Limiting oxygen concentrations of gases
Bibliographic record
Abstract
This article discusses technical and practical aspects of the limiting oxygen concentration (LOC) including a detailed appraisal of possible sources of measurement error. LOC values obtained in the “classical” 5‐cm diameter, vertical tube apparatus have been described as nonconservative owing to flame quenching. However, much of the difference between classical LOC values and those obtained in closed vessels using ASTM E2079 is due to the change in the definition of “ignition.” Formerly defined as propagation of a visible flame through a vertical tube at least 1.5 m tall, which in a closed tube would typically generate an overpressure in excess of 100%, ignition is now defined as an overpressure that exceeds 7%. The small pressure rise criterion tends to underestimate the LOC in smaller test vessels, especially for tests made at elevated pressures and temperatures. More use should be made of ASTM E2079's flexibility regarding the use of larger pressure rise criteria. Having lost sight of the flame, it is important not to lose sight of the objective, namely to select test criteria that correctly predict the flammable limits pertaining to large volumes of gas mixture. Additional “reference quality” LOC data are needed both to improve the definition of “ignition” with respect to test vessel volume and to investigate the accuracy of LOC estimation techniques. Any large‐scale reference quality test program should investigate the effect of low levels of forced turbulence on measured LOC values. The NFPA “Explosion Protection Systems” Technical Committee recently supported the development of two Tentative Interim Amendments (TIAs) to NFPA 69 “Standard on Explosion Prevention Systems.” The first TIA resolves issues with the table of LOC values for gases, which had been modified and over‐corrected in the 2008–2014 editions. The second TIA introduces a simple method for estimating the LOCs of single gases and gas mixtures based on the equation LOC = LFL × S, where LFL is the lower flammable limit and S the molar stoichiometric ratio of oxygen to fuel. An Appendix describes the two TIA recommendations while a second Appendix describes how the estimating methods have been incorporated into ASTM's “CHETAH” Program. © 2016 American Institute of Chemical Engineers Process Saf Prog 35: 107–114, 2016
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".