UNlTEDSTATESENVlRONMENTALPROTECTlONAGENCY 

Office of Air Quality Planning and Standards 

Research Triangle Park, North Carolina 27711 

r 6 JUN ngo 

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MEMORANDUM 

SUBJECT: Performance Test Calculation 

FROM: William G. Laxton, Director 

Technical Support Division, -wf A, ' '- 

John S. Seitz, Director 

Stationary Source Compliant 

TO: New Source Performance Standards/National Emission 

Standards for Hazardous Pollutants Compliance Contacts 

The following guidelines should be used in calculating and reporting 

emission rates and concentrations when determining compliance with the new 

source performance standards (NSPS) and national emission standards for 

hazardous pollutants (NESHAP). These guidelines can also be used for State 

implementation plans (SIP's). The areas addressed in this memorandum concern 

metric and English measurement systems, significant figures (SF's) in the 

emission standards, SF's to carry in intermediate calculations, and the 

rounding of final emission value numbers to the proper SF's. 

1. Use only the emission standard in the metric units to determine 

compliance. 

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The policy of using the metric system was established back in the early * 

1970's. When the Environmental Protection Agency (EPA) proposed standards for 

seven source categories (38 FR 15406) on June 11, 1973, EPA stated: 

"The Environmental Protection Agency has adopted a policy of 

expressing standards in the metric rather than English system. 

Although technical terms in test methods 10 and 11 are expressed in 

metric units, many of those in test methods 1 through 9 are 

expressed in English units. Test results derived through 

calculations in test methods 1 through 9 must be converted to metric 

units to agree with the form of the proposed standards." 

In keeping with this policy, EPA promulgated amendments to Subparts D, E, 

F, G, and tl on June 14, 1974 (39 FR 20790). In the preamble of this 

rulemaking, EPA stated: 

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. . . . Also; to be consistent with the Administrator's policy of 

converting to the metric system, the standards of performance and 

other numerical entries, which were originalJy expressed in English 
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units, are converted to metric units. Some of the numerical entries are 

rounded after conversion to metric units. It should be noted that the 

methods in the appendix will be changed to metric units at a later date." 

The change to metric units for the test methods were proposed on 

June 8, 1976 (41 FR 23060) and promulgated on August 18, 1977 (42 FR 41754). 

Clearly, EPA's intent was to use the metric units to determine compliance. 

However, on a practical scale, the use of the metric or the English 

system of units would not make any difference in determining whether a source 

is in compliance or out-of-compliance. Only in very rare cases will separate 

calculations in the metric and in the English systems with the same source 

test measurement values result in one system showing compliance while the 

other does not. Therefore, it is not necessary to reouire source testing 

firms to submit the results in the metric units on a routine basis. It is 

suggested that if the value in English units is within 1 percent of the 

emission standard and if such a difference is 'of concern in your enforcement 

strategy, then the value should be converted to metric to determine 

compliance. The numerical value in English units (in parentheses) are to be 

considered as close approximations of the metric and should not be used to 

determine compliance in borderline cases. 

2. Consider all emission standards to have at least two SF’s, but no 

more than three SF's. 

As a review, an SF is any digit that is necessary to define the specific 

value or quantity. Zeros may be used either to indicate a specific value, 

like any other digit, or to indicate the magnitude of a number. Examples are 

given below: 

Ex. 1: 1300 or 1.3 x lo3 

Ex. 2: 

hgs two SF's. 

1300. or 1.300 x 10 as four SF's. 

Ex. 3: 1300.0 or 1.3000 x 

Ex. 4: 13040 or 1.304 x 1 4 

4 0 9 has five SF's. 

has four SF's. 

Ex. 5: 0.034 or 3.4 x lo- ha2 two SF's. 

Ex. 6: 0.03400 or 3.40 

Ex. 7: 0.03 or 3 x lo- !I 

x 10 has four SF's. 

has one SF. 

Because the emission standards were not written with consideration to the 

rules of SF's, especially with the use of zeros, all existing emission 

standards are considered to have at least two SF's, but no more than three 

SF's, under this guideline., Thus: 

Case 1: 9d mg/dscm (Subpart I) is considered to be 90. (two SF's). 

Case 2: 520 rig/J (Subpart Da) is considered to be 520. (three-SF's). 

Case 3: 0.05 kg/Mg (Subpart S) is considered to be 0.050 (two SF's). 

Case 4: 0.1 g/kg (Subpart BB) is considered to be 0.10 (two SF's). 

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Case 5: 0.005 g/kg (Subpart BB) is considered to be 0.0050 

(two SF's). 

Case 6: 3870 kg/28 days (Subpart BBB) is considered 

to have (three SF's). 

The above rule differs from the previous guidance given by the 

predecessor of the Stationary Source Compliance Division. In his 

August 20, 1980 memorandum to Ms. Louise Jacobs, Director of Enforcement 

Division of Region VII, Mr. Edward E. Reich, Director of the Division of 

Stationary Source Enforcement interpreted the emission standards as being 

absolute, i.e., as having an infinite number of SF's. Using the example of an 

emission standard of 0.04 gr/dscf, Mr. Reich wrote: 

"As a legal matter, anything greater than 0.04 is a violation (e.g., 

0.0401). However, since the third digit was not established, most 

engineers in reporting results would tend to round off and therefore 

0.044 would be reported as 0.04 and 0.045 would be reported as 0.05. 

A better guide would be that anything showing greater than a ten 

percent excess is worth considering for enforcement action." 

Although the new guidance appears to be a major shift in Agency policy, 

it is not, because only very limited cases, if any, would be involved. For 

example, if the emission standard is 90 mg/dscm, 90.0000001 mg/dscm would be 

in violation according to the August 20, 1980 guidance. However, such an 

occurrence would be highly unlikely. (Note also that strict adherence to an 

infinite number of SF's would require an infinite number of SF's in conversion 

factors, which is a practice that is impossible to follow.) Therefore, the 

effect of the change to two or three SF's in the emission standards would be 

practically no different from the initial guidance. 

3. Carry at least five significant digits in intermediate calculations. a 

Since the measurement of variables and sample quantities during source 

performance tests are recorded in metric or English units or a combination of - 

both, specifying specific rules to handle SF's in addition, subtraction, 

multiplication, and division would become cumbersome. To keep things on a 

practical basis for emission standards with two or three SF's, English or 

metric may be used provided that at least five SF's are retained (most 

calculators retain nine digits) in a17 intermediate calculations. The final 

calculation after averaging all the runs should produce the metric units, if 

necessary. Then the final number should be rounded off as described below to 

determine compliance. 

4. Round off calculated emission nu&ers to the number of SF’s 

determined by the rule stated in (2) above. 

When rounding off a figure, the following procedure, based on practices 

given under the American Society for Testing and Materials (ASTM) (Standard 

for Metric Practice E 380), should be used: If the first digit to be 

discarded is less than five, the last digit retained should not be changed. 

When the first digit discarded is greater than five, or if it is a five 
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followed by at least one digit other than 0, the last figure retained should 

be increased by one unit. When the first digit discarded is exactly five, 

followed only by zeros, the last digit retained should be rounded upward if it 

is an odd number, but no adjustment made if it is an even number. 

For example, if the emission standard is 90, then 90.357 would be rounded 

to 90, 90.639 would be rounded to 91, 90.500 would be rounded to 90, and 

91.500 would be rounded to 92. '. 

Consideration was given to round upward when the last digit to be 

discarded is five; however, because the occurrence of the first digit to be 

discarded being exactly five followed by zeros is rare, it was decided to be 

consistent with the ASTM practice. 

cc: Jack R. Farmer, ESD (MD-13) 

John Calcagni, AQMD (MD-15) 

J.E. McCarley, EMB (MD-14) 

Rodney M. Midgett, QAD (MD-77A) 

Louis R. Paley, SSCD (EN-341) 

Ron Shafer, SSCD (EN-341) 

Roger T. Shigehara, EMB (MD-19) 

John J. Silvasi, AQMD (MD-15) 

Gilbert H. Wood, EMB (MD-14) 

Susan R. Wyatt, CPB (MD-13) 

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