Section 27. No testing method shall be used for the determination of milk-fat or other component content of milk or cream for the purpose of inspection or as a basis for payment in buying or selling unless it has been jointly approved by the director and the chief of the bureau of milk marketing created under the provisions of section six of chapter twenty and duly regulated under the provisions of section twenty-nine A. No person shall operate any mechanical equipment for such determination unless he has first obtained from the director or his agent a certificate indicating that he is competent in such operation and has thereafter been licensed under the provisions of section twenty-eight. The fee for such a certificate of competency shall be set by and be payable to the director. The director shall establish such requirements with respect to the application for and granting of said certificates as he deems appropriate. A copy of each such certificate shall be furnished to the chief of the bureau of milk marketing created under the provisions of section six of chapter twenty.