To estimate the efficiency of the heat evacuation system designed for the new nuclear power plant special 3D codes are used. It is very important to verify the codes in concern with the modeling of new safety measures. Therefore the experimental data obtained at the conditions similar to actual ones are needed. Such conditions are realized at the specially constructed test rig. Measurement of the air velocity, cooling water flow rates and temperatures has to be carried out. For the measurement of the velocity field the multipoint LDA system is build, which uses laser diode and self-mixing receiving technique.