When any meter becomes inoperative or does not operate properly, water will be charged in accordance with the average consumption during the three months immediately prior to the failure of the meter; provided, however, allowance shall be made for weather and seasonal changes, and in the event that the three previous months would not be a fair measure of water consumption for the period in question, the period used as a measure shall either be less than the three months period or three months other than the immediately prior three months.