A nephelometric turbidimeter vial arrangement includes a vial and a separate vial cap. The vial comprises a transparent cylindrical vial body configured to enclose a vial interior, a bottom inlet window, and a top vial opening configured to be circular. The separate vial cap comprises a light trap cavity. The separate vial cap is configured to close the top vial opening. The light trap cavity comprises an inner surface which comprises a light absorbing surface. The light trap cavity is configured to be open to the vial interior.