through 5.3.3 of Method 6C. shall

be used. For O₂ monitors that cannot analyze zero gas. a calibration gas concentration equivalent to less than 10 percent of the span may be used in place of zero gas. 6. Measurement System Performance Test Procedures. Perform the following procedures before measurement of emissions (Section 7). 6.1 Calibration Concentration Verification. Follow Section 6.1 of Method 6C. except if calibration gas analysis is required, use Method 3 and change the acceptance criteria for agreement among Method 3 results to 5 percent (or 0.2 percent by volume. whichever is greater). 6.2 Interference Response. Conduct an interference response test of the analyzer prior to its initial use in the field. Thereafter. recheck the measurement system if changes are made in the instrumentation that could alter the interference response (e.g., changes in the type of gas detector). Conduct the interference response in accordance with