The U9 ACPD instrument is one of a series of NDT instruments manufactured by TSC for the detection and sizing of defects in electrically conducting material. The U9 is a portable instrument designed primarily for site use.

Alternating Current Potential Difference (ACPD) is a crack sizing technique which can be used for monitoring and measuring fatigue crack length and depth.

In TSC equipment the current input and voltage measurement functions are kept apart. This allows the current input points on probes to be widely separated which produces a more uniform electric field (and have more accurate sizing), and also allows many voltage probes to be associated with a single input current for monitoring purposes.

The U9 ACPD instrument can be used for a variety of crack sizing applications, including:

• Monitoring of cracks to give crack growth data
• Monitoring of critical regions for crack detection

The U9 ACPD instrument is designed for use with one set of inputs and is generally used on site for depth profile mapping of cracks that have previously been located. In situations where multiplexing is required with a capability of extension to 128 channels and 4 output currents, the U10 ACPD instrument is recommended.