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PERFORMANCE SPECIFICATION 12A—SPECIFICATIONS AND TEST PROCEDURES FOR TOTAL VAPOR 

PHASE MERCURY CONTINUOUS EMISSION MONITORING SYSTEMS IN STATIONARY SOURCES

1.0 Scope and Application

1.1 Analyte. The analyte measured by these procedures and specifications is total vapor 

phase mercury (Hg) in the flue gas, which represents the sum of elemental Hg (Hg°, CAS 

Number 7439-97-6) and oxidized forms of gaseous Hg (Hg+2), in concentration units of 

micrograms per cubic meter (μg/m3).

1.2 Applicability.

1.2.1 This specification is for evaluating the acceptability of total vapor phase Hg 

continuous emission monitoring systems (CEMS) installed at stationary sources at the time of or 

soon after installation and whenever specified in the regulations. The Hg CEMS must be capable 

of measuring the total concentration in μg/m3of vapor phase Hg, regardless of speciation, and 

recording that concentration at standard conditions on a wet or dry basis. These specifications do 

not address measurement of particle bound Hg.

1.2.2 This specification is not designed to evaluate an installed CEMS's performance over 

an extended period of time nor does it identify specific calibration techniques and auxiliary 

procedures to assess the CEMS's performance. The source owner or operator, however, is 

responsible to calibrate, maintain, and operate the CEMS properly. The Administrator may 

require, under section 114 of the Clean Air Act, the operator to conduct CEMS performance 

evaluations at other times besides the initial performance evaluation test. See §§60.13(c) and 

63.8(e)(1).

1.2.3 Mercury monitoring approaches not entirely suited to these specifications may be 

approvable under the alternative monitoring or alternative test method provisions of §60.13(i) 

and §63.8(f) or §60.8(b)(3) and §63.7(f), respectively.

2.0 Summary of Performance Specification

Procedures for determining CEMS relative accuracy, linearity, and calibration drift are 

outlined. CEMS installation and measurement location specifications, data reduction procedures, 

and performance criteria are included.

3.0 Definitions

3.1 Continuous Emission Monitoring System (CEMS) means the total equipment required 

to measure a pollutant concentration. The system generally consists of the following three major 

subsystems:
3.2 Sample Interface means that portion of the CEMS used for one or more of the 

following: sample acquisition, sample transport, sample conditioning, and protection of the 

monitor from the effects of the stack effluent.

3.3 Hg Analyzer means that portion of the Hg CEMS that measures the total vapor phase 

Hg mass concentration and generates a proportional output.

3.4 Data Recorder means that portion of the CEMS that provides a permanent electronic 

record of the analyzer output. The data recorder may provide automatic data reduction and 

CEMS control capabilities.

3.5 Span Value means the measurement range as specified in the applicable regulation or 

other requirement. If the span is not specified in the applicable regulation or other requirement, 

then it must be a value approximately equivalent to two times the emission standard. Unless 

otherwise specified, the span value may be rounded up to the nearest multiple of 10.

3.6 Measurement Error Test means a test procedure in which the accuracy of the

concentrations measured by a CEMS at three or more points over its measurement range is 

evaluated using reference gases. For Hg CEMS, elemental and oxidized Hg (Hg0and mercuric 

chloride, HgCl2) gas standards of known concentration are used for this procedure.

3.7 Measurement Error (ME) means the absolute value of the difference between the 

concentration indicated by the CEMS and the known concentration of a reference gas, expressed 

as a percentage of the span value, when the entire CEMS, including the sampling interface, is 

challenged.

3.8 Calibration Drift (CD) means the absolute value of the difference between the CEMS 

output response and either an upscale Hg reference gas or a zero-level Hg reference gas, 

expressed as a percentage of the span value, when the entire CEMS, including the sampling 

interface, is challenged after a stated period of operation during which no unscheduled 

maintenance or repair took place.

3.9 Relative Accuracy Test Procedure means a test procedure consisting of at least nine 

test runs, in which the accuracy of the concentrations measured by a CEMS is evaluated by 

comparison against concurrent measurements made with a reference method (RM). Relative 

accuracy tests repeated on a regular, on-going basis are referred to as relative accuracy test audits 

or RATAs.

3.10 Relative Accuracy (RA) means the absolute mean difference between the pollutant 

concentrations determined by the CEMS and the values determined by the RM plus the 2.5 

percent error confidence coefficient of a series of tests divided by the mean of the RM tests. 

Alternatively, for sources with an average RM concentration less than 5.0 micrograms per 

standard cubic meter (μg/scm), the RA may be expressed as the absolute value of the difference 

between the mean CEMS and RM values.

4.0 Interferences [Reserved]
5.0 Safety

The procedures required under this performance specification may involve hazardous 

materials, operations, and equipment. This performance specification may not address all of the 

safety problems associated with these procedures. It is the responsibility of the user to establish 

appropriate safety and health practices and determine the applicable regulatory limitations prior 

to performing these procedures. The CEMS user's manual and materials recommended by the 

RM should be consulted for specific precautions to be taken.

6.0 Equipment and Supplies

6.1 CEMS Equipment Specifications.

6.1.1 Data Recorder Scale. The Hg CEMS data recorder output range must include the full 

range of expected Hg concentration values in the gas stream to be sampled including zero and 

the span value.

6.1.2 The Hg CEMS design should also provide for the determination of CD and ME at a 

zero value (zero to 20 percent of the span value) and at upscale values (between 50 and 100 

percent of the span value). The Hg CEMS must be constructed to permit the introduction of 

known concentrations of Hg and HgCl2 separately into the sampling system of the CEMS 

immediately preceding the sample extraction filtration system such that the entire CEMS can be 

challenged.

6.2 Reference Gas Delivery System. The reference gas delivery system must be designed 

so that the flowrate exceeds the sampling system flow requirements of the CEMS and that the 

gas is delivered to the CEMS at atmospheric pressure.

6.3 Other equipment and supplies, as needed by the reference method used for the Relative 

Accuracy Test Procedure. See Section 8.6.2.

7.0 Reagents and Standards

7.1 Reference Gases. Reference gas standards are required for both elemental and oxidized 

Hg (Hg and mercuric chloride, HgCl2). The use of National Institute of Standards and 

Technology (NIST)-traceable standards and reagents is required. The following gas 

concentrations are required.

7.1.1 Zero-level. 0 to 20 percent of the span value.

7.1.2 Mid-level. 50 to 60 percent of the span value.

7.1.3 High-level. 80 to 100 percent of the span value.

7.2 Reference gas standards may also be required for the reference methods. See Section 

8.6.2.
8.0 Performance Specification Test Procedure

8.1 Installation and Measurement Location Specifications.

8.1.1 CEMS Installation. Install the CEMS at an accessible location downstream of all 

pollution control equipment. Place the probe outlet or other sampling interface at a point or 

location in the stack (or vent) representative of the stack gas concentration of Hg. Since the Hg 

CEMS sample system normally extracts gas from a single point in the stack, a location that has 

been shown to be free of stratification for Hg or, alternatively, SO2 is recommended. If the cause 

of failure to meet the RA test requirement is determined to be the measurement location and a 

satisfactory correction technique cannot be established, the Administrator may require the CEMS 

to be relocated. Measurement locations and points or paths that are most likely to provide data 

that will meet the RA requirements are described in Sections 8.1.2 and 8.1.3 below.

8.1.2 Measurement Location. The measurement location should be (1) at least two 

equivalent diameters downstream of the nearest control device, point of pollutant generation or 

other point at which a change of pollutant concentration may occur, and (2) at least half an 

equivalent diameter upstream from the effluent exhaust. The equivalent duct diameter is 

calculated according to Method 1 in appendix A-1 to this part.

8.1.3 Hg CEMS Sample Extraction Point. Use a sample extraction point either (1) no less 

than 1.0 meter from the stack or duct wall, or (2) within the centroidal velocity traverse area of 

the stack or duct cross section. This does not apply to cross-stack, in-situ measurement systems.

8.2 Measurement Error (ME) Test Procedure. Sequentially inject each of at least three 

elemental Hg reference gases (zero, mid-level, and high level, as defined in Section 7.1), three 

times each for a total of nine injections. Inject the gases in such a manner that the entire CEMS is 

challenged. Do not inject the same gas concentration twice in succession. At each reference gas 

concentration, determine the average of the three CEMS responses and subtract the average 

response from the reference gas value. Calculate the measurement error (ME) using Equation 12-

1 by expressing the absolute value of the difference between the average CEMS response (A) 

and the reference gas value (R) as a percentage of the span (see example data sheet in Figure 

12A-1). For each elemental Hg reference gas, the absolute value of the difference between the 

CEMS response and the reference value must not exceed 5 percent of the span value. If this 

specification is not met, identify and correct the problem before proceeding. Repeat the 

measurement error test procedure using oxidized Hg reference gases. For each oxidized Hg 

reference gas, the absolute value of the difference between the CEMS response and the reference 

value shall not exceed 10 percent of the span value. If this specification is not met, identify and 

correct the problem before proceeding.

8.3 Seven-Day Calibration Drift (CD) Test Procedure.
8.3.1 CD Test Period. While the affected facility is operating normally, or as specified in 

an applicable regulation, determine the magnitude of the CD once each day (at 24-hour intervals, 

to the extent practicable) for 7 consecutive unit operating days according to the procedures in 

Sections 8.3.2 and 8.3.3. The 7 consecutive unit operating days need not be 7 consecutive 

calendar days. Use either Hg0or HgCl2 standards for this test.

8.3.2 The purpose of the CD measurement is to verify the ability of the CEMS to conform 

to the established CEMS response used for determining emission concentrations or emission 

rates. Therefore, if periodic automatic or manual adjustments are made to the CEMS zero and 

upscale response settings, conduct the CD test immediately before these adjustments, or conduct 

it in such a way that the CD can be determined.

8.3.3 Conduct the CD test using the zero gas specified and either the mid-level or highlevel gas as specified in Section 7.1. Sequentially introduce the reference gases to the CEMS at 

the sampling system of the CEMS immediately preceding the sample extraction filtration system. 

Record the CEMS response (A) for each reference gas and, using Equation 12A-2, subtract the 

corresponding reference value (R) from the CEMS value, and express the absolute value of the

difference as a percentage of the span value (see also example data sheet in Figure 12A-2). For 

each reference gas, the absolute value of the difference between the CEMS response and the 

reference value must not exceed 5 percent of the span value. If these specifications are not met, 

identify and correct the problem before proceeding.

8.4 Relative Accuracy (RA) Test Procedure.

8.4.1 RA Test Period. Conduct the RA test according to the procedure given in Sections 

8.4.2 through 8.4.6 while the affected facility is operating normally, or as specified in an 

applicable subpart. The RA test may be conducted during the CD test period.

8.4.2 Reference Methods (RM). Unless otherwise specified in an applicable subpart of the 

regulations, use Method 29, Method 30A, or Method 30B in appendix A-8 to this part or 

American Society of Testing and Materials (ASTM) Method D6784-02 (incorporated by 

reference, see §60.17) as the RM for Hg concentration. For Method 29 and ASTM Method 

D6784-02 only, the filterable portion of the sample need not be included when making 

comparisons to the CEMS results. When Method 29, Method 30B, or ASTM D6784-02 is used, 

conduct the RM test runs with paired or duplicate sampling systems and use the average of the 

vapor phase Hg concentrations measured by the two trains. When Method 30A is used, paired 

sampling systems are not required. If the RM and CEMS measure on a different moisture basis, 

data derived with Method 4 in appendix A-3 to this part must also be obtained during the RA 

test.

8.4.3 Sampling Strategy for RM Tests. Conduct the RM tests in such a way that they will 

yield results representative of the emissions from the source and can be compared to the CEMS 

data. The RM and CEMS locations need not be immediately adjacent. Locate the RM 
measurement points in accordance with section 8.1.3 of Performance Specification 2 (PS 2) in 

this appendix. It is preferable to conduct moisture measurements (if needed) and Hg 

measurements simultaneously, although moisture measurements that are taken within an hour of 

the Hg measurements may be used to adjust the Hg concentrations to a consistent moisture basis. 

In order to correlate the CEMS and RM data properly, note the beginning and end of each RM 

test period for each paired RM run (including the exact time of day) on the CEMS chart 

recordings or other permanent record of output.

8.4.4 Number and Length of RM Test Runs. Conduct a minimum of nine RM test runs. 

When Method 29, Method 30B, or ASTM D6784-02 is used, only test runs for which the paired 

RM trains meet the relative deviation criteria (RD) of this PS must be used in the RA 

calculations. In addition, for Method 29 and ASTM D6784-02, use a minimum sample time of 2 

hours and for Methods 30A and 30B use a minimum sample time of 30 minutes.

NOTE: More than nine sets of RM test runs may be performed. If this option is chosen, RM 

test run results may be excluded so long as the total number of RM test run results used to 

determine the CEMS RA is greater than or equal to nine. However, all data must be reported 

including the excluded test run data.

8.4.5 Correlation of RM and CEMS Data. Correlate the CEMS and the RM test data as to 

the time and duration by first determining from the CEMS final output (the one used for 

reporting) the integrated average pollutant concentration for each RM test period. Consider 

system response time, if important, and confirm that the results are on a consistent moisture basis 

with the RM test. Then, compare each integrated CEMS value against the corresponding RM 

value. When Method 29, Method 30B, or ASTM D6784-02 is used, compare each CEMS value 

against the corresponding average of the paired RM values.

8.4.6 Paired RM Outliers.

8.4.6.1 When Method 29, Method 30B, or ASTM D6784-02 is used, outliers are identified 

through the determination of relative deviation (RD) of the paired RM tests. Data that do not 

meet the RD criteria must be flagged as a data quality problem and may not be used in the 

calculation of RA. The primary reason for performing paired RM sampling is to ensure the 

quality of the RM data. The percent RD of paired data is the parameter used to quantify data 

quality. Determine RD for paired data points as follows:

Where:

Ca and Cb are the Hg concentration values determined from the paired samples.

8.4.6.2 The minimum performance criteria for RM Hg data is that RD for any data pair 

must be ≤ 10 percent as long as the mean Hg concentration is greater than 1.0 μg/m3. If the mean 

Hg concentration is less than or equal to 1.0 μg/m3, the RD must be ≤ 20 percent or ≤ 0.2 μg/m3
absolute difference. Pairs of RM data exceeding these RD criteria should be eliminated from the 

data set used to develop a Hg CEMS correlation or to assess CEMS RA.

8.4.7 Calculate the mean difference between the RM and CEMS values in the units of 

micrograms per cubic meter (μg/m3), the standard deviation, the confidence coefficient, and the 

RA according to the procedures in Section 12.0.

8.5 Reporting. At a minimum (check with the appropriate EPA Regional Office, State or 

local Agency for additional requirements, if any), summarize in tabular form the results of the 

CD tests, the linearity tests, and the RA test or alternative RA procedure, as appropriate. Include 

all data sheets, calculations, charts (records of CEMS responses), reference gas concentration 

certifications, and any other information necessary to confirm that the CEMS meets the 

performance criteria.

9.0 Quality Control [Reserved]

10.0 Calibration and Standardization [Reserved]

11.0 Analytical Procedure

For Method 30A, sample collection and analysis are concurrent. For the other RM, post-run 

sample analyses are performed. Refer to the RM employed for specific analytical procedures.

12.0 Calculations and Data Analysis

Calculate and summarize the RA test results on a data sheet similar to Figure 12A-3.

12.1 Consistent Basis. All data from the RM and CEMS must be compared in units of 

micrograms per standard cubic meter (μg/scm), on a consistent and identified moisture basis. 

The values must be standardized to 20 °C, 760 mm Hg.

12.1.1 Moisture Correction (as applicable). If the RM and CEMS measure Hg on a 

different moisture basis, they will need to be corrected to a consistent basis. Use Equation 12A4a to correct data from a wet basis to a dry basis.

Use Equation 12A-4b to correct data from a dry basis to a wet basis.

Where:

Bws is the moisture content of the flue gas from Method 4, expressed as a decimal fraction (e.g., 

for 8.0 percent H2O, Bws= 0.08).
12.2 Arithmetic Mean. Calculate d, the arithmetic mean of the differences (di) of a data set 

as follows:

Where:

n = Number of data points.

12.3 Standard Deviation. Calculate the standard deviation, Sd, as follows:

12.3 Confidence Coefficient (CC). Calculate the 2.5 percent error confidence coefficient 

(one-tailed), CC, as follows:

12.4 Relative Accuracy. Calculate the RA of a set of data as follows:

Where:

 = Absolute value of the mean of the differences (from Equation 12A-5)

|CC| = Absolute value of the confidence coefficient (from Equation 12A-7)

 = Average reference method value
13.0 Method Performance

13.1 Measurement Error (ME). For Hg0, the ME must not exceed 5 percent of the span 

value at the zero-, mid-, and high-level reference gas concentrations. For HgCl2, the ME must

not exceed 10 percent of the span value at the zero-, mid-, and high-level reference gas 

concentrations.

13.2 Calibration Drift (CD). The CD must not exceed 5 percent of the span value on any of 

the 7 days of the CD test.

13.3 Relative Accuracy (RA). The RA of the CEMS must be no greater than 20 percent of 

the mean value of the RM test data in terms of units of μg/scm. Alternatively, if the mean RM is 

less than 5.0 μg/scm, the results are acceptable if the absolute value of the difference between the 

mean RM and CEMS values does not exceed 1.0 μg/scm.

14.0 Pollution Prevention [Reserved]

15.0 Waste Management [Reserved]

16.0 Alternative Procedures [Reserved]

17.0 Bibliography

17.1 40 CFR part 60, appendix B, “Performance Specification 2—Specifications and Test 

Procedures for SO2 and NOX Continuous Emission Monitoring Systems in Stationary Sources.”

17.2 40 CFR part 60, appendix A, “Method 29—Determination of Metals Emissions from 

Stationary Sources.”

17.3 40 CFR part 60, appendix A, “Method 30A—Determination of Total Vapor Phase 

Mercury Emissions From Stationary Sources (Instrumental Analyzer Procedure).

17.4 40 CFR part 60, appendix A, “Method 30B—Determination of Total Vapor Phase 

Mercury Emissions From Coal-Fired Combustion Sources Using Carbon Sorbent Traps.”

17.5 ASTM Method D6784-02, “Standard Test Method for Elemental, Oxidized, ParticleBound and Total Mercury in Flue Gas Generated from Coal-Fired Stationary Sources (Ontario 

Hydro Method).”
18.0 Tables and Figures

TABLE 12A-1—T-VALUES

n

a

t0.975 n

a

t0.975 n

a

t0.975

2 12.706 7 2.447 12 2.201

3 4.303 8 2.365 13 2.179

4 3.182 9 2.306 14 2.160

5 2.776 10 2.262 15 2.145

6 2.571 11 2.228 16 2.131

a

The values in this table are already corrected for n-1 degrees of freedom. Use n equal to the number of 

individual values.

FIGURE 12A-1—ME DETERMINATION

 Date Time

Reference

gas value (μg/m3)

CEMS

measured

value (μg/m3)

Absolute

difference

(μg/m3)

ME

(% of span value)

Zero level

 

 

 Average

Mid level

 

 

 Average

High level

 

 

 Average
FIGURE 12A-2—7-DAY CALIBRATION DRIFT DETERMINATION

 Date Time

Reference

gas value (μg/m3)

CEMS

measured

value (μg/m3)

Absolute

difference

(μg/m3)

CD

(% of span value)

Zero level

 

 

 

 

 

 

 

Upscale

(Mid or High)

 

 

 

 

 

 

 
FIGURE 12A-3—RELATIVE ACCURACY TEST DATA

Run 

No. Date

Begin 

time

End 

time

RM value 

(μg/m3)

CEMS value 

(μg/m3)

Difference

(μg/m3)

Run used? 

(Yes/No) RD1

1

2

3

4

5

6

7

8

9

10

11

12

Average Values

Arithmetic Mean Difference:

Standard Deviation:

Confidence Coefficient:

T-Value:

% Relative Accuracy:

| (RM)avg − (CEMS)avg | :

1Calculate the RD only if paired samples are taken using RM 30B, RM 29, or ASTM 6784-08. Express RD 

as a percentage or, for very low RM concentrations (≤ 1.0 μg/m3), as the absolute difference between Ca

and Cb.