Relative Accuracy Test Audit (RATA): How to Plan, Execute, and Pass
- Jan 2, 2025
- 8 min read
A Relative Accuracy Test Audit (RATA) is a field audit that validates the accuracy of your Continuous Emissions Monitoring System (CEMS) by comparing its data against reference method measurements taken simultaneously by certified stack testers. It answers one fundamental question: is your CEMS producing data that accurately represents actual emissions?
Unlike a cylinder gas audit (CGA), which tests your analyzers with known-concentration gases, a RATA tests the entire CEMS — from probe to data output — against independent measurements at the stack. The results determine whether your system is in control and whether your compliance data can be used.
A failed RATA means your CEMS is out-of-control, your data becomes invalid, and you’re looking at out-of-control periods, substitution data, and significant reporting headaches. A missed RATA — one that wasn’t scheduled, notified, or conducted at the right load level — can be just as costly.
Getting a RATA right is not complicated, but it requires systematic preparation. This guide covers what a RATA is, which regulations govern it, and exactly what to do before, during, and after your test.
When Is a RATA Required? Part 60 vs. Part 75
RATA requirements differ depending on which regulation governs your facility. Many facilities are subject to both, which creates additional complexity. Use the table below to identify what applies to you.
40 CFR Part 75 | 40 CFR Part 60 (Appendix F, Procedure 1) | Dual Compliance Note | |
Applies To | Electric generating units (EGUs) under Acid Rain Program, CSAPR, and related NOx programs | Broader range of stationary sources: industrial boilers, cement kilns, glass furnaces, chemical plants, and others subject to NSPS | Many facilities use the same CEMS to satisfy both. Title V permits often specify that a Part 75-compliant RATA satisfies Part 60 requirements — verify this in your permit. |
Pollutants | SO₂, NOₓ, CO₂, O₂, volumetric flow | Varies by subpart: SO₂, NOₓ, CO₂, O₂, CO, PM, VOCs, HAPs | Confirm pollutant-specific requirements in your applicable subpart and permit. |
RATA Frequency | At least once every four QA operating quarters (each with ≥168 operating hours); annually for flow monitors | At least once every four calendar quarters; successive audits no closer than 2 months apart | QA operating quarters (Part 75) vs. calendar quarters (Part 60) can create different scheduling obligations. |
Load Requirements | Gas RATAs: at the normal or next-normal load level. Flow RATAs: at each of three load levels every 5 years, separated by ≥25% of operating range | At normal operating conditions as defined by the applicable subpart | Load level errors are among the most common RATA mistakes — and are usually discovered after the test. |
Minimum Runs | 9 runs minimum; 10 recommended | Per applicable Performance Specification in Appendix B | More runs give you more data to bring RA down if early results are trending high. |
Advance Notice | At least 21 days to EPA and state agency, unless exempt under §75.61(a) | Per permit or state requirements — varies by jurisdiction | Check your permit and local agency requirements. Missing notification deadlines can invalidate a RATA. |
Key Regulatory References |
|
|
If your facility is subject to both Part 75 and Part 60, review your Title V Operating Permit carefully. It will typically specify whether satisfying Part 75 RATA standards also satisfies Part 60 — but this must be explicitly stated. If it is not, confirm with your state or local regulatory authority before conducting a combined test.
Before the RATA: Pre-Test Planning Checklist
Most RATA problems originate before the test even begins. Poor scheduling, wrong load levels, inadequate notification, or equipment that hasn’t been serviced are all avoidable with a structured pre-RATA checklist.
1. Determine When Your RATA Is Due
Review your facility’s in-house RATA procedures and your Title V permit for applicable frequency requirements
Generate a QA Test Status Report from your DAS to confirm where you stand in your current QA cycle
Cross-check Part 75 QA operating quarters vs. Part 60 calendar quarters if both apply
2. Verify Operating Quarters
Each of the last four QA operating quarters must have at least 168 operating hours for Part 75
Confirm the unit will be operating during the planned test window at appropriate load levels
3. Identify the Correct Load Level
Load level errors are one of the most common — and most costly — RATA mistakes. They are typically discovered after the test has been conducted, resulting in invalid data and the need to retest.
Gas RATAs must be performed at the normal load level (highest percentage of operating hours in the last four quarters), or the next-normal load if the unit cannot operate at normal load for economic reasons
Flow RATAs must be conducted at each of three load ranges every five years, with each range separated by at least 25% of the operating range
Perform a load analysis before scheduling to confirm the correct operating level — do not rely on assumptions from the previous RATA cycle
4. Schedule Testing and Submit Notifications
Schedule your stack testing contractor well in advance — weeks to months, depending on your testing window and contractor availability
Submit written notification to EPA and applicable state agencies at least 21 days before the test date under Part 75, unless your facility qualifies for an exemption under §75.61(a)
Verify state or local notification requirements, which may differ from or exceed federal requirements
Confirm the notification includes the correct unit, stack, pollutant(s), and intended test date
5. Verify Stack Tester Qualifications
This step is frequently overlooked but carries real compliance implications. EPA requires that a “Qualified Individual” be present during RATA testing. For many source categories, this means your stack testing contractor must hold current Air Emissions Testing Body (AETB) certification.
Confirm AETB certification when issuing your RFP and before the contractor arrives on site
Verify that the individual who will be on site holds the certification, not just the firm
Include certification requirements explicitly in your testing contracts
6. Service All CEMS Equipment Before the Test
Complete any scheduled maintenance, calibration, or repairs before the test window — do not leave these for the week of the test
Verify all analyzers are operating within calibration drift limits
Check probe, filter, and umbilical condition; replace or clean as needed
Confirm your DAS is configured correctly and that all CEMS parameters are being logged as expected
Day of the RATA: What to Do During Testing
By the time test day arrives, most of the variables should already be controlled. Day-of execution is about maintaining stable operating conditions, staying engaged with the testing team, and catching problems early enough to act on them.
1. Establish and Maintain Stable Operating Conditions
Operate at the correct load level as determined in your pre-RATA planning — stable and consistent, at 90% or more of maximum load if that is your normal range
Ensure all pollution control equipment is running in its normal operational state
Avoid process changes, cleanings, or adjustments during the test window
2. Confirm Sampling Points Before Testing Begins
A stratification test is required under Part 75 Appendix A §§6.5.5–6.5.6.1 to determine appropriate sampling points
Flow RATAs require traverse points selected per Part 75 Appendix A and Method 1 under Part 60
Confirm sampling locations with the stack testing team before runs begin — disputes about sampling points mid-test create delays and can compromise results
3. Communicate Actively with the Stack Testing Team
Be on site and accessible throughout testing — do not leave the stack testing team to operate without facility personnel present
Review and compare results with the stack testers every 2–3 runs
After 6–7 completed runs, evaluate whether your relative accuracy is trending toward passing or whether additional runs are advisable
Part 75 requires a minimum of 9 runs; running 10 provides additional data that can help bring RA below the 7.5% threshold if early runs are running high
4. Monitor CEMS Performance Throughout
Watch for calibration drift, analyzer alarms, or data gaps in real time during the test
Document any anomalies, equipment events, or operational changes that occur during the test window — your DAS should be capturing all of this automatically, but manual notes provide additional context
If a significant equipment issue arises mid-test, consult with your regulatory contact before deciding whether to continue or reschedule
After the RATA: Post-Test Requirements
The test is not complete when the stack testers leave the site. Post-RATA steps are where documentation, data entry, and DAS updates take place — and where errors can still compromise an otherwise successful test.
1. Review Test Results and Documentation
Review the stack tester’s results and compare them against your CEMS data before the testers leave the site if at all possible
Identify and resolve any discrepancies with the testing contractor while they are still on site
Confirm that all required runs are documented, that sampling points are correct, and that the test was conducted at the appropriate load level
2. Enter RATA Results in Your DAS
Enter all RATA results into your DAS accurately and promptly
Confirm that the DAS is correctly recording the RATA as a QA certification event
Verify that the applicable out-of-control flags are cleared once the RATA passes
3. Submit Updated Monitoring Plan (If Required)
If the RATA requires changes to your monitoring plan — new load level ranges, updated sampling point definitions, or modified calculation methods — update and resubmit the monitoring plan before your next quarterly submission
For Part 75 facilities, monitoring plan changes must be submitted via ECMPS before the quarter in which the change takes effect
4. Include RATA Results in Your Quarterly Submission
RATA results must be included in your QA file for the quarter in which the test was conducted
Do not wait until the submission deadline to verify that your RATA data is correctly captured in your electronic data report (EDR) — generate and review your QA file as soon as results are entered
If the RATA resulted in an out-of-control period, confirm that substitution data is correctly applied for the relevant period in your quarterly submission
5. What to Do If Your RATA Fails
A failed RATA means your CEMS is out-of-control from the time of the most recent prior passing test. Out-of-control data cannot be used for compliance and must be replaced with substitute data per your applicable regulation.
Take corrective action on your CEMS — diagnose the root cause of the accuracy failure before retesting
Following corrective action, a RATA must be used to return the system to in-control status — a CGA or RAA is not sufficient to close an out-of-control period triggered by a failed RATA
Document the out-of-control period, the corrective actions taken, and the results of the retest
Apply substitute data for the full out-of-control period in your quarterly submission per Part 60 or Part 75 substitution methodology
Need Support for Your Next RATA?
Alliance Technical Group's field service and regulatory teams work with facilities across industries to prepare for, conduct, and document RATAs under Part 60, Part 75, and applicable state programs. Our technicians support CEMS-side RATA execution — ensuring your analyzers are operating correctly, calibrations are current, and the system is ready before reference method testing begins. From pre-RATA load analysis and notification support through post-test DAS entry and quarterly submission review, we help facilities navigate the full RATA process without compliance exposure.
Our StackVision DAS and 8864 Data Controller collect, monitor, QA, and report on RATA data in real time — so you have full visibility into results as testing progresses and a clean, audit-ready record when it's done.
There's a better way to manage your RATA.