An observer comprised of a non-linear simplified mathematical model of the LD-converter and a non-linear feedback is employed in order to obtain real-time estimates of the molten metal analysis. Measurements of the oxygen flow through the lance as well as the analysis and the flow of the off-gas are used for updating the process model. The observer design method is based on evaluating the sensitivity functions of the process output signals, i. e. the off-gas analysis, with respect to the process states, i. e. the carbon and silicon content. The observer is implemented and run on a process control computer monitoring regular steel production in three steel converters at two steel mills. The whole test is scheduled to take six months and is already underway. A detailed account of the experiments will be included in the final paper.