Stack testing is used to determine a facility’s compliance with emission limits, or with capture or control efficiencies [1]. EPA calls the exercise a performance test, run under conditions the Administrator specifies based on the facility’s representative performance [2]. The procedures live in the reference methods EPA publishes in Appendix A to 40 CFR Part 60, often several per pollutant [3]. Part 60 is the federal rule titled Standards of Performance for New Stationary Sources [3].

A crew typically measures velocity, moisture, molecular weight, and pollutant concentration; together these can give a mass emission rate [3]. Along with continuous monitor data, the result is usually the best representation of that outlet’s emissions on the day of the test. On its own, it does not measure the rest of the year.

This post covers the four measurements a test combines, why a result shows normal operation only, and how reconciliation makes it inventory evidence.

Quick Answer: A stack test is a performance test that determines and demonstrates compliance with an emission standard, using promulgated or approved test methods [1]. Unless the applicable subpart says otherwise, each test is three separate runs, and compliance uses the arithmetic mean of the three [2]. The general provisions exclude startup, shutdown, and malfunction from representative conditions, unless the applicable standard specifies otherwise [2].

A Stack Test Combines Four Separate Measurements

A mass emission rate needs both concentration and flow, so a rate-based test stacks several methods on top of each other. Method 1 sets the sample and velocity traverse points, which fixes where in the duct the crew samples [3]. Method 2 determines stack gas velocity and volumetric flow rate using a Type S pitot tube [3].

A raw velocity is not yet a flow, so two more methods supply the corrections. Method 3 is the gas analysis for determining dry molecular weight [4]. Method 4 determines moisture content in stack gases [5].

Flow diagram showing how a stack test becomes one number: Method 1 traverse points fix where in the duct the crew samples, Methods 2, 3 and 4 combine into the volumetric flow, and the pollutant method concentration multiplies with that flow into a mass emission rate such as pounds per hour, with compliance on the arithmetic mean of three runs

Methods 1 to 4 can build the flow and a pollutant method the concentration; Part 60 compliance uses a three-run mean unless a subpart differs. [3] [2]

The Pollutant Method Depends on the Pollutant

Concentration comes from a separate method chosen for the analyte. Method 7E determines nitrogen oxides emissions by instrumental analyzer procedure [6]. Method 10 determines carbon monoxide emissions from stationary sources [6]. Method 25A determines total gaseous organic concentration using a flame ionization analyzer [7].

For methane and ethane, the total is not enough. Method 18 measures gaseous organic compound emissions by gas chromatography, which is how individual hydrocarbons such as methane and ethane get their own numbers [8]. Method 3C determines carbon dioxide, methane, nitrogen, and oxygen by gas chromatography in samples from municipal solid waste landfills and other sources a subpart specifies [4]. Method 25A reads the organic total; Method 18 names individual organics, and Method 3C measures methane at landfills or where a subpart specifies.

Particulate matter has a method of its own. Method 5 determines particulate matter emissions from stationary sources [5]. Four methods build the flow, and one more is chosen for the pollutant being measured.

MethodWhat it determinesRole in the rate
Method 1Sample and velocity traverse pointsFixes where in the duct sampling and velocity traverses are taken
Method 2Stack gas velocity and volumetric flow rate, by Type S pitot tubeFlow
Method 3Dry molecular weight, by gas analysisFlow: corrects the velocity
Method 4Moisture content in stack gasesFlow: corrects the velocity
Method 5Particulate matter emissionsConcentration: filterable PM
Method 7ENitrogen oxides, by instrumental analyzerConcentration: NOx
Method 10Carbon monoxide emissionsConcentration: CO
Method 18Individual gaseous organics, by gas chromatographyConcentration: speciated hydrocarbons such as methane and ethane
Method 25ATotal gaseous organic concentration, by flame ionization analyzerConcentration: total hydrocarbons
Method 3CCarbon dioxide, methane, nitrogen, and oxygen, by gas chromatography with a thermal conductivity detectorConcentration: methane in landfill gas, or where a subpart specifies it

*Each method name links to its appendix of 40 CFR Part 60 Appendix A. *

The calculator below runs that combination for your own numbers, three runs to a mean.

What the Operator Has to Supply

Testing obligations reach the facility itself, not just the crew. The owner or operator must provide ports adequate for the test methods, a safe platform, safe access to it, and utilities for the equipment [2]. The stack or duct also has to be free of cyclonic flow during testing, as demonstrated by the applicable test methods [2].

The rule sets the test deadlines, and allows force majeure extensions and rescheduled dates by notice or mutual agreement [2]. Absent force majeure, tests come within 60 days of reaching maximum production rate, by 180 days after startup, and as the part or Administrator requires [2]. Unless another subpart says otherwise, the operator gives at least 30 days prior notice so the Administrator can observe [2].

A Stack Test Result Describes Normal Operation Only

The rule that requires the test also narrows what the test is allowed to see. The agency sets the operating conditions for the test, anchored to how the unit performs representatively [2]. Unless the applicable standard says otherwise, startup, shutdown, and malfunction periods do not count as representative conditions for a performance test [2]. The test sees normal operation only.

That exclusion is deliberate and reasonable, since a compliance test should not judge a unit on a bad morning. It also means the measurement and the annual inventory are answering different questions.

The Same Boundary Applies to Emission Factors

Published factors inherit a similar boundary, because AP-42 factor tests are usually conducted at normal operating conditions. EPA writes that source-specific tests or continuous monitors determine a source’s actual contribution better than emission factors can [9]. Even then, the results apply only to the conditions existing at the time of the testing or monitoring [9]. What the factors themselves assume is walked in AP-42 Emission Factors: Gas Engines, Turbines, Boilers, Flares.

Two EPA documents written for different purposes, the test rule and the factor compilation, draw a similar boundary. Neither method claims to represent abnormal operation, and neither is wrong to decline.

Reconciliation Turns an Outlet Result Into Inventory Evidence

A stack test and an inventory can disagree for structural reasons, even when neither of them is faulty. Unless the applicable subpart says otherwise, the test consists of three separate runs [2]. The inventory is meant to cover the site’s sources across a full year, including intermittent and abnormal events. Three runs cannot cover a year.

OGMP 2.0, the Oil and Gas Methane Partnership, makes an analogous reconciliation the core of Level 5: site-level measurement against source-level estimates [10]. OGMP 2.0 is a measurement-based methane reporting framework for the oil and gas industry [11]. OGMP 2.0 Level 5 reconciles site-level measurements statistically with source-level estimates and reports the uncertainty; findings may extend to similar facilities [10].

A Stack Test Is the Level 5 Problem in Miniature

The gap between a measured number and a built-up estimate is the same gap at both scales. At one outlet, the reconciliation is tractable, because the measured stream and the estimated stream describe the same pipe. Across a whole site, the measurement can capture sources the estimate missed, and the attribution is no longer obvious.

Closing that gap needs an estimate that can represent conditions no test observed. A mechanistic model simulates the equipment and the process rather than multiplying an activity by a constant [12].

The Mechanistic Air Emissions Simulator is a physics-based simulator of that kind, with its methodology developed at CSU-METEC [13]. It generates an expected emission range rather than a single value, so a short field measurement can be checked against it [14]. TetraSoft’s founders built MAES as research scientists at CSU-METEC, and the platform offers commercial access to that published methodology [13].

For the satellite view of the same reconciliation problem, see Why Satellites Show About 2x More Methane Than Inventories. For a single equipment class where one component dominates a modeled total, see Glycol Pumps: 90% of Modeled Emissions at Six TEG Dehydrators.

Frequently Asked Questions

How many runs does a stack test require?

Under federal Part 60, which a delegated state or local agency may administer, a performance test is three separate runs unless the subpart says otherwise [2]. Compliance is determined from the arithmetic mean of the three runs [2].

A sample may be accidentally lost, or a run discontinued by forced shutdown, sample-train failure, extreme weather, or other circumstances beyond the operator’s control [2]. The Administrator may then approve using the mean of the other two runs [2]. Each run lasts for the time and under the conditions the applicable standard specifies [2].

Can an operator use a method other than the reference method?

Yes, if the Administrator approves; 40 CFR 60.8(b) lists five routes, from minor method changes to a waiver of the test [2]. The Administrator may specify or approve a reference method with minor changes in methodology [2]. Approving an equivalent method, or an alternative method adequate for indicating compliance, is also allowed [2].

The Administrator may also approve shorter sampling times and smaller sample volumes when process variables require it [2]. The Administrator may also waive the performance test if the operator demonstrates compliance by other means to the Administrator’s satisfaction [2].

What happens if a scheduled stack test has to be moved?

Rescheduling runs on notice, and the rule gives two ways to do it [2]. The operator can provide at least 7 days notice of the new date [2]. Alternatively, the operator can arrange a rescheduled date with the agency by mutual agreement [2]. That shorter window contrasts with the 30 days required ahead of the original date.

What happens if a test has to be delayed by an event outside the operator’s control?

The rule provides a force majeure route: the operator gives written notice, and the Administrator alone decides whether to extend [2]. The operator notifies the Administrator in writing as soon as practicable, and before the test deadline unless a force majeure event delays the notice [2]. The notice describes the event, the rationale for the delay, the measures taken to minimize it, and a proposed test date [2]. Until an extension is approved, the facility remains strictly subject to the requirements of the part [2].

Which assets must reach OGMP 2.0 Level 5?

OGMP 2.0 does not require assets without material emissions to reach Level 5 reporting [15]. Material assets are those accounting for 95 per cent of an operator’s total emissions [15]. The Gold Standard, the status the framework works toward, requires all material assets at Level 4 and 5, on set timelines, with an implementation plan [11]. The Gold Standard timeline is three years for operated assets and five for non-operated assets [11].

Interested in building a Measurement-Informed Inventory for your operations? Contact us to learn about our MAES-based estimation services.

References

  1. EPA, Clean Air Act National Stack Testing Guidance, April 27, 2009. United States Environmental Protection Agency. https://www.epa.gov/sites/default/files/2013-09/documents/stacktesting_1.pdf
  2. 40 CFR 60.8, Performance tests, current as of July 29, 2026. Electronic Code of Federal Regulations, Title 40, Part 60, Subpart A. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/subpart-A/section-60.8
  3. 40 CFR Part 60 Appendix A-1, Test Methods 1 through 2F, current as of July 29, 2026. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/appendix-Appendix%20A-1%20to%20Part%2060
  4. 40 CFR Part 60 Appendix A-2, Test Methods 2G through 3C, current as of July 29, 2026. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/appendix-Appendix%20A-2%20to%20Part%2060
  5. 40 CFR Part 60 Appendix A-3, Test Methods 4 through 5I, current as of July 29, 2026. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/appendix-Appendix%20A-3%20to%20Part%2060
  6. 40 CFR Part 60 Appendix A-4, Test Methods 6 through 10B, current as of July 29, 2026. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/appendix-Appendix%20A-4%20to%20Part%2060
  7. 40 CFR Part 60 Appendix A-7, Test Methods 19 through 25E, current as of July 29, 2026. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/appendix-Appendix%20A-7%20to%20Part%2060
  8. 40 CFR Part 60 Appendix A-6, Test Methods 16 through 18, current as of August 26, 2026. Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/appendix-Appendix%20A-6%20to%20Part%2060
  9. EPA AP-42, Introduction to AP-42 Volume I, Fifth Edition, January 1995. United States Environmental Protection Agency, Compilation of Air Pollutant Emission Factors. https://www.epa.gov/sites/default/files/2020-09/documents/c00s00.pdf
  10. UNEP OGMP 2.0 Module 2, The OGMP 2.0 reporting levels, August 2025. https://www.ogmpartnership.org/sites/default/files/resources/2025-08/OGMP%20module%20-%202.pdf
  11. UNEP OGMP 2.0 Module 1, Introduction to OGMP 2.0 reporting and mitigation framework, August 2025. https://www.ogmpartnership.org/sites/default/files/resources/2025-08/OGMP%20module%20-%201.pdf
  12. Mollel et al., ACS ES&T Air 2025, 2, 723-735, DOI 10.1021/acsestair.4c00168. Using Prototypical Oil and Gas Sites to Model Methane Emissions in Colorado’s Denver-Julesburg Basin Using a Mechanistic Emission Estimation Tool. https://doi.org/10.1021/acsestair.4c00168
  13. TetraSoft, MAES Platform. https://tetrasoftco.com/maes-platform/maes-landing.html
  14. CSU METEC, Measurement Informed Inventory, accessed November 4, 2026. Colorado State University, Methane Emissions Technology Evaluation Center. https://metec.colostate.edu/measurement-informed-inventory/
  15. UNEP OGMP 2.0, OGMP 2.0 Level 5 Assessment, March 2026. https://www.ogmpartnership.org/sites/default/files/resources/2026-03/OGMP%202.0%20Level%205%20Assessment.pdf

This post is for informational purposes only and does not constitute legal or compliance advice. Consult qualified legal counsel or a compliance professional for guidance specific to your operations and jurisdiction.

Interactive tool

Stack test emission rate calculator

A test report states a concentration; a permit limit is usually a mass rate. Enter the three run concentrations and the dry standard flow, and read each run's rate and the mean the rule compares against the limit.

Three-run arithmetic mean

3.64 lb/hr

Arithmetic mean of three runs, the figure 60.8 compares against the limit.

RunConcentrationMass rate (lb/hr)
Run 1 42 ppmvd 3.61 lb/hr
Run 2 45 ppmvd 3.87 lb/hr
Run 3 40 ppmvd 3.44 lb/hr

lb/hr = ppmvd × 10−6 × MW × dscfm × 60 ÷ 385.3, the molar volume at the 40 CFR 60.2 standard conditions (20°C, 101.3 kPa). Three runs and the arithmetic mean per 40 CFR 60.8. kg/hr and g/s convert at 0.45359237 kg per lb, exact by definition. No diluent correction is applied. A starting point, not a test report. Nothing leaves your browser.