A process for preparing a reactor (1), said reactor defining an opening (13) for the passage of a device for dispensing solid particles (3) suitable for charging the reactor with solid particles, the process comprising:—providing a sensor support device (2) to which a sensor is attached, said sensor being intended for collecting information on the charging of the reactor,—attaching the sensor support to a component of the reactor (5) different from the device for dispensing solid particles.