Plant trials with radio-isotopes and thermography failed to provide precise lining thickness quantitatively required for corrective action. Extensive research in the application of laser led to the development of an electro-optical He-Ne laser system for the purpose. The system is able to perform measurements inside a hot vessel upto 1600°C and can physically measure lining thickness to an accuracy of ±5 mm from a remote and arbitrary position in front of the vessel through its mouth opening.

TATA STEEL have acquired one such state-of-the art technology and made extensive use of the same. In this paper the authors made an attempt to describe their experience of the same.

The IMS 1600 measuring system, as commercially designated, is basically a precision surveying instrument which uses He-Ne laser light source from a measuring head.

The instrument emits laser beam on to the hot refractory lining inside the steel making vessel and the diffuse reflection from the object is detected by the measuring head in the instrument console. The slope distance of the object, the vertical and the horizontal angles are evaluated. The raw data received from the measuring head are processed in a desk-top computer. The results are displayed and printed almost immediately by an impact printer. The captive software in the computer, evaluates the lining thickness at the measured spots and presents the same in the form of profiles showing outline of the vessel along with the measured points.

For the purpose of measurement of slope distance the laser beam is sinusoidally amplitude modulated by means of a modulator at precisely two known frequencies, used in sequence namely at 299.7 kHz and 2997.0 kHz. By comparing the phase angles between the transmitted and the received light beams, it is possible to calculate the travelling time and hence the slope distance. The horizontal and vertical angles are measured by sensing the position of two pulse discs, one vertical and one horizontal. Some of the following important applications made at Tata Steel are shared by the authors in this paper.

• Remaining working lining thickness measurement in LD vessel.

• Assessment of progressive wear in the length of canned elements in "Bath Agitation Process" in LD vessel.

The He-Ne laser technique has been found to be useful for the reliable and safe operation of the oxygen steel making vessel. Timely assessment of lining condition followed by gunniting repair helps in increasing the campaign life of the vessel. Thus condition monitoring by this technique not only improves reliability of operation but also increases production and productivity.