Method for inspecting prestressed concrete pressure pipes based on remote field eddy current/transformer coupling and use of non-coaxial coils

A method for detecting discontinuities in at least one prestressing wire in prestressed concrete pressure pipe (PCPP) with an embedded metallic cylinder having an inner surface and a central longitudinal pipe axis, comprising:

• passing an RFEC/TC remote field probe, comprising at least one exciter means and at least one detector means, each being adjacent the inner surface of said PCPP, through said PCPP;
• energizing said exciter means with low frequency ac, so as to create an energy flow path, externally of said metallic cylinder, between said exciter means and said detector means and a transformer coupling through said prestressing wire, inductively linking said exciter and detector means;
• and
• receiving a signal in said detector means, indicative of discontinuities in said prestressing wire,
• said signal comprising at least a portion of the signal arising from the transformer coupling linking said exciter means and said detector means and
• wherein at least one of said exciter and detector means comprises a non-coaxially wound coil.