Calibration Gas Requirements for CEMS Users
The concept of calibrating analyzers and other equipment seems simple enough. However, when we start talking about calibrating a CEMS, enough acronyms and percentages are thrown around to make things complex. It is important that your calibration gas bottles are properly maintained and monitored to avoid system issues. This article will discuss and clarify the calibration requirements for a CEMS user.
System Calibration
When discussing CEMS, the first priority is understanding what is being calibrated. While users know that adjustments are made to the gas analyzers (measuring NOx, oxygen, CO, etc.), EPA regulations require calibration of the entire CEMS. This means calibration gas must be introduced as close as possible to the sample gas entry point, allowing it to follow the same path through the system.
For a CEMS this means that the cal gas is sent up the umbilical and introduced at the front of the probe, where it then travels the same entire pathway as the stack gas, down to the analyzers. There are two different regs that apply to the majority of the CEMS in the US – Part 60 and Part 75. Although there are other specific state and local regs, this article will concentrate on these main two.
A. Daily Calibration Checks
EPA regulations require that CEMS be “checked” daily. This automated process uses a controller (PLC or datalogger) to open and close solenoids, allowing calibration gas to enter the system. The Data Acquisition System (DAS) records the readings and compares them to the known calibration gas values to determine pass or fail.
Each day, a zero and span check is performed. The zero gas must be between 0–20% of the analyzer’s range, while the span gas must be:
50–100% of range for Part 60 systems
80–100% of range for Part 75 systems.
For example, a NOx analyzer with a 0–500 ppm range would typically use a span gas concentration between 250 ppm and 500 ppm.
For Part 60 CEMS, daily calibration must be within 10% of the calibration gas value. An error over 10% is a failed calibration. Errors between 5–10% are considered “bad” calibrations; four consecutive bad calibrations are treated as a failure.
For Part 75 CEMS, the daily calibration must be within 5% of the calibration gas value.
In both systems, if dual-range analyzers are used, both ranges must be checked. A failed calibration typically requires operator intervention to adjust the analyzer.
B. Quarterly Calibration Audits
EPA regulations require CEMS users to perform quarterly calibration audits to verify system performance. These audits can be fully automated with the push of a button, using a controller and solenoids to introduce calibration gases.
Part 60 Systems:
A Cylinder Gas Audit (CGA) is performed, challenging the CEMS 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 Systems:
A Linearity Test is required, challenging the CEMS with three gas levels: low, mid, and high.Low gas: 20–30% of range
Mid gas: 50–60% of range
High gas: 80–100% of range
Passing error limit: 5%
To learn more about performing linearity checks, read our blog post.
Calibration Gas Bottles
Calibration gas is available in various configurations, blends, and concentrations based on CEMS requirements. To save money and streamline calibration, CEMS typically use blended gas cylinders. Many gases can be combined in a single cylinder and remain stable.
A. Blends
A common blend example would be a NOX/CO/O2 CEMS where only two bottles would be needed for the daily zero and span of all three analyzers. The NOX and CO can be blended in a single bottle with nitrogen which allows for checking the span of the NOX and CO analyzer, and also checking the zero of the oxygen analyzer. A bottle with oxygen can check both the oxygen analyzer span and also act as a zero bottle for the NOX and CO analyzers.
B. Selecting a Grade
Cal gas manufacturers are under strict EPA control for the manufacturing of EPA Protocol bottles. To make EPA Protocol bottles a facility must be audited and have a registration number. There are NIST standards that must be adhered to and traceability protocols that must be followed. In an effort to differentiate themselves, cal gas manufacturers have introduced some confusion in the industry with regard to the different grades of gases by creating their own tradenames.
Part 60 and Part 75 regulations both require the use of Protocol gas. The EPA regulationss state:
EPA Protocol gas concentrations must be certified by an EPA Protocol gas production site to have an analytical uncertainty (95-percent confidence interval) to be not more than plus or minus 2.0 percent (inclusive) of the certified concentration (tag value) of the gas mixture.
Manufacturers have created names like:
- EPA Protocol Gas
- EPA Certified Gas
- RATA Class Protocol Gas
- Compliance Class Protocol Gas
- Emission Credit Gas
- And more…
The bottom line is that CGA’s and Linearities for a CEMS must be performed with an EPA Protocol gas that is manufactured within ±2%. Many companies make 1% and 2 % Protocol Gases while some just sell the 1%. Either are acceptable.
In many faciltiies a NIST traceable Certified gas (1% or 2%) can be used for daily cals. Sometimes the price difference warrants the use of different grades – especially for the daily bottles which are used in greater quantities.
C. Bottle Life
Cal Gas bottles come with expiration dates on their certification certificates. Depending on the type of gas, these dates ranges from 1 to 8 years. Quarterly bottles tend to last for many years as they are only used 4 times a year. Typically they are certified for 2 years. The regs have been modified over the years and now allow for recertification of bottles to extend the usability range. It is often less expensive to send a bottle back to the manufacturer for recertification than purchasing a new bottle.
D. Bottle Placement
Cal gas bottles can be placed indoors or outdoors. Placing them close to the CEMS cabinet or shelter is best and placing on a ground floor is always better because of the routine replacement required.
When placed indoors, personnel protection must be taken into account in the event of a leak. Ambient gas monitors are most commonly used when bottles are placed inside a shelter. Outdoor placement requires the use of some type of hood or shelter to protect the regulators from freezing in the event of ice or snow buildup. The bottles themselves don’t need protection.
The cal gas bottles should be plumbed to the CEMS fittings via Teflon or stainless steel lines. Regulators are chosen based on the type of gas typically – corrosive or non-corrosive – and are made of brass or stainless.
Common Issues with Calibration Gas Cylinders
The following are some useful questions to run through when assessing issues with your calibration gas cylinders:
Do you have the correct cylinders?
- Concentrations must be within the specified ranges
- Do you need additional concentrations for testing/calibrating?
Is there enough gas available?
- Enough to troubleshoot, calibrate and run CGAs/linearities
- Spare(s) in the event of a leak or bad bottle
What level is your cylinder pressure at?
You should regularly monitor the pressure of your cylinders to prevent leaks.
Are your cylinders at risk of contamination?
It is important to not run gas cylinders under 150 psi to prevent contamination. The proper level to exchange them at is 200 psi.
How ESC Spectrum Can Help
Interested in learning a more comprehensive overview of how CEM Systems work? Read Understanding Continuous Emissions Monitoring Systems (CEMS): A Comprehensive Guide. This guide will give you a complete understanding of all the components in the flow of a CEMS.
