Volume 7, Number 6
A Gossman Consulting, Inc. Publication
December 2002

6-Minute Block Associated

With Opacity Requirements


David Gossman, Dave Constans and Jim Woodford, Gossman Consulting, Inc.

Recently, GCI ran into a regulatory interpretation question concerning performance testing related to PC MACT. When calculating the 6-minute block(s) associated with the opacity requirement should a facility calculate the 6-minute blocks beginning at the beginning of the run as discrete 6-minute blocks or as a 6 minute rolling average? After all, the regulation [63.1350(c)(3)] does state that a facility must ensure that  "the 6-minute average opacity for any 6-minute block average period does not exceed 20 percent."  The operative word here would seem to be “any” since logic would suggest that the only way you can ensure that “any 6-minute block average period does not exceed 20 percent” is through use of a rolling average rather than looking at discrete 6 minute blocks. This question required regulator interaction.


Regulator response provided a detailed but different explanation/interpretation. The key word turned out to be "block" rather than "any".  However, even with that focus, it was pointed out that the GCI interpretation concerning "any six-minute block" was actually the definition of a rolling average.


Nonetheless, Subpart LLL doesn't refer to a rolling average when discussing the opacity limit compliance.  It was also pointed out that Subpart LLL does require the use of a rolling average for the temperature limit, and in 63.1350(f)(3)-(5), describes how this is calculated.  Again, there is no similar discussion for opacity. 


Much of PC MACT ends up sending one back into existing regulations and regulatory language, which has been around for some time. Consequently, GCI did do additional research into the question only to learn that 40CFR60.2 definitions: states that a "six-minute period means any one of the 10 equal parts of a one-hour period." Clearly this was not particularly helpful. This research also led GCI into a possible explanation for how the 6-minute block came about in the first place.


It appears to derive from the original method of evaluating opacity, by human eye, 6 readings per 1 minute for 6 minutes for a total of 36 readings averaged together (6 minutes may be the limit a man could stand there and observe without succumbing to practice effect or some such).  But opacity monitors eventually came along and produced a value every minute … forever.  EPA appears to have stuck with the 6-minute blocks.   No new regulations have been written to challenge the old ways at this time (although look at the evolution of the waste fuel regulations that were ostensibly conservative back in 1991).  Since the old way did not specify when to start a visual evaluation period no start time seems to have been specified for the monitors either. 


It would be great if it came down to correct or incorrect. But as many industry representatives have found out, no matter the intent of a regulation, the enforcement group always finds new and novel ways to perform and apply their own interpretations, which often results in costing the industry money (in out of court settlements to avoid the cost of lengthy court battles if nothing else - right is not always the issue unfortunately).

The industry can work with whichever definition the regulators desire. Either way of calculating makes little difference in the results, but enforcement, yea, there’s the rub. Since "block" is not specifically defined anywhere in the regulation and the word "any" was used instead of "each," hopefully some sort of correction or guidance will be forthcoming before enforcement weighs in and clarifies the definition for us.