Approved. Utilities may test meters by other methods approved by the Commission or by use of other working standards approved by the Commission. Accuracy. The approved working standards shall be maintained in good condition and correct adjustment, shall have a high degree of repeatability, and shall be capable of determining the actual proof of the meter under test to within ±0.5 percent at rates of flow between 15 percent and 100 percent of rated capacity of the meter under test. Test Period. The approved working standards shall be tested for accuracy by comparison with an acceptable standard at least once each 3 years.