Each instrument utilized for leakage surveys and investigations must be tested against a known sample or in accordance with the manufacturer's recommended instructions as follows:
(a) after any repair or replacement of parts other than normal maintenance;
(b) once every three months at intervals not exceeding 105 days for CGI's and solid-state leak detection instruments and once each calender year not exceeding 15 months for hydrogen flame ionization (HFI) instruments; and
(c) any time it is suspected that the calibration has changed.