the ratio of length to diameter of the cavity is greater than or equal to 1 and less than or equal to 750 (1 = L/D = 750) and where the length of the cavity is not greater than 1500 mm (L = 1500 mm); or, alternatively, instruments with an open cavity on both sides where the ratio of length to diameter of the cavity is greater than or equal to 2 and less than or equal to 1500 (2 = L/D = 1500 ...