Required. Each utility shall own and maintain, or have access to, one or more calibrated meter provers of appropriate size, and all other equipment necessary to test meters. Temperature Control of Bell Provers. Means shall be provided to maintain the temperature of the liquid in the bell provers at substantially the same level as the air temperature in the prover room. The maximum difference in temperature should not be more than 2°F. Accuracy. Each meter prover shall be maintained in good condition, in correct adjustment, and accurate to within ±0.3 percent at each point used in testing meters. A prover shall be calibrated at least once every 3 years and if moved or disassembled. The accuracy of a bell prover shall be determined by one or more of the methods indicated below: By comparison to a 1 cubic foot standard calibrated by the National Institute of Standards and Technology; By comparison to an approved transfer standard for a bell prover in excess of 5 cubic feet; or By strapping the bell prover with a calibrated tape whose accuracy is traceable to the National Institute of Standards and Technology. The accuracy of any other type of prover shall be determined by appropriate standards traceable to the National Institute of Standards and Technology. Attachments. A device used in conjunction with a prover is calibrated to the following accuracy standards: If the accuracy of the attachment is determined independently of the system accuracy it shall be within ±0.2 percent; or If the accuracy of the system includes the accuracy of the attachment the system accuracy shall be within ±0.3 percent at each point used in testing meters.