Abstract: A method of withdrawing a mobile inspection sensor assembly from the lower header of a vertical tubular heat-exchanger includes the steps of introducing into a guide tube of the assembly and into a water tube of the heat-exchanger aligned therewith an end portion of a dummy sensor in the form of a tube of a relatively rigid material, which may nevertheless be slightly bent, securing the end of the dummy sensor to the water tube by actuating means operable at the other end of the sensor tube, and sliding the sensor assembly along the tube of the dummy to an access opening of the header.A dummy sensor for carrying out this method includes an expansible head which may be mechanically or pneumatically operated to fix the sensor in a water tube.