thickness (upto 500µm) of the tubes and pipes. The system has the capability to indicate the reduction in wall thickness either in outer diameter or inner diameter or both. It also reveals uniform corrosion or localised pitting.

The ultrasonic transducer is supported on a probe carrier which fits into and is centred in the tube. The ultrasonic pulses are emitted along a path parallel to the probe axis. These pulses are reflected by a 45° mirror so that they are directed normal to the tube wall. Reflections from the inner and outer walls follow the same path to the transducer. The mirror is supported on a water driven turbine that spins on an axis parallel to the probe axis. As the mirror turns the point of impingement of successive ultrasonic pulses is advanced along the circumference of the tube wall, so that whole circumference of the tube wall is covered with each revolution of the mirror.

All the wall thickness measurements made during a scan around the circumference of the tube are displayed on an oscilloscope screen. The image produced is a stationary rectilinear picture of the circumferential cross section (B scan) of the tube.

The paper describes the results obtained on various experiments conducted on tubes of various materials having artificial defects and also tubes used in process industries having ID and OD corrosion and severe pittings. The wall thickness profile obtained by IRIS on a corroded pipe was compared with destructive metallography. The reading accuracy obtained by IRIS measurements is 100µm. By comparing the IRIS results with metallography it was possible to enhance the reading accuracy to 50µm. This can be taken as the corrective factor to be applied to IRIS results. IRIS is an instrument best used for failure prediction and prevention, rather than post mortem analysis. It avoids untimely and expensive shutdown of plants.