A continuous emissions monitor is only trusted as long as someone periodically proves it is still telling the truth, and the test that provides that proof is the RATA. The name stands for relative accuracy test audit, and it is the quality-assurance procedure that compares a CEMS against an independent reference measurement. This guide explains what a RATA does, the pass and fail criteria and how often it must be run, and how a monitoring platform helps by tracking monitor data availability and scheduling the QA tests that keep a certified emissions monitor in compliance.
Relative Accuracy Test Audit (RATA) in one line: A relative accuracy test audit, or RATA, is a quality-assurance test that validates a continuous emissions monitoring system by comparing its readings to a certified reference method measuring the same stack gas at the same time. Relative accuracy is expressed as the difference between the CEMS and the reference method relative to the reference value, and the monitor passes if that figure stays within a specified limit, commonly on the order of twenty percent of the reference or an equivalent absolute criterion. RATAs are run on a defined schedule, often annually, to keep the CEMS certified for regulatory use.
A CEMS produces a continuous stream of emissions readings, but the instrument can drift, foul, or lose calibration over time, so its output cannot be taken on faith indefinitely. A RATA answers the question of whether the monitor is still accurate by running a certified reference method alongside it: a testing team measures the stack gas independently using an approved manual or instrumental reference method, and each reference run is paired with the CEMS reading over the same interval so the two can be compared directly.
The comparison is expressed as relative accuracy, which relates the difference between the CEMS and the reference method to the magnitude of the reference measurement, rather than as a raw absolute error. This framing matters because a fixed absolute error means something very different at a high concentration than at a low one. The test typically consists of multiple paired runs, and the results are combined statistically, accounting for both the average difference between monitor and reference and the scatter among the runs, to yield a single relative-accuracy figure and an assessment of any consistent bias.
The reference method is the arbiter in this comparison because it is a recognized, validated way of measuring the pollutant that does not depend on the CEMS being correct. If the CEMS and the reference method agree closely enough, the monitor is confirmed accurate; if they diverge beyond the allowed limit, the monitor has failed the audit and cannot be relied on for compliance reporting until it is corrected and retested.
A RATA passes when the calculated relative accuracy falls within the criterion set by the applicable quality-assurance requirements, which for many monitors is on the order of twenty percent of the reference method value, with alternative absolute criteria available where the reference values are very low and a percentage limit would be unreasonably tight. The exact criterion depends on the pollutant and the governing procedure, but the principle is constant: the monitor must agree with the reference closely enough that its continuous data can be trusted for compliance.
The quality-assurance framework also examines bias, meaning a consistent tendency of the monitor to read high or low relative to the reference. A monitor can be precise yet biased, and where a significant bias is found, a bias adjustment factor may be applied to the monitor's data so that reported values are corrected for the known offset. This keeps the reported emissions conservative and accurate even when the raw instrument has a systematic tilt.
RATAs are conducted on a defined schedule rather than once. A common cadence is annual, though the required frequency can tighten or relax depending on how the monitor has performed on prior audits, with strong performers sometimes eligible for reduced frequency and marginal ones facing more frequent testing. Missing a scheduled RATA, or continuing to report data from a monitor that has failed one, is itself a compliance problem, which is why keeping track of when each monitor is due is as important as the test result.
Between RATAs, a certified monitor still has to meet ongoing requirements, and the one operators most often stumble on is data availability: the monitor must actually be producing valid data for a sufficient fraction of the time, and extended outages can jeopardize compliance even when the instrument is accurate when it runs. A cloud SCADA platform such as Merobix, receiving the monitor's output continuously, can track uptime and flag gaps as they accumulate, so an operator sees a developing availability shortfall rather than discovering it in a quarterly summary.
Scheduling is the other place a platform earns its keep. Because RATAs and the routine daily and quarterly calibration checks all fall due on their own cadences, and because the frequency can change based on past performance, keeping the calendar straight across a fleet of monitors is genuinely hard to do by memory. A system that records each monitor's last audit and its required interval can surface which monitors are approaching their due dates, turning a set of scattered deadlines into a managed schedule.
Tying the QA schedule to the same platform that holds the emissions data also keeps the record coherent. When the RATA date, the availability history, and the reported emissions all live together, an operator responding to a regulator's question can show not only what the monitor read but that it was properly and timely audited and that its data availability met the requirement. For a company running certified monitors at multiple sites, that unified view is what makes it practical to keep every monitor continuously in good standing rather than firefighting one lapse at a time.
A RATA compares the CEMS readings against a certified reference method measuring the same stack gas at the same time. The reference method is a recognized, validated way of measuring the pollutant that does not depend on the CEMS being correct, so it acts as the arbiter. Multiple paired runs are combined statistically to produce a single relative-accuracy figure and an assessment of any bias.
RATAs are run on a defined schedule, commonly annually, though the required frequency can change based on how the monitor performed on prior audits. Strong performers may be eligible for reduced frequency, while marginal ones may face more frequent testing. Missing a scheduled RATA or continuing to report data from a monitor that failed one is itself a compliance problem.
If the CEMS and reference method diverge beyond the allowed relative-accuracy limit, the monitor has failed the audit and cannot be relied on for compliance reporting until it is corrected and retested. Where a consistent bias is found but the monitor otherwise passes, a bias adjustment factor may be applied so that reported values are corrected for the known offset.
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