Section 10. A customer of a company or corporation subject to this chapter, or such company or corporation, may apply to the department for an examination and test of any water meter to determine the accuracy of the same. The department shall forthwith cause such examination and test as in its judgment is practicable and reasonable to be made by a competent and disinterested person, and shall furnish to the company or corporation and to the customer a certificate of the result and expense thereof. The person designated to make such examination and test may at any reasonable time enter upon the premises where the meter to be inspected is placed for the purpose of making the inspection. If upon examination and test, it appears the appliance does not register correctly, the department may order the company or corporation to correct or remove such meter and substitute a correct meter. The department may establish such rules and regulations, fix such standards, prescribe such fees, and employ such means and methods in, and in connection with, such examinations and tests of water meters as it deems most practicable, expedient and economical, and may purchase such materials, apparatus and standard measuring instruments for such examinations and tests as it deems necessary. All fees for examination and tests shall be in the first instance paid by the person, company or corporation making the application therefor; but if the examination or test is made at the request of the customer, and the meter is found to be incorrect because too fast, the company or corporation shall pay such fees to the department, to be repaid by it to the applicant. Whoever being engaged in the sale of water maintains upon the premises of a customer for the purpose of determining the charge to be made for water supplied to him a meter which is found upon examination and test to register incorrectly as against such customer, shall refund to him such an amount as if not agreed upon, shall, upon application of the customer and after opportunity given to the vendor to be heard, be determined by the department. A meter shall be deemed to be correct for the purposes of this section if it appears from such examination or test that it does not vary more than two per cent from accuracy. This section shall not authorize or prohibit differential prices for water supplied by any such company or corporation.