Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

What is Ultrasonic Immersion Testing?


Ultrasonic immersion testing is the inspection by ultrasound of engineering parts and components submerged in water.


How Does Immersion Ultrasonic Testing Work?

The test object, part or component is positioned in a water-filled tank. An ultrasound probe is then moved over the surface of the part. Sound travels though the water and into the part. Following the sound’s interaction with the interior microstructure, any flaws that may exist, or with the inner surface of the component, the echoed sound wave returns to the probe. The arrival time of the echo and its amplitude enable interpretation of where and what it is originating from in the material.

The ultrasonic probe is usually mounted on a robotic arm that has translational and sometimes also rotational movement. Coverage of the part can then be automated through the use of pre-programmed trajectories, which plot multiple passes over a given area. Data is then collected through the incremental scanning of these passes. The acquired data is most commonly viewed as a C-scan, but B-scans, D-scan images can also be viewed.

The use of water as a coupling medium removes the variability of coupling quality that may occur in contact inspection. It also enables rougher surfaces to be inspected without a loss of coupling, and curved surfaces to be inspected without the requirement of custom wedges. Ultrasonic immersion testing is highly versatile and can be configured for single element probes, phased array probes and for advanced techniques such as FMC/TFM. High frequency probes are generally used for narrow component thicknesses and materials that are easy to penetrate such as carbon steel, whereas lower frequencies are typically required for thick sections and attenuating materials such as stainless steels and plastics.

Typical inspection tasks using this test technique include corrosion mapping of steel sheets and pipes, weld inspection of ex-situ process piping, quality control of automotive parts, and damage characterisation of post-impacted composites. 

What are the Advantages?

  • Efficient inspection of large components, with high resolution compared to conventional ultrasonic inspection
  • High versatility of the inspected component’s shape, size and material
  • High spatial precision scanning, enabled by the automated probe movement and pre-set data collection increments
  • High repeatability, due to the consistency of the water as a coupling medium
  • Permanent record of the inspection with associated scan images
  • Causes no damage to the test object, unlike destructive testing

What are the Limitations?

  • Not applicable for in-service inspections due to the immersion tank required
  • Components must be submerged in water, and may be susceptible to corrosion, depending on the material type
  • Probe access limitations may inhibit inspections of complex geometries

TWI Resources

  • Large immersion tank with seven axes of freedom
  • Small immersion tank with Utex UT340 pulser receiver

More Information

Ultrasonic inspection system for offshore mooring chains

Seven axis ultrasonic immersion system

For more information please email: