Tonnage is numeric value for rating graphite. The higher the graphite tonnage the stiffer the graphite is and the more vibrations are transmitted through the rod. As tonnage increases less material is required to achieve the same action reducing the weight of the rod and increasing sensitivity. The stiffer a material is the more brittle it becomes and because of this high tonnage graphite rods require the use of specialized resins and construction techniques to maintain durability.