Is Your CEMS Calibration Gas Up to Code?

The concept of calibrating analyzers seems straightforward. When applied to a full CEMS, however, enough acronyms and percentage thresholds are involved to make things complex fast. This article clarifies the calibration gas requirements for CEMS users under Part 60 and Part 75 — including system calibration, gas grades, bottle management, and common issues.

System Calibration

The first thing to understand is what EPA regulations actually require to be calibrated. While the focus is often on individual gas analyzers (NOx, O2, CO, etc.), EPA requires calibration of the entire CEMS. That means calibration gas must be introduced as close as possible to the sample gas entry point, so it travels the same path through the system as actual stack gas — up the umbilical, in at the probe, and all the way through to the analyzers.

Two regulations cover the majority of CEMS in the U.S.: 40 CFR Part 60 and 40 CFR Part 75. While state and local rules also apply in some cases, this article focuses on these two.

Daily Calibration Checks

EPA regulations require daily calibration checks for CEMS. This is an automated process: a controller (PLC or datalogger) opens and closes solenoids to introduce calibration gas into the system. The DAS records the readings and compares them against the known calibration gas values to determine pass or fail.

Each day, a zero and span check is performed. Zero gas must be within 0–20% of the analyzer's measurement range. Span gas concentration requirements differ by regulation:

• Part 60: 50–100% of range

• Part 75: 80–100% of range

For example, a NOx analyzer with a 0–500 ppm range would typically use a span gas in the 250–500 ppm range.

Part 60 requires daily calibration to land within 10% of the calibration gas value. Errors over 10% are a failed calibration. Errors between 5–10% are considered bad calibrations; four consecutive bad calibrations are treated as a failure.

Part 75 requires daily calibration to be within 5% of the calibration gas value.

For both programs, if dual-range analyzers are in use, both ranges must be checked. A failed calibration typically requires operator intervention to adjust the analyzer.

Quarterly Calibration Audits

In addition to daily checks, EPA requires quarterly calibration audits to verify system performance. These audits can be fully automated using a controller and solenoids to introduce calibration gases.

Part 60 — Cylinder Gas Audit (CGA)

The CEMS is challenged three times each with low- and mid-level gases:

• Low gas: 20–30% of analyzer range

• Mid gas: 50–60% of analyzer range

• Passing error limit: 15%

Part 75 — Linearity Test 

The CEMS is challenged with three concentration levels:

• Low gas: 20–30% of range

• Mid gas: 50–60% of range

• High gas: 80–100% of range

• Passing error limit: 5%

Calibration Gas Cylinders

Blends

Calibration gas is available in various configurations, blends, and concentrations. To reduce cost and simplify logistics, CEMS typically use blended cylinders — many gases can be combined in a single cylinder and remain stable.

A common example: for a NOx/CO/O2 CEMS, only two cylinders are needed for daily zero and span checks across all three analyzers. A NOx/CO blend in nitrogen handles the span for NOx and CO while also serving as the O2 zero. A separate oxygen cylinder handles the O2 span and doubles as the zero for NOx and CO.

Selecting a Gas Grade

Cal gas manufacturers producing EPA Protocol bottles must be audited and registered with EPA. Their products must meet NIST traceability standards. To differentiate their offerings, manufacturers use various tradenames — EPA Protocol Gas, EPA Certified Gas, RATA Class Protocol Gas, Compliance Class Protocol Gas, Emission Credit Gas, and others — which can create confusion.

The bottom line: CGAs and linearity tests under both Part 60 and Part 75 must use EPA Protocol gas certified to an analytical uncertainty of no more than ±2% of the tag value (95% confidence interval). Both 1% and 2% Protocol gases are acceptable.

For daily calibration checks, many facilities use NIST-traceable certified gas at 1% or 2%. The price difference between grades can make this practical, particularly for daily bottles that are consumed more frequently than quarterly audit cylinders.

Bottle Life

Cal gas cylinders come with expiration dates on their certification certificates. Depending on the gas type, certification periods range from 1 to 8 years. Quarterly audit bottles are used infrequently and often last for many years; most are certified for 2 years. Current regulations allow for recertification of bottles to extend usability — in many cases, sending a bottle back to the manufacturer for recertification is less expensive than purchasing a new one.

Bottle Placement

Cal gas bottles can be placed indoors or outdoors. Positioning them close to the CEMS cabinet or shelter is preferred, and ground-floor placement simplifies routine cylinder exchanges.

Indoor placement requires personnel protection measures in case of a leak — ambient gas monitors are standard when bottles are housed inside a shelter. Outdoor placement requires a hood or shelter to protect regulators from freezing in icy or snowy conditions. The cylinders themselves do not require weather protection.

Cal gas bottles should be plumbed to the CEMS using Teflon or stainless steel lines. Regulator selection — brass or stainless — is based on gas type: corrosive or non-corrosive.

Common Issues with Calibration Gas Cylinders

When troubleshooting calibration problems, run through these questions:

Do you have the correct cylinders? Concentrations must fall within the specified ranges for your regulation. Confirm you have all concentrations needed for testing and calibration.

Is there enough gas available? You need enough to troubleshoot, calibrate, and complete CGAs or linearities, plus spare cylinders in case of a leak or failed bottle.

What is the cylinder pressure? Monitor cylinder pressure regularly. Do not run cylinders below 150 psi — this risks contamination. Exchange cylinders at 200 psi.

Are cylinders at risk of contamination? Running a cylinder too low is the most common contamination pathway. Staying above the 200 psi exchange threshold prevents this.

How Alliance Technical Group Can Help

Alliance Technical Group provides comprehensive CEMS support — field maintenance, regulatory and reporting services, training, and more. Whether you need help troubleshooting a calibration issue, setting up a new system, or managing ongoing QA requirements, our team covers both the CEMS side and the data side. There's a Better Way.

Contact us to discuss your CEMS needs or to request a site assessment. →