The limiting oxygen index (LOI) is a measure of a material's flammability and its ability to support combustion in the presence of oxygen. It is determined by conducting a specific test called the oxygen index test. Here's how it is typically performed:

1. Specimen Preparation: Prepare samples of the material in the form of bars or sheets with specific dimensions and conforming to relevant standards or testing protocols.

2. Test Apparatus: Set up the oxygen index test apparatus, which usually consists of a combustion chamber, oxygen supply system, flow control, and ignition source.

3. Calibration: Calibrate the apparatus according to the testing standards, ensuring accurate measurements of oxygen concentration and flow rate.

4. Testing Procedure: Place the prepared sample vertically in the combustion chamber and adjust its position. Start the oxygen flow into the chamber at a specified flow rate.

5. Ignition: Using a controlled ignition source, ignite the sample near the bottom end. The sample should catch fire and start to burn.

6. Oxygen Concentration Adjustment: Gradually increase or decrease the oxygen concentration inside the chamber while monitoring the sample's behavior. The oxygen concentration at which the sample stops burning and self-extinguishes is recorded.

7. Calculation: The limiting oxygen index (LOI) is determined by dividing the minimum oxygen concentration required to sustain combustion by the total oxygen concentration in the test atmosphere and multiplying it by 100. LOI = (Minimum oxygen concentration for sustained burning / Total oxygen concentration) × 100.

The LOI value indicates the minimum percentage of oxygen in the mixture that is required to support sustained burning of the tested material. Higher LOI values indicate that the material has better fire resistance and is less likely to support combustion.

It's worth noting that the specific testing procedure and apparatus may vary depending on the testing standards or protocols being followed.