In order to remember the grade of Natural Barium Sulfate easily , we have amended the grade of Natural Barium Sulfate :

Original  Grade             →                       New Grade

HTN-OP                      →                        HTN-NO

HTN-TF                      →                         HTN-NP

HTN-FK                       →                       HTN-NQ

HTN-ST                       →                       HTN-NR

HTN-EC                       →                       HTN-NS