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Eddy Current testing uses the principals of electromagnetism as the base for conducting inspections. When alternating current is applied to a conductor a magnetic field develops in and around the conductor. When a second conductor is introduced into the magnetic field an induced current flow is created. Eddy currents are a type of induced current.
In the case of eddy current inspection, eddy currents are created using a probe. Inside the probe is a length of electrical conductor which is formed into a coil. AC flows in the coil at a chosen frequency and a dynamic expanding and collapsing magnetic field forms around the coil as the ac flows through the coil. When electrically conductive material is placed in the coil's magnetic field, electromagnetic induction occurs, and eddy currents are induced. These eddy currents flowing in the material generate their own "secondary" magnetic field that opposes the coil's primary magnetic field. The strength of the generated currents, known as the standard depth of penetration vary depending on probe frequency, material conductivity and permeability.
Major advantages of this inspection are the ability to increase inspection information collected from one probe pull, comparison of same discontinuity signal at different frequencies, mixing of frequencies that helps to reduce or eliminate sources of noise and improves detection, interpretation and sizing capabilities of the NDT methods used in nuclear power plants, ultrasonic testing accounts for the most commonly applied as it is primarily used to detect leaking fuel rods and end plug weld control.
While quality control activities (detection/monitoring and action) occur during and after data collection, the details should be carefully documented. A clearly defined communication structure is a necessary pre-condition for establishing monitoring systems. There should not be any uncertainty about the flow of information between principal investigators and staff members following the detection of errors in data collection. A poorly developed communication structure encourages lax monitoring and limits opportunities for detecting errors.
Tubing Bobbin Probes.
We use tubing bobbin probes for inspection steam generators and heat exchangers. Coils placed inside of probes generate magnetic field of electrical currents. Loops of electrical currents induced within conductors by a changing magnetic field in the conductor due to Faraday's law of induction.
Steam generators probes are built for flexibility, allowing for optimal navigation through tubing u-bends, while maintaining high data quality.
Probes for heat exchangers are designed different way. Probes have sharper leading edges and they are designed to punch through tubes with significant impediments. Both probes utilize stainless steel front and back bearings to protect the coils and ensure long probe life.