I'd definitely recommend using the material_dir setting of the piping_material_source. The material_dir option will automatically calculate the WT_LEN if you use a different unit systems and generally MUCH easier to deal with.
The material_dir option links the material of the pipeline to the listed material in the pro_material_dir directory.
The weight_length option is a bit trickier and requires some calculations beforehand, particularly when using IPS unit system. If you're going with this option, in Creo the mass and density values are explicitly not the weight (force) - gravity needs to be accounted for in any calculations. It is VERY important to note the difference between the Creo Parametric Default unit system and IPS unit system. IPS is a measure of pound-force and Creo Parametric Default is a measure of pound-mass. You will need to convert any standard "lb/in³" densities to lbf by dividing by (32.174 ft/s2 x 12 in/ft).
John Piotrowski, Co-Founder & Product Manager CadActive Technologies - www.cadactive.com