The sound measuring set, consisting of the sound level meter, microphone, calibrator, and any cables, shall be submitted to the instrument manufacturer's designated calibration and repair facility or an approved laboratory for calibration, adjustment, and repair at least once every two years. The tachometer shall be maintained in calibration in accordance with the tachometer manufacturer's calibration instructions. Exhaust system manufacturers may repair and calibrate their own instruments when the calibration standards are traceable to the National Bureau of Standards in the United States or the national standards body in the country in which the laboratory is located.