BAR-97 gas specifications
BAR-97 Emissions Inspection System (BAR-97 EIS) gas benches were originally designed and validated using calibration and audit gases from petrochemically-sourced materials. These materials have a stable carbon-12 (12C) to carbon-13 (13C) ratio.
Some gas suppliers now use carbon from non-petrochemical sources, which may have different 12C to 13C ratios. When calibration or audit gases are produced from these alternative sources, the different ratios may affect the accuracy of the BAR-97 EIS gas bench measurements. In some cases, this may result in equipment audit failures.
To support accurate and consistent BAR-97 measurements, BAR recommends that calibration and audit gas blends meet the purity and carbon isotope specifications below.
Note: BAR is currently evaluating updates to the BAR-97 gas blender specifications. The information below is provided to help gas blenders and their customers understand how gas blend source material purity can affect BAR-97 gas bench accuracy.
Specifications
| Component | Minimum purity | Additional requirements |
|---|---|---|
| Propane | 99.5% | O2 < 50 ppm |
| Carbon monoxide (CO) | 99.8% | THC < 50 ppm O2 < 10 ppm Fe (CO)5 + Ni (CO)4 < 4 ppm 13C Content 4,5: δ13C = -38 ‰ ± 20 ‰ |
| Carbon dioxide (CO2) | 99.8% | 13C Content 4,5: δ13C = -38 ‰ ± 10 ‰ |
| Nitric oxide (NO) | 99.0% | No additional requirements |
| Nitrogen (N2) | 99.9997% | O2 < 1 ppm |
| Full mixture | N/A | Total O2 < 1 ppm for all blends containing NO |
What gas blenders need to know
Gas manufacturers and blenders are encouraged to verify that carbon monoxide (CO) and carbon dioxide (CO2) used in BAR-97 calibration and audit gas blends are consistent with the carbon isotope specifications in the table above.
Compliance may be demonstrated by:
Obtaining CO and CO2 from geological petroleum sources that meet the isotope specifications.
Having CO and CO2 source materials tested by qualified laboratory capable of measuring stable carbon isotope ratios.
Gas blends that do not meet these specifications may affect BAR-97 gas bench accuracy and could contribute to inaccurate measurements or equipment audit issues.
Technical notes
Carbon isotope content varies by source
The 13C isotope content of carbonaceous materials such as hydrocarbons, carbon monoxide and carbon dioxide varies widely in nature due to how they are formed. In addition, CO and CO2 materials are commercially available with the 13C isotope intentionally removed.
Gas blends used for BAR-97 calibration and audits should be produced from carbonaceous raw materials that meet the 13C isotope ranges listed in the specification table.
Carbon isotope ratios affect CO and CO2 measurements
BAR-97 gas benches use non-dispersive infrared analyzers to measure CO and CO2. Because high concentrations of CO2 can interfere with the CO measurement, the infrared absorption features used for these measurements are sensitive to both 13CO and 13CO2 enhancement or depletion.
Using CO and CO2 with 13C isotope signatures within the specified ranges ensures that analysis errors due to isotope variation will not exceed ± 0.5% (one-half of the allowed accuracy budget of 1%).
Understanding δ13C values
δ13C is the standard notation for measurement of stable isotope ratios. The δ value, which is usually negative, indicates the relative degree of 13C depletion compared to the international standard for stable carbon isotope analysis.
The primary reference standards used by the Bureau of Automotive Repair to qualify audit gases contain CO and CO2 with 13C isotope ratios noted in the table.