A nephelometric turbidimeter with a cylindrical turbidimeter vial. The cylindrical turbidimeter vial includes a transparent vial body and a circular optical shielding configured to optically block an inside from an outside of the turbidimeter vial. The vial body comprises a transparent and flat bottom inlet window, and a transparent vial cylinder body. The vial cylinder body comprises a circular outlet window. The optical shielding is arranged axially above the outlet window of the vial cylinder body, over a part of an axial length of the vial cylinder body, and axially adjacent to a non-shielded part of the vial cylinder body which serves as the outlet window.