Test Methods. For both calibration error methods in Regulation .09C(7) of this chapter, three filters shall each be placed in the light path five times and the monitor responses shall be determined from the permanent data recorder. The low-, mid-, and high-range calibration error results are computed as the main difference and the 95 percent confidence interval for the difference between the expected and actual responses of the monitor difference as corrected to the stack exit conditions. An owner or operator shall use neutral density filters with values that have been determined according to 7.1.3 Attenuator Calibration of Performance Specification 1, which is incorporated by reference under Regulation .03B(1) of this chapter. Attenuator Values. The stability of the attenuator values shall be checked at least once per year according to the procedure specified in Performance Specification 1, which is incorporated by reference under Regulation .03B(1) of this chapter. The attenuator shall be recalibrated if the stability checks indicate a change of 2 percent opacity or greater. Calibration attenuators that produce simulated opacities (as corrected to stack exit conditions) in the ranges listed in §C of this regulation shall be used. Where the use of the specified audit filter values is not practical, alternate filter ranges may be used subject to the approval of the Department. Filter Ranges for Opacity Performance Audits. The filter ranges for opacity performance audits are as provided in §C(2) of this regulation or, as applicable, as provided in Performance Specification 1 (PS1) in Appendix B of 40 CFR Part 60. Audit Points and Filter Range are as follows: 8—15% Opacity (low) 20—30% Opacity (mid) 40—50% Opacity (high)