What is the oxygen index tester?

An oxygen index tester is an instrument used to measure the combustion properties of materials. In the field of fire safety, understanding the combustion characteristics of materials is crucial for preventing and reducing fire risks. An oxygen index tester primarily measures the oxygen concentration (volume percentage) required by a material during combustion, using this data to assess its combustion performance. Simply put, it examines how a material burns in environments with varying oxygen content. So what is the oxygen index? The oxygen index (OI) refers to the minimum oxygen concentration required for a material to sustain flaming combustion in an oxygen-nitrogen mixture under specified conditions. It is expressed as the volume percentage of oxygen. For example, a material with an OI of 28% means that it can sustain flaming combustion in an oxygen-nitrogen mixture only when the oxygen content reaches 28%. A higher OI indicates a less flammable material and better flame retardancy. Conversely, a lower OI indicates a more flammable material. Generally, an OI of less than 22 is considered flammable, an OI between 22 and 27 is considered combustible, and an OI greater than 27 is considered flame-retardant. For example, the polypropylene (PP) that we come into contact with in daily life has an oxygen index of about 21%, which is a combustible material; while the oxygen index of polyvinyl chloride (PVC) can reach 30%, which is relatively difficult to burn.

Working Principle

The oxygen index meter operates based on the combustion characteristics of materials under varying oxygen concentrations. The instrument primarily consists of a combustion chamber, a combustion gas flowmeter, a sample holder with aeration holes, and an oxygen content meter. During testing, the sample is first secured to the sample holder and the aeration holes are adjusted to adjust the oxygen concentration. Next, the oxygen flowmeter and ignition device are activated to ignite one end of the sample. High-temperature heating and the introduction of high-purity oxygen in the combustion chamber remove impurities from the air, ensuring test accuracy. The combustion gas flowmeter measures the oxygen flow rate in the combustion chamber to calculate the oxygen content. The oxygen content meter measures the oxygen concentration in real time. The oxygen index value is calculated by observing data such as the sample's ignition time and combustion time.

Operation Instructions

Before starting the instrument, complete preparations. First, check the instrument's status, confirming that the combustion tube is vertical and there are no leaks in the piping. Ensure that the oxygen and nitrogen pressures are maintained between 0.2 and 0.3 MPa and 0.2 and 0.4 MPa, respectively. Next, prepare the specimens and cut them according to standards (such as GB/T 2406.2-2009) to ensure that the dimensions meet the specifications. Ensure environmental control, maintaining a laboratory temperature of 23 ± 2°C and a humidity of 50 ± 5%, and ensuring that the temperature inside the combustion tube stabilizes at the set value. After completing pre-startup preparations, proceed to parameter setup and calibration. Use flowmeters to adjust the oxygen flow rate to 10 to 100 L/min and the nitrogen flow rate to 40 to 100 L/min to adjust the mixing ratio. The total flow rate is typically maintained at 10 L/min. Next, perform oxygen concentration calibration, using standard materials to verify the consistency between the digital display and the actual oxygen concentration. Adjust the "full scale" knob as necessary. After parameter setup and calibration, the sample is installed and tested. The sample is placed vertically in the center of the combustion tube, with the top ≥100mm from the tube opening and the bottom 100mm above the gas dispersion device. There are two ways to ignite the sample: the top ignition method, which maintains a flame length of 16±1mm and ignites the top of the sample within 30 seconds. The diffusion ignition method is suitable for flame-retardant materials. Observe and record the test process. If the sample burns for 3 minutes or spreads 50mm, adjust the oxygen concentration and repeat the test until the critical value is found. Finally, record the data and shut down the machine. Read the oxygen index value from the display and record the environmental parameters (temperature, humidity, etc.).

Core Component Maintenance

Oxygen sensor maintenance, along with inspection of the air system, operating environment management, daily usage guidelines, and a regular maintenance plan, are key steps in ensuring stable instrument operation.

Regarding oxygen sensor maintenance, if the instrument is not used for an extended period, remove the oxygen sensor and store it in a refrigerator (0-4°C) to prevent sensor aging due to high temperatures or humidity. Calibrate the sensor every 3-6 months depending on frequency of use, or every 1-3 months for high-frequency or harsh environments. If abnormal values, slow response, or calibration failure occur, replace the sensor immediately (typical lifespan is 1-2 years). After testing, clean the sensor air inlet with a dedicated cotton swab to avoid oil or chemical residue. When disinfecting in medical settings, wipe with a 75% alcohol pad, avoiding areas around the display and buttons.

Regarding gas system inspections, when using gas sources such as liquefied petroleum gas and natural gas, regularly check all connections for leaks. Deteriorating vent pipes should be replaced promptly to prevent gas leaks from affecting measurement accuracy. Disassemble and clean components such as filters and moisture separators to remove scale and deposits. If necessary, soak with an organic solvent to ensure clear piping.

Regarding operating environment management, during installation and storage, the instrument should be installed in a dry, well-ventilated space away from sources of fire, flammable materials, and strong electromagnetic interference. The recommended storage temperature is 0-40°C, with a humidity of less than 85%. Avoid direct sunlight and corrosive gases. During long-term storage, remove the battery to prevent leakage and corrosion of the circuitry. Regarding safety precautions, when testing materials that release toxic or hazardous gases, work in a fume hood and wear protective glasses and gloves. The laboratory should be equipped with fire extinguishers. Operators are strictly prohibited from smoking or lighting fires to prevent fires.

Daily use specifications include pre-operation inspections, using the specified voltage (e.g., DC24V), ensuring the instrument is properly grounded to prevent electric shock or signal interference, preparing the sample plate as required, and adjusting the sample holder angle to ensure flatness and stability to avoid test deviations. During operation, monitor the instrument display and operating sounds in real time. If any abnormality is detected, stop the test immediately and investigate the cause (e.g., shifting of the pressure regulating valve spring, screw stripping, etc.). Operators must not leave the test site to prevent accidents. After operation, clean the sample holder with a brush or soft cloth, avoiding acidic or alkaline detergents. If the nozzle is clogged, use compressed air to clear it. After the test, turn off the power and air supply in sequence to prevent the equipment from idling or gas leaks.

The regular maintenance plan includes monthly maintenance to check whether the instrument's self-test function is normal, whether the alarm function is sensitive, clean the instrument surface, and wipe it with a slightly damp soft cloth to prevent liquid from seeping into the interior; quarterly maintenance requires checking whether the oxygen sensor, circuits and components are damaged or aging, and replacing them in time, cleaning the dust inside the instrument, replacing the filter, and ensuring good heat dissipation; annual maintenance requires a comprehensive performance test of the instrument, including temperature accuracy, sensitivity, response time and other indicators to ensure compliance with requirements, check the insulation performance of the protective ground wire, and immediately shut down the instrument for inspection and repair if any problems are found.

In summary, oxygen index testers are of great importance in the field of fire safety. Their proper operation relies on a comprehensive and meticulous process, from oxygen sensor maintenance and gas system inspection to operating environment management, daily usage specifications, and a regular maintenance schedule. Strictly adhering to these operating and maintenance guidelines not only ensures the accuracy and reliability of test results, providing a solid basis for evaluating the combustion performance of materials, but also effectively extends the instrument's service life and reduces equipment failure rates, providing strong support for fire prevention and risk reduction efforts, and playing a key role in protecting life and property.