires after some appropriate modifications.