To assure that the burning qualities of the distributed gas do not vary sufficiently to adversely affect the safety and/or efficiency of any appliances, each company shall maintain and operate suitable test equipment, during the periods when its manufactured gas plant and/or gas mixing station is operating, to compare the burning qualities of the distributed gas with those of the gas with which its consumers' appliances have been adjusted. The types and adjustments of said test equipment shall be representative of those of its consumers' appliances. The performance of said test equipment shall be observed at frequent intervals whenever supplement, substitute or diluent gas is added to the gas usually distributed and be considered to be indicative of the actual interchangeability of the gases involved. Records shall be kept of the periodic observation of the performance of said test equipment.