TAPPI TIP 0402-21 has been developed by a TAPPI Task Group to offer a basis for evaluating the skills of technicians who measure the thickness of boiler tubes from the external surface using manual instruments. Use of this procedure is prudent when surveying the thickness of recovery boiler tubes, where high reliability is important to avoid a potential smelt-water reaction and explosion from a water leak. The evaluation includes a background interview of each technician, as well as an operative test to objectively determine the technician's ability to accurately calibrate an ultrasonic thickness unit and measure the thickness of boiler tubes in a test panel. This procedure is written for bare carbon steel tubes but can be applied to other materials (e.g. chromized, composite, weld overlaid) if representative test panels are prepared. Technician skills for using specialized ultrasonic test equipment, such as semi-automated scanners, could be addressed by appropriate modifications to this procedure.