The three-gas analyzer uses the non-dispersive infrared (NDIR) principle: CO, CO₂, and CH₄ molecules respectively absorb infrared light of specific wavelengths, and the amount of absorption is related to the concentration. After the instrument measures the contents of the three gases, the carbon potential can be calculated based on the furnace temperature; the process factor (PF) can be obtained by comparing it with the controller display value, and the residual CH₄ reflects whether the atmosphere is close to equilibrium. It is a common tool for CQI-9 carbon potential calibration and atmosphere diagnostics.
- NDIR: Different gases absorb different wavelengths of infrared light
- Measure CO, CO₂, CH₄ (some optional O₂)
- CO + CO₂ + Temperature → Carbon potential; compare with controller → PF
- A high CH₄ indicates that the atmosphere is out of balance
- Sampling system (filtration, cooling, leak prevention) determines data quality
NDIR principle
Many gas molecules have characteristic absorption bands in the infrared region: CO is around 4.6 μm, CO₂ is around 4.3 μm, and CH₄ is around 3.3 μm. The basic structure of an NDIR analyzer is:
Infrared light source → Measuring gas chamber (the measured gas flows through) → Filter (selecting a specific wavelength) → Detector
The higher the concentration, the more infrared light is absorbed and the weaker the detector signal. Through standard gas calibration, the relationship between signal and concentration can be established. Diatomic homonuclear molecules such as N₂ and H₂ do not absorb infrared light and therefore do not interfere with measurements.
Measurement content and calculation
Taking the Gauss Control FAA-B portable three-gas analyzer as an example, the official website introduces that it can measure CO, CO₂, and CH₄ (O₂ optional) in the atmosphere, and use it to calculate process values such as carbon potential and PF value, which can be used to diagnose the atmosphere in the furnace and reduce the impact of human factors on carbon potential measurement.
Carbon potential calculation: Based on the boundary reaction equilibrium, use the measured CO, CO₂ and the input furnace temperature to calculate the carbon activity, and then convert the carbon potential.
Process Factor (PF): Compare the carbon potential calculated by the analyzer and the carbon potential displayed by the controller to obtain the correction coefficient.
CH₄ Diagnosis: High residual methane usually means too much enriched gas, the atmosphere is not balanced, or the furnace temperature is too low. At this time, the carbon potential calculation based on the equilibrium assumption will be biased, and the atmosphere problem should be solved first.
Sampling system
No matter how accurate the analyzer's reading is, it's useless if the sample gas is wrong. Key points of the sampling system:
- sampling point: Represents the workpiece area, away from the air inlet and furnace door;
- filter: Remove carbon black and dust to prevent contamination of the air chamber;
- Cooling and water removal: Prevent condensation water from entering the instrument;
- Leak proof: Any leakage will suck in air, making CO₂ higher and carbon potential lower;
- flow: Stable flow rate according to instrument requirements;
- Safety: The sample gas contains CO and H₂, and the exhaust must be handled safely.
Use
- Daily carbon potential verification (meets CQI-9 backup verification requirements);
- New furnace commissioning and atmosphere diagnosis;
- Generator outlet gas quality inspection;
- Compare the atmosphere conditions of different furnaces and different carrier gases.
Frequently asked questions
Can the carbon potential energy measured by the three-gas analyzer directly replace the oxygen probe control?
Fixed analyzers can be used for control, but there is a delay in sampling. The oxygen probe is usually still used for real-time control, and the analyzer does calibration and CO correction.
Why is there a big difference between the CO measured by the analyzer and the set value?
It means that the CO% set in the controller is inconsistent with the actual situation, which is common in nitrogen-methanol atmosphere. The settings should be corrected based on actual measurements, or online CO measurement should be used.
How often is the analyzer calibrated?
Determine according to the manufacturer's recommendations and frequency of use, and perform a zero point check before each measurement (if supported by the instrument). The key is to have calibration records and traceable standard gases.