Oxygen Index (LOI): Measurement Principle, ASTM D2863 and ISO 4589-2 Standards

The Limiting Oxygen Index (LOI) determines the minimum oxygen concentration required to sustain the combustion of a polymeric material. The method is widely used in research, flame-retardant formulation development and quality control.

Automatic EA04 system for Limiting Oxygen Index determination.

What is the Oxygen Index?

The Oxygen Index, referred to as LOI (Limiting Oxygen Index) or OI (Oxygen Index), expresses the minimum volumetric concentration of oxygen in an oxygen/nitrogen mixture capable of supporting the combustion of a small vertical test specimen under specified test conditions.

The result is expressed as a percentage of oxygen. In general, a higher value indicates that the material requires a greater oxygen concentration to continue burning. The value is therefore useful for comparing materials, formulations and the effect of flame-retardant additives.

CORRECT INTERPRETATION OF THE RESULT

LOI is a comparative index obtained under laboratory conditions. It should not, on its own, be interpreted as a complete prediction of a product’s fire behaviour in its final application.

How the LOI Test is Performed

The specimen is positioned vertically inside a transparent Pyrex glass column. A controlled oxygen/nitrogen mixture flows upward through the column. Once the gas composition has stabilized, the specimen is ignited and the flame behaviour is observed.

Fase	Action	Controlled variable
1	Specimen preparation and mounting	Specimen geometry and orientation
2	Setting the O₂/N₂ mixture	Oxygen concentration
3	Flow stabilization	Flow rate and uniformity
4	Controlled ignition	Flame application method
5	Combustion observation	Burning time and flame propagation
6	Determination of the limiting value	Minimum concentration that sustains combustion

Example of specimen positioning and ignition in the test column.

Meaning of the Measured Value

The measured value represents the transition point between sustained combustion and self-extinguishment according to the method criteria. Reliable and comparable results require strict control of the gas mixture, flow conditions, specimen geometry, material conditioning and ignition procedure.

Reference Standards

Standard	Subject	Scope
ASTM D2863-23e1	Minimum oxygen concentration that supports candle-like combustion	Plastic materials
ISO 4589-2:2017	Determination of the Oxygen Index at ambient temperature	Small vertical specimens

Noselab ATS Automatic System Technology

In the EA04 system, the oxygen concentration is measured using a paramagnetic transducer. Microprocessor-based control enables direct reading of the measured values, automatic flow control, and oxygen/nitrogen mixing. Testing and calibration procedures are managed via a 7-inch color touchscreen.

FUNCTION	DECLARED CONFIGURATION
O₂ Detection	Paramagnetic transducer, O₂ accuracy < 0.1%
Flow Control	Automatic via mass flow controller
Mixing	Automatic oxygen/nitrogen control
Control	Microprocessor and 7” color touchscreen
Calibration	Automatic at 0% and 100%
Gas Optimization	Option to add air for measurements at high oxygen concentrations
Connection	USB output for PC connection
Column	Type A Pyrex glass, Ø100 × 450 mm

Why use a paramagnetic transducer?

Oxygen has paramagnetic properties that can be used to determine its concentration. In the context of the LOI test, continuous measurement of the O₂ percentage makes it possible to verify the composition of the mixture actually present in the column, rather than relying solely on nominal flow-rate values.

METROLOGICAL ADVANTAGE

The combination of concentration measurement and automatic flow control reduces the need for manual adjustments and helps ensure more stable and repeatable test conditions.

Left: optional Data Link INDOX software. Right: holders for different types of specimens.

Applications in Laboratories and Industry

The LOI test is suitable for the comparative characterization of insulating materials, cable jackets, elastomeric and plastomeric fillers used in electrical cables. The principle can also be applied to research and quality control of other polymeric materials within the scope of the relevant standard.

Industry	Materials / Components	Use of Results
Electrical Cables	Insulation, jackets and fillers	Comparison of combustion behavior
Polymers	Thermoplastics and flame-retardant formulations	Development of flame-retardant materials
Elastomers	Compounds and flexible components	Comparative evaluation of formulations
Foams	Self-supporting cellular materials	Product control and research
R&D Laboratories	Experimental samples	Screening and formulation optimization
Quality Control	Production batches	Verification of process consistency

Conclusions

The Limiting Oxygen Index provides a quantitative parameter for comparing the combustion behaviour of polymeric materials. Reliable results depend on accurate control of the gas mixture, stable flow conditions and strict compliance with the standardized procedure. The NOSELAB ATS automatic system integrates paramagnetic oxygen measurement, automatic flow regulation, calibration and digital test management in a configuration designed for research and quality control laboratories.

Technical references: NOSELAB ATS Technical Datasheet rev. 3-2019; ISO 4589-2:2017; ASTM D2863-23e1.

Fire safety standards and test methods