When not otherwise provided by law, the commissioner, or his inspectors at his direction, shall have the power to inspect and test to ascertain if they are correct all weights and measures kept, offered, or exposed for sale. It shall be the duty of the commissioner, as he may deem necessary, to inspect and test to ascertain if they are correct all weights and measures commercially used (1) in determining the weight, measurement, or count of commodities or things sold, or offered or exposed for sale, on the basis of weight, measure, or of count, or (2) in computing the basic charge or payment for services rendered on the basis of weight, measure, or of count; provided, however, that with respect to single-service devices (that is, devices designed to be used commercially only once and to be then discarded) and with respect to devices uniformly mass-produced, as by means of a mold or die and not susceptible of individual adjustment, tests may be made on representative samples of such devices. The lots of such samples which are representative shall be held to be correct or incorrect upon the basis of the results of the inspections and tests on such samples.