Relative Accuracy Test Audits (RATA) Explained

A continuous emissions monitoring system can generate data every minute, but data volume does not establish data quality. Relative Accuracy Test Audits (RATA) provide the independent performance check that compares a facility’s CEMS results with results obtained through approved reference test methods. For regulated sources, that comparison is central to demonstrating that reported emissions data are representative, traceable, and defensible.

RATAs are commonly required under regulatory programs that rely on CEMS data, including U.S. EPA Part 75 applications and certain permit, provincial, or facility-specific monitoring requirements. The exact audit frequency, acceptance criteria, and calculation procedures depend on the applicable regulation and monitoring plan. The underlying principle remains consistent: a CEMS must be evaluated against a higher-order reference measurement under representative operating conditions.

What a Relative Accuracy Test Audit Measures

A RATA assesses agreement between a CEMS and a reference method test. During the audit, trained field personnel collect a series of reference method samples while the CEMS measures the same source gas stream over corresponding periods. The paired results are then used to calculate relative accuracy.

The audit is not simply a calibration check. A daily calibration verifies that an analyzer responds appropriately to known calibration gases. A RATA evaluates the complete monitoring system in its installed operating environment, including the sample interface, probe, conditioning equipment, analyzer response, data acquisition system, and the effect of actual stack gas conditions.

For sources monitoring pollutants such as nitrogen oxides, sulfur dioxide, carbon monoxide, carbon dioxide, oxygen, or flow, the applicable reference methods and calculations vary. EPA Methods 3A, 6C, 7E, 10, and 19 are frequently relevant to gaseous emissions monitoring programs, although the correct method must always be selected based on the pollutant, source type, permit conditions, and governing rule.

Why RATA Results Matter for Compliance

CEMS data may support emissions reporting, allowance programs, operating limits, greenhouse gas inventories, compliance certifications, and internal process decisions. If the monitoring system has not demonstrated acceptable relative accuracy, a large body of reported data can become difficult to defend.

A failed RATA does not necessarily mean a facility is out of compliance with an emissions limit. It does, however, signal that the CEMS may not be measuring emissions within the required performance range. Depending on the governing program, the facility may need to investigate the cause, apply prescribed substitute-data procedures, complete corrective action, and conduct a successful retest before returning the monitor to full compliance status.

The operational consequences can be significant. Invalid data periods may increase administrative workload, complicate quarterly or annual reporting, and create uncertainty during regulatory review. Planning the audit carefully is generally less costly than managing an avoidable data-quality issue after the fact.

Preparing for a RATA Without Compromising the Test

A successful audit begins before the test crew arrives. The facility should confirm that the CEMS is operating normally, calibration records are current, maintenance activities are documented, and the data acquisition system is recording valid data. Known analyzer drift, plugged sample lines, moisture carryover, unstable dilution ratios, or failed components should be addressed before scheduling the audit.

Operating conditions also matter. A RATA should be performed during conditions that are representative of normal source operation and that meet the applicable rule’s requirements. Testing during an unusual low-load period, transient startup, shutdown, or atypical fuel condition may produce results that do not satisfy the monitoring plan or provide a meaningful assessment of routine emissions performance.

Site access and safety planning are equally important. Reference testing requires suitable sampling ports, safe platform access, adequate power, clear communication with operations personnel, and attention to weather and work-at-height requirements. For remote industrial facilities across Western Canada, logistics, cold-weather readiness, and equipment redundancy can be decisive factors in completing a test window as scheduled.

Common Causes of Poor Relative Accuracy

Poor RATA performance is often caused by a system issue that has developed gradually rather than a single obvious failure. Sample transport problems can delay or alter the sample reaching the analyzer. Probe fouling, leaks, inadequate heat tracing, condensation, and degraded filters may affect measurement integrity. Analyzer interference, incorrect calibration gas values, data alignment errors, and improper flow or moisture corrections can also create unacceptable bias or variability.

The reference method side of the comparison requires the same level of discipline. Test locations, traverse points, sampling durations, leak checks, analytical procedures, and quality-control records must conform to the applicable method. A defensible audit depends on both measurement systems being executed correctly.

When results are marginal or fail acceptance criteria, the most productive response is a structured root-cause review. Compare individual test runs, examine CEMS calibration and maintenance history, review operating conditions, and verify the calculation inputs. Retesting without resolving the underlying issue can repeat the same failure and extend the period of data uncertainty.

RATA Planning Should Be Part of CEMS Management

A RATA should not be treated as an isolated annual field event. It is one component of a broader quality-assurance program that includes daily calibrations, quarterly audits where required, preventive maintenance, calibration gas management, data review, and timely corrective action.

Facilities that track CEMS performance between required audits are better positioned to identify drift or process-related changes early. That discipline supports more reliable reporting and reduces the risk that a scheduled audit becomes an urgent compliance event. The strongest RATA programs combine qualified reference testing, careful documentation, and practical coordination with plant operations.