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D2685-11 – Standard Test Method Technical Guide
📐 Test Method Overview
This test method, designated ASTM D2685-11 (Reapproved 2019), provides a gas chromatographic procedure for the determination of air and carbon tetrafluoride (CF₄) as impurities in sulfur hexafluoride (SF₆). Air is defined as nitrogen, oxygen, or any mixture thereof, and commercial grade air or nitrogen is used for standardization. The method is essential for ensuring the quality of SF₆, particularly in electrical insulating applications where these contaminants can degrade performance.
The standard is issued under the fixed designation D2685 and includes references to other ASTM standards such as D2472 for SF₆ specifications, and E260 and E355 for gas chromatography practices.
⚙️ Apparatus and Calibration Details
The gas chromatograph must be equipped with a sample inlet system, adsorption column, flow meter, detector, and data handling system. The column material must be compatible with the sample components, and the system must completely separate air, carbon tetrafluoride, and sulfur hexafluoride with baseline return between peaks.
| 🟦 Component | 📏 Requirement |
|---|---|
| Gas Chromatograph Setup | Includes sample inlet, column, flow meter, detector, and data handling |
| Column Material | Must be compatible with sample components (e.g., SF₆, CF₄, air) |
| Separation Quality | Complete separation with baseline return between air, CF₄, and SF₆ peaks |
| Reproducibility | Successive runs of a reference standard agree within 5% on peak area or height |
For calibration, a reference standard mixture with known percentages of air and carbon tetrafluoride in helium or SF₆ is used. The concentration of each component in the reference should be between 50% and 300% of the corresponding component in the unknown sample. Operating conditions should be optimized according to manufacturer instructions and in line with Practices E260 and E355.
💡 Tip: Ensure the reference standard matrix matches the sample matrix (e.g., in helium or SF₆) for accurate comparison.
⚠️ Safety Note: Handle sulfur hexafluoride and all gases in a well-ventilated area. Refer to Material Safety Data Sheets for proper handling procedures.
❓ Frequently Asked Questions
🔍 What is the scope of ASTM D2685?
This test method covers the determination of air (N₂, O₂, or mixtures) and carbon tetrafluoride (CF₄) as impurities in sulfur hexafluoride (SF₆) using gas chromatography.
💡 Why are these impurities significant?
Both air and CF₄ can significantly reduce the insulating properties of SF₆, and their maximum levels are specified in ASTM D2472 for electrical insulation applications.
⚡ What are the calibration requirements?
A reference standard mixture with known concentrations of air and CF₄ is required, with each component’s concentration between 50% and 300% of that in the unknown sample. Chromatograms must be reproducible within 5%.
📌 How is the analysis performed?
The sample is injected into the gas chromatograph, and peak areas or heights for air, CF₄, and SF₆ are compared to those from a reference standard under similar conditions to calculate composition.