What to Expect During Your CEMS RATA

What is a RATA?

RATA is short for Relative Accuracy Test Audit. It is a test performed regularly (usually annually but may also be semi-annual) to ensure that the Continuous Emissions Monitoring System (CEMS) operating at your facility is providing accurate data. The test consists of a minimum of 9 test runs during which the output of the CEMS will be compared to the results of testing performed by Air Measurement.

Pretest Protocol

A Pretest Protocol is submitted to your state regulatory agency at least 30 days prior to performance of the test program. This Protocol defines the test methodologies to be utilized as well as process operations expected during the test program. If a protocol must be submitted to US EPA it is required at least 60 days prior to performance of the test program.

Test Crew Arrival

The test crew will usually arrive on-site the afternoon prior to the first day that testing is scheduled to set up the test equipment. Most analyzers perform much more consistently if allowed to warm up and stabilize overnight; so hooking up power to the Environmental Mobile Laboratory (EML) the day preceding testing is critical. Please plan to have an electrician standing by and ready to connect power to the EML as soon as the test crew arrives onsite. Delays here can incur additional costs.

Test Conditions

In order for a RATA test to be considered valid, process load must be documented to be at least 50% of maximum rated (permitted) load. It is essential to arrange process conditions to meet this requirement, as failure to do so may result in the test results being rejected by the reviewing agency.

Test Length

A normal RATA test consists of between 9 and 12 tests of typically 21-minutes each. In certain rare circumstances, longer tests may be required. With time for calibrations between tests, this means that a minimum of 4.5 hours of continuous runtime at greater than 50% load is required to perform the RATA. If this is not possible alternative arrangements should be made in advance and defined in the Pretest Protocol.

If it can be verified on-site that the CEMS is in compliance, testing can be halted after 9 tests. In the case that test results are not available on-site (i.e. laboratory testing must be performed) a full suite of 12 tests should be performed.

Process Data Recording

It is important for CEMS average data to be provided to the test crew at the conclusion of each test run, for comparison to reference method results. This can help spot compliance problems early in the test when corrective action can be taken, and ensures that the test crew will not leave the site until the compliance status of the CEMS is determined. Please plan to have personnel available to perform this function, as the test crew will be very busy during testing and unable to leave the test site to gather data.

Test Administrators

A representative of your state’s Department of Environmental Protection (or equivalent) will likely elect to observe all or part of the test program. This test administrator may or may not be your regular department contact. While on-site, the administrator will likely be most interested in observing how the testing is conducted. Air Measurement personnel are highly experienced in dealing with test administrators. He/she may also wish to examine the CEMS and/or plant operations. It is incumbent upon your facility to provide the administrator with access to whatever information and records he/she deems necessary. Depending on your plant’s security policy, you may also need to provide the administrator with an escort while he/she is on site.

Test Crew Departure

As soon as all parties involved are assured that the test program was a success, the test crew will begin demobilizing. Please have an electrician available to disconnect power to the EML within one half hour from the conclusion of testing. Be sure to provide copies of all CEMS and process data necessary to the test crew before the crew leaves your facility.

Test Report

Shortly after testing is completed (generally within two weeks) you will receive a draft copy of Air Measurement’s test report to review. Please promptly communicate your feedback on the test report to the Air Measurement engineer responsible for your project, who will take it under advisement before submitting the final test report. Unless you desire otherwise, Air Measurement will submit the test report directly to the appropriate personnel at the state agency. Reports are submitted in PDF format, with printed and bound copies available on request.


If you have any questions concerning your upcoming RATA, don’t hesitate to contact your Air Measurement Project Engineer by phone or e-mail. Air Measurement personnel have years of experience performing similar testing and will work diligently to ensure that you are satisfied with the results of your test program.

Pretest Preparations

In order for your RATA to be completed as quickly and efficiently as possible, please:

☐ Arrange access to any required site-specific safety training for the test crew. If this can be performed online in advance of test crew arrival, send any required information to your Air Measurement Project Engineer.

☐ Inform plant security personnel of the test crew’s anticipated arrival, if necessary.

☐ Check all sampling ports for blockage and ensure all port covers can be removed by hand. Significant extra charges can be incurred if testing is delayed due to port blockage.

☐ Ensure that safe access to the test site is possible. Remove ice and snow from the work area, if applicable.

☐ If testing requires an aerial lift to access the sampling location, please ensure that it is available the day prior to testing for equipment setup and port inspection.

☐ Ensure that the process to be tested is operating normally and that there is sufficient production scheduled on the test date(s) to ensure adequate load.

☐ Ensure that the CEMS system is operating normally and all daily calibrations have been successfully performed.

☐ Ensure there is a level, secure location at least 10’x25’ within 50’ of the electrical power hookup and within 100’ of the test location for trailer parking.

☐ Have a qualified electrician ready upon test crew arrival to connect electrical power to the EML. Air Measurement’s EMLs require 220-volt, single phase power, on a 100-amp circuit. 208-volt power (2 hot legs from a 3-phase circuit) can suffice as well. The trailer power cord has 4 bare wire leads (0-gauge) that will need to be wired into a junction box.