Abstract: To prevent the tube to a pressure measuring means from being blocked by solid material mobile in a pressure container, a small part of the clean pressure medium from a source of such medium is injected into the tube so that the tube is continuously purged with clean pressure medium. By locating the pressure medium input at a suitable position along the tube relative to the pressure measuring means, the desired purging of the tube is obtained without appreciably increasing the pressure in the tube, and thus minimizing possible errors in measurement of the pressure. If, for example, pressure medium is passed into a pressurized combustion chamber through a surrounding pressurized container, and the tube to the pressure-measuring means passes through said surrounding pressurized container, the clean pressure medium can be bled into the tube through a hole bored in the side of the tube within the pressurized container.