Guide to Fully Automatic Oxygen Index Testers

A fully automatic oxygen index tester is a precision testing instrument used to determine the flame-retardant properties of polymers. Its core function is to simulate various oxygen environments by precisely controlling the concentration of an oxygen-nitrogen gas mixture, thereby determining the minimum volume percentage of oxygen (i.e., the oxygen index, OI) required for a material to sustain stable combustion. This allows for the evaluation of a material’s flame-retardant properties and fire hazard, making it widely used in industries such as plastics, rubber, textiles, and building materials.

(1) Definition

The fully automatic oxygen index tester (also known as a limiting oxygen index tester or LOI tester) is a specialized testing device based on combustion kinetics and gas ratio control principles. By automatically regulating the mixing ratio of oxygen and nitrogen under specified test conditions, it determines the minimum oxygen concentration (oxygen index) required to sustain combustion in various materials such as polymer materials, textiles, and rubber. The oxygen index is expressed as a volume percentage. Its value is positively correlated with the material’s flame-retardant performance, and the material’s fire hazard can typically be assessed by determining its combustion rating based on the oxygen index value.

(2) Applications

This equipment serves not only as a testing method for evaluating the flame-retardant properties of polymers but also as a research tool, providing an efficient testing method for flame retardant formulation research and the development of new flame-retardant materials. The equipment is easy to operate and offers stable, reliable performance. Its testing system utilizes an imported ferromagnetic oxygen sensor and is controlled by a computer.

By simulating environments with different oxygen concentrations, the critical oxygen concentration required for a material to sustain combustion is determined.

(1) Gas Supply System

Serving as the instrument’s “gas supply assurance,” this system primarily includes oxygen cylinders, nitrogen cylinders (to be provided by the user, with a purity of >99%), pressure regulators, precision flow meters, gas mixers, and gas piping. The pressure regulator is used to adjust the output pressure of the cylinders, ensuring stable gas pressure; The precision flowmeter employs a stepper proportional valve for high-precision control, allowing for accurate adjustment of oxygen and nitrogen flow rates, typically within a range of 0–20 L/min; the gas mixer is used to thoroughly blend the oxygen and nitrogen gases, ensuring uniform concentration of the mixed gas; the gas piping features a sealed design and is equipped with a gas connection error alert function.

(2) Combustion Test Apparatus

The core component is the combustion chamber, typically made of transparent quartz glass or high-temperature-resistant glass to facilitate observation of the entire combustion process of the specimen; the bottom of the combustion chamber is filled with glass beads or ceramic balls to ensure uniform airflow; a metal mesh is installed above the combustion chamber to prevent flame splatter and ensure test safety; additionally, the combustion test apparatus includes a draft shield to isolate external airflow interference and ensure a stable test environment.

(3) Sample Clamping System

Used to secure the sample, this system offers two types of clamps based on sample characteristics: First, the self-supporting material clamp, suitable for rigid plastics, building materials, and other self-supporting materials, which secures the sample at the center of the combustion tube; second, the non-self-supporting material clamp (optional), suitable for textiles, soft plastics, and other flexible, non-self-supporting materials, which secures both vertical edges of the sample to the frame simultaneously.

(4) Igniter

The igniter manufactured by UTSTESTER is a product specifically designed for testing instruments, featuring a compact size, precise adjustment, easy maintenance, and user-friendly operation. A compatible cartridge gas cylinder can be purchased from our store and connected for use.

(5) Control System

Operated via a full-color touchscreen, the system integrates test control, parameter settings, data logging, and fault alerts. Operators can directly set parameters such as oxygen concentration and test duration on the touchscreen. It is equipped with an imported oxygen sensor and features data storage, historical data retrieval, and data clearing functions.

(6) Auxiliary Safety System

To ensure test safety, the instrument is equipped with various safety devices, including an overpressure relief valve, a fire extinguishing system, and a windshield.

Measurement Range: 15% – 60%.

Resolution: 0.1%.

Measurement Accuracy: (±0.2%).

Response Time: < 2 seconds.

Output Drift: < 5%/year.

Time Resolution: 0.1 seconds.

The fully automatic oxygen index tester has a wide range of applications and can be used to test the Flammability of various solid materials, including the following:

1. Plastics: Polyethylene, polypropylene, polycarbonate, PVC, plastics for electronic and electrical enclosures, etc.;

2. Rubber: Natural rubber, synthetic rubber, automotive tires, power cable sheaths, etc.;

3. Textiles: Woven fabrics, knitted fabrics, non-woven fabrics, fire-resistant clothing fabrics, hotel carpets, airport curtains, etc.;

4. Other materials: Foam materials (polyurethane foam, polystyrene foam), flame-retardant wood, thermal insulation materials, artificial leather, films, fire-resistant doors, etc.

(I) Sample Preparation

Collect at least 15 standard samples. Mark a line 50 mm from one end of each sample and insert the other end into the sample holder of the combustion column.

(2) Instrument Inspection and Calibration

Before turning on the instrument, check that the power supply and gas line connections are normal; turn on the instrument power, start the control system, and perform a self-test to verify that components such as the oxygen sensor, igniter, and flow meters are functioning properly; use standard gas to calibrate the oxygen sensor, and verify the accuracy of the oxygen and nitrogen flow meters by comparing them against a standard flow meter.

(3) Parameter Settings and Gas Adjustment

Set the test parameters on the touchscreen, including the estimated oxygen concentration, gas flow rates, and combustion criteria (such as combustion time and combustion length); based on the estimated oxygen concentration, start the gas supply system and adjust the oxygen-to-nitrogen gas ratio; the instrument will automatically mix the gases. Once the oxygen concentration has stabilized (indicated by an audible alert), purge the combustion chamber with gas for a specified duration to remove residual air and ensure a stable test environment.

(4) Sample Loading and Ignition

Select the appropriate fixture based on the sample type, and secure the sample vertically and centrally on the axis of the combustion chamber to ensure it is firmly fixed and does not shift; Close the combustion chamber protective cover, activate the igniter, adjust the flame length to meet standard requirements, insert the ignition nozzle into the combustion chamber, and ignite the top of the specimen. Immediately remove the ignition nozzle after ignition; simultaneously, the instrument automatically starts the timer, and the operator observes the combustion status in real-time through the viewing window.

(5) Result Evaluation and Adjustment

Based on the preset combustion criteria, the instrument automatically determines whether the specimen can sustain stable combustion at the given oxygen concentration: If the specimen’s burning time or length exceeds the specified values, the oxygen concentration is too high, and the test must be repeated after reducing the oxygen concentration; if the specimen cannot sustain combustion or the burning time or length does not meet the standards, the oxygen concentration is too low, and the test must be repeated after increasing the oxygen concentration; Adjust the oxygen concentration repeatedly until the critical oxygen concentration is found; this value represents the oxygen index of the material. Multiple tests must be conducted on the same material, and the average value is taken as the final test result.

(6) Test Conclusion and Cleaning

After the test is complete, shut off the gas supply system, close the valves on the oxygen and nitrogen cylinders, and release any residual pressure in the piping; turn off the instrument’s power supply, and after the instrument has cooled down, promptly clean the combustion chamber, sample holder, and igniter to remove soot and combustion residues; thoroughly record information such as test conditions, ambient temperature and humidity, gas flow rates, and test results; compile the test report and properly store the test data.

1. GB/T 2406.2-2009 “Plastics—Test Methods for Flammability—Oxygen Index Method”

2. GB/T 5454-1997 “Textiles—Testing of Flammability—Oxygen Index Method”

4. ISO 4589-2-2006 “Plastics—Determination of burning behavior by the oxygen index method—Part 2: Test at room temperature”

5. ASTM D2863-2000 “Test method for the minimum oxygen concentration (oxygen index) of plastics on a wick”