There is a definite need to determine the time required for the water in a process tube to boil out after loss of inlet flow. This information is required to evaluate the reactivity and power transients following such an occurrence. Such information will also partially determine the incentive for improvements in safety circuits and equipment, accelerated safety rods in particular. A secondary objective is to learn more about the mechanism of the boil-out. Boil-out times have been theoretically predicted in the past with very little experimental data to back them up. A scanning capacitance method is presented for measurement of the water distribution within a process tube under transient boiling conditions. Preliminary results of exploratory runs are included. The runs simulate inlet pigtail failures on process tubes operating at 725 and 1,000 KW. Curves of water distribution over the tube with time and total tube water content are included. Recommendations are made that the tests be continued to provide valid data for assessing the reactor hazards involved in cases of cooling water supply failure.