DEPARTMENT OF 

ENVIRONMENTAL CONSERVATION 

Performance Audits for COMS 

Adopted by Reference in 18 AAC 50.030 

April 1, 2002 

Revised August 20, 2008 
 

Performance Audits for COMS April 1, 2002 

Revised August 20, 2008 1 

Alaska Department of Environmental Conservation 

Performance Audits for COMS 

Performance audits. The following elements shall be included in performance audits for 

Continuous Opacity Monitoring Systems (COMS), unless the department gives written approval for 

unit-specific audit procedures. 

1 Optical Alignment Assessment. The status of the optical alignment of the monitor 

components shall be checked and recorded according to the procedures specified by the monitor 

manufacturer. Realign as necessary. 

2 Zero and Upscale Response Assessment. The zero and upscale response errors shall be 

determined and recorded according to the calibration drift procedures of 8.1(4)(i) and (ii) in 

40 C.F.R. 60, Appendix B, Performance Specification 1 (PS-1), adopted by reference in 

18 AAC 50.040(a). The error is defined as the difference (in % opacity) between the correct 

value and the observed value for the zero and high-level calibration checks. 

3 Zero Compensation Assessment. The value of the zero compensation applied at the time of 

the audit shall be calculated as equivalent opacity, corrected to stack exit conditions as 

necessary, according to the procedures specified by the manufacturer. Record the compensation 

applied to the effluent recorded by the monitor system. 

4 Calibration Error Check. Conduct a three-point calibration error test using three calibration 

attenuators that produce outlet pathlength corrected, single-pass opacity values shown in 

ASTM D 6216-98, section 7.5, adopted by reference in 18 AAC 50.035(c). If the applicable 

limit is less than 10 percent opacity, use attenuators as described in ASTM D 6216-98, 

section 7.5 for applicable standards of 10 to 19 percent opacity. Confirm the external audit 

device produces the proper zero value on the COMS data recorder. Separately, insert each 

calibration attenuator (low, mid, and high-level) into the external audit device. While 

inserting each attenuator, (1) ensure that the entire light beam passes through the attenuator; 

(2) minimize interference from reflected light; and (3) leave the attenuator in place for at 

least two times the shortest recording interval on the COMS data recorder. Make a total of 

five nonconsecutive readings for each attenuator. At the end of the test, correlate each 

attenuator insertion to the corresponding value from the data recorder. Subtract the singlepass calibration attenuator values corrected to the stack exit conditions from the COMS 

responses. Calculate the arithmetic mean difference, standard deviation, and confidence 

coefficient of the five measurements value using equations 1-3, 1-4, and 1-5 of PS-1. 

Calculate the calibration error as the sum of the absolute value of the mean difference and the 

95 percent confidence coefficient for each of the three test attenuators using equation 1- 6 of 

PS-1. Report the calibration error test results for each of the three attenuators. 
 

Performance Audits for COMS April 1, 2002 

Revised August 20, 2008 2 

5 Zero Alignment Assessment. Compare the COMS simulated zero to the actual clear path zero 

of the installation. The assessment may be conducted in conjunction with, but prior to, other 

performance audit elements. 

A. Primary Zero Alignment Method. The primary zero alignment shall be performed under 

clear path conditions. This may be accomplished if the process is not operating and the 

monitor pathlength is free of particulate matter or the monitor may be removed from its 

installation and set up under clear path conditions. The absence of particulate matter shall 

be demonstrated prior to conducting the test at the installed site. No adjustment to the 

monitor is allowed other than the establishment of the proper monitor pathlength and correct 

optical alignment of the monitor components. Record the monitor response to a clear path 

condition and to the monitor's simulated zero condition as percent opacity corrected to stack 

exit conditions as necessary. For monitors with automatic zero compensation, disconnect or 

disable the zero compensation mechanism or record the amount of correction applied to the 

monitor's simulated zero condition. The response difference in percent opacity to the clear 

path and simulated zero conditions shall be recorded as the zero alignment error. Adjust the 

monitor's simulated zero device to provide the same response as the clear path condition. 

Restore the COMS to its operating mode. 

B. Alternate Zero Alignment Method. Monitors capable of allowing the installation of an 

external, removable zero-jig may use the equipment for an alternative zero alignment 

provided that the zero-jig setting is established for the monitor pathlength and recorded for 

the specific COMS by comparison of the COMS responses to the installed zero-jig and to 

the clear path condition. The zero-jig is shown to be capable of producing a consistent zero 

response when it is repeatedly (i.e., three consecutive installations and removals prior to 

conducting the final zero alignment check) installed on the COMS. The zero-jig setting 

shall be permanently set at the time of the initial COMS zeroing to the clear path zero value 

and protected when not in use to ensure that the setting equivalent to zero opacity does not 

change. The zero-jig setting shall be checked and recorded prior to initiating the zero 

alignment. Emission unit owners and operators that employ a zero-jig shall perform a 

primary zero alignment audit once every three years. 

C. Failure Criteria for Zero Alignment. The zero alignment is acceptable if the error at 

the simulated zero check is less or equal than 2% opacity prior to adjustment (i.e. if the 

zero alignment error is 0% the analyzer does not need servicing solely based on this test).