Air Research Group Inc. RATA Relative Accuracy Test Audits

A CEMS can operate continuously and still produce data that does not withstand regulatory scrutiny. Air Research Group Inc. - RATA - Relative Accuracy Test Audits address that risk by comparing continuous emissions monitoring system results against carefully executed reference-method testing under representative operating conditions.

For industrial facilities, a Relative Accuracy Test Audit is not simply a field exercise or a reportable event. It is a structured quality assessment of whether CEMS data can support compliance decisions, emissions reporting, permit obligations, and internal performance management. The audit must be planned around the applicable regulatory framework, source conditions, process operation, safety requirements, and the monitor configuration at the site.

What a RATA Measures

A RATA evaluates the agreement between a CEMS and an approved reference method. The CEMS provides continuous measurements, while the reference method uses independent sampling and analysis procedures performed at the same source during the audit. The resulting comparison establishes relative accuracy, typically expressed as a percentage of the reference-method value or applicable emissions standard.

Depending on the unit, regulated parameters may include nitrogen oxides, sulfur dioxide, carbon monoxide, carbon dioxide, oxygen, moisture, flow, or other permit-specific pollutants. The applicable acceptance criteria, test frequency, calculation approach, and reporting requirements depend on the governing program. Requirements may arise from a facility approval, federal standards, provincial requirements, MSAPR obligations, or a site-specific compliance plan.

That distinction matters. A technically sound test program must be built to the rule that applies to the source, rather than relying on a generic testing scope.

RATA Relative Accuracy Test Audits Require Representative Data

Relative accuracy depends on more than collecting paired data points. The test must capture operating conditions that are representative of normal or required compliance operation. If a combustion source is operating far below its usual load, experiencing unstable fuel quality, or transitioning through a process upset, the resulting data may not provide a meaningful assessment of monitor performance.

Before mobilization, the testing team should review the source process, emissions control equipment, CEMS data availability, recent calibration records, analyzer maintenance history, sampling system condition, and expected unit load. Coordination with operations is essential because a successful audit requires stable conditions while preserving safe plant operation.

A pre-audit review also identifies practical constraints, including access limitations, safe work procedures, sampling port condition, electrical requirements, turnaround schedules, and whether the CEMS has been taken out of service or adjusted recently. Finding those issues before the test window reduces the chance of invalid runs, lost production time, or delayed compliance reporting.

Field Execution: Reference Methods and CEMS Coordination

During the audit, certified personnel perform the selected reference methods while facility staff maintain the agreed operating conditions and provide CEMS data for the corresponding test periods. Sampling locations, traverse requirements, calibration procedures, leak checks, analyzer performance checks, and chain-of-custody controls must follow the applicable method and project quality plan.

For many sources, reference testing may involve EPA methods for gaseous pollutants, oxygen or carbon dioxide, moisture, velocity, volumetric flow, and other parameters needed to calculate emissions correctly. The correct method suite depends on the measurement being audited and the source configuration. A flow monitor, for example, may require a different testing and calculation approach than a gas concentration analyzer.

The audit team must also synchronize timing. Reference-method data and CEMS values need to correspond to the same operating periods. Small errors in time alignment, data averaging, unit conversion, dry-to-wet basis adjustments, or oxygen correction can materially affect the final relative accuracy calculation.

How Facilities Should Prepare for a RATA

The most useful preparation is deliberate and documented. Plant teams should confirm that the monitor is operating normally, routine maintenance is complete, and data acquisition systems are retaining the required records. They should also ensure that the audit is not scheduled during foreseeable abnormal conditions unless the governing requirement specifically calls for that operating scenario.

Before the test date, confirm these items:

• Current CEMS calibration, drift, maintenance, and quality-control records are available.

• Source operating ranges, fuel information, control device status, and planned process changes are communicated to the testing team.

• Sampling ports, platforms, utilities, access routes, and fall-protection provisions are safe and ready for use.

• Applicable permits, prior RATA reports, regulatory correspondence, and reporting templates have been reviewed.

Preparation does not guarantee a passing result, but it ensures that the audit produces credible information. That information is valuable even when corrective action is needed.

When Relative Accuracy Does Not Meet Requirements

A failed or marginal RATA should be treated as a technical investigation, not a paperwork problem. The underlying cause may involve analyzer calibration bias, probe contamination, sample conditioning issues, moisture measurement error, flow-monitor configuration, data-handling logic, reference-method variability, or an operating condition that was not representative.

Corrective action should begin with a review of the paired data, calibration documentation, field notes, process records, and CEMS maintenance history. In some cases, the appropriate response is a monitor adjustment and retest. In others, the issue may require sampling system repair, analyzer service, reconfiguration of calculation parameters, or a broader evaluation of the monitor's quality assurance program.

Air Research Group Inc. supports facilities with RATA planning, reference-method execution, emissions data review, and technically defensible reporting. A well-managed audit gives plant and environmental teams more than a pass-or-fail result: it provides evidence that the emissions data used for compliance decisions can be trusted.