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 calendar year at intervals not exceeding 15 months for Hydrogen Flame Ionization (HFI) instruments; and
(c) any time it is suspected that the calibration has changed.