Section 111. The unit of measure for the sale of gas by meter shall be the cubic foot, containing sixty-two and two thousand nine hundred and ninety-three ten-thousandths pounds avoirdupois weight of air-free distilled water at sixty degrees Fahrenheit when weighed in dry air at the same temperature and at a barometric pressure of thirty inches of mercury, or a thermal unit of measure, such as the ”therm” or ”British Thermal Unit”, as the department shall determine and approve.