Overview - ISO 6615:1993 (Conradson method)
ISO 6615:1993 specifies the Conradson carbon residue method for determining carbonaceous residue left after evaporation and pyrolysis of petroleum products. The method is a destructive-distillation, laboratory test that quantifies residue as a mass percentage of the original test portion and is applicable over the range 0.01 % (m/m) to 30.0 % (m/m). It is used to indicate relative coke-forming tendency and to rank fuels and oils by deposit propensity.
Key topics and technical requirements
- Principle: A weighed test portion is heated in a crucible under controlled, severe heating; vapours are burned off and the remaining carbonaceous residue (coke) is weighed and reported as a percentage of the original mass.
- Apparatus: Includes a glazed porcelain or silica crucible, Skidmore iron crucible, sheet‑iron hood, insulator (ceramic block or metal box), wire support, and a Meker-type burner (or equivalent).
- Test procedure:
- Test portion is weighed and heated under the sheet‑iron hood; vapour pre‑ignition and burning periods are controlled and timed.
- Total heating period is controlled (specified tolerance), and the residue is cooled in a desiccator before final weighing.
- Calculation: residue % = (mass of residue / mass of test portion) × 100.
- Result expression and precision:
- Results reported as “Carbon residue - Conradson”.
- Express results to 0.01 % (m/m) for values up to 9.99 % and to 0.1 % for 10.0–30.0 %.
- The standard includes precision data and repeatability/reproducibility guidance.
- Interferences and notes:
- Ash‑forming additives (ISO 6245) and alkyl nitrates can raise Conradson carbon residue values and may not correlate with deposit propensity.
- Annex A provides a normative procedure for preparing a 10 % (V/V) distillation residue prior to Conradson testing (when applicable).
- Safety: The standard warns that hazardous materials and operations are involved and that users must follow appropriate safety and regulatory practices.
Practical applications and users
ISO 6615 is commonly used by:
- Petroleum and fuel testing laboratories for quality control and specification testing.
- Refineries and fuel producers to evaluate heavy residues, coker feedstocks and residual fuel oils.
- Lubricant analysts for comparing base oil coke tendency (finished lubricants with ash‑forming additives require caution).
- Research and process engineers assessing deposit formation propensity in combustion systems.
Keywords: ISO 6615, Conradson carbon residue, carbon residue test, petroleum products testing, fuel deposit propensity.
Related standards
- ISO 10370 (micro carbon residue method) - similar applications for many products.
- ISO 4262 (Ramsbottom method) - different numerical results; no universal correlation with Conradson results.
- ISO 3405 (distillation), ISO 3170 / ISO 3171 (sampling), ISO 1772 (crucibles), ISO 6245 (ash in lubricants).
This concise guide helps labs and engineers understand the purpose, setup, limitations and typical uses of ISO 6615:1993 for carbon residue determination.
