It shall be unlawful for any person to operate a milk or cream testing apparatus to determine the percentage of milk fat in milk or cream for the purpose of purchasing the same, either for himself or another, without first securing a license from the commissioner, who shall issue such license upon a form prepared by him, upon payment of a fee of $1.00, which license shall be valid, unless sooner revoked, until the next succeeding January 1. The applicant for such license shall pass a satisfactory personal examination that shall satisfy said commissioner that he is competent and qualified to operate and use such apparatus and make an accurate test with the same. The license may be revoked by the commissioner when it shall be shown that such licensed person is incompetent or unreliable. Any person making application for a tester's license may be issued a permit by the commissioner to do testing until such time as a personal examination can be given by the commissioner or his duly authorized agent. The testing of each lot of milk or cream by any unlicensed person shall constitute a separate offense under this article. Any licensed person may, for a valid reason satisfactory to said commissioner, appoint a substitute for a period not to exceed 15 days, subject to the approval of said commissioner.