Thermal hydraulic experiments at system pressures close to the reference reactor are difficult and costly. The pressure chamber technology described in the present paper was developed to improve the accessibility of the test fluid for an application of advanced instrumentation. The main idea consists in accommodating the actual test rig inside a pressure tank and operate it in pressure equilibrium with a filling gas in the chamber. In this way, the walls of the test equipment do not have to be designed as a pressure-bearing boundary. An important feature is a passive approach to maintain the needed very good pressure equilibrium between the fluid in the test setup and the filling gas of the chamber. It is in particular needed in experiments requiring high mass flows of steam supply. Auxiliary systems for pressurizing and cooling the chamber are described. Furthermore, several useful solutions of particular problems are presented, such as the accommodation of delicate devices, like high-speed cameras and infrared cameras, in the pressure chamber, a blurring-free imaging through large observation windows, the sealing of the latter, auxiliary equipment for an efficient handling of the test equipment and the design of penetrations for multiple electric signals through the pressure boundary. Finally, an overview of the experimental programs conducted by now is given, whereby the emphasis is put on examples of measuring results illustrating the potentials of the pressure chamber technology rather than on the scientific output of the given experimental programs.