There are five commonly used carbon potential calibration methods: the fixed carbon chip is the most accurate but time-consuming and depends on the operator; the hot wire resistance method is faster, with an accuracy of about ±0.03 in the carbon potential range of 0.7–1.2; the three-gas analyzer produces results in about 5 minutes and has no consumables, but the results are affected by the atmosphere and equipment type; the dew point meter cannot directly measure the carbon potential; the carbon rod requires composition analysis and the measurement is complicated. It is recommended to use quick methods for daily calibration and regular carbon sheets for comparison and arbitration.
- Fixed carbon sheet: the most accurate, time-consuming, and has high operational requirements
- Hot Wire Resistor: Faster, 0.7–1.2 CP Accurate to approximately ±0.03
- Three gas analysis: about 5 minutes, no consumables, affected by atmosphere type
- Dew point meter: cannot directly obtain carbon potential
- Combination strategy: daily use of fast method, regular use of fixed carbon tablets for comparison
A quick look at five methods
| Method | Principle | Speed | Accuracy | Operational requirements | Main limitations |
|---|---|---|---|---|---|
| Shim Stock | The thin low carbon steel sheet is balanced in the furnace, and the weight gain is measured or the carbon content is analyzed during combustion. | slow | High (recognized as more accurate) | high | Time-consuming; placement, cooling, cleaning, and weighing affect the results |
| hot wire resistance method | The change in resistivity of special material wire after carburization is related to the carbon content | faster | 0.7–1.2 CP range approximately ±0.03 | Low | There is an optimal measurement range; special instruments are required |
| Three gas analyzer | Infrared measurement of CO, CO₂, CH₄, combined with temperature to calculate carbon potential | Quick (about 5 minutes) | Good, needs calibration | Low | Atmosphere type and equipment type can affect results |
| Dew point meter | Measure H₂O content | quick | indirect | middle | Carbon potential cannot be measured or calculated directly |
| carbon rod | Analysis of carbon content after carburizing of rods | slow | depends on analysis | high | Requires component analysis and complex measurement |
(The characteristics of each method are summarized from the article on carbon potential management of Gauss Control CQI-9.)
Method details
Fixed carbon sheet Place a low-carbon steel sheet with a thickness of about 0.05–0.1 mm into the workpiece area of the furnace and keep it balanced with the atmosphere for sufficient time. When taking it out, it must be prevented from oxidation and decarburization. The carbon is then measured by weighing method (weight gain converted to carbon content) or combustion infrared method. Its advantage is that it directly reflects "how much carbon iron can absorb in this atmosphere", which is consistent with the definition of carbon potential.
hot wire resistance method After the special material wire is carburized in the furnace, the resistivity can be directly measured to convert the carbon potential. The Gauss Control article pointed out that its key components and core algorithms have been domestically produced; cleaning, drying and other treatments have little impact on the results and can be placed in the furnace together with the carbon verification foil for comparison.
Three gas analysis Measure CO and CO₂ in real time, calculate the carbon potential at a given temperature, and measure residual CH₄ to determine the quality of the atmosphere. The calibration method is simple, third-party accuracy reports can be provided, it is applicable to many furnace types and processes, the operation is simple, and there are no consumables.
Combination suggestions
| Business situation | Recommended combination |
|---|---|
| Few stoves and too many people | Mainly based on carbon tablets + three gas analysis auxiliary diagnosis |
| There are many furnaces and they need to be calibrated daily. | Three-gas analysis or daily calibration of hot wire resistance + weekly/monthly comparison of carbon verification foil |
| Dispute or customer review | Calibration tablets or third-party laboratory results as arbitration |
How to use the verification results
- Calculated deviation = calibration value − controller display value;
- Deviation within allowable range: record;
- Deviation exceeds limit: start reaction plan (check probe, temperature, carrier gas, evaluate work in progress);
- Stable system deviation: Adjust process factors after review and record the basis.
Frequently asked questions
How long should the carbon sheets be kept?
It takes enough time to reach equilibrium with the atmosphere, which is usually related to the thickness of the sheet, temperature and carbon potential. The time should be fixed according to the instrument or company procedures to ensure comparability.
The carbon potential measured by the three-gas analyzer and the oxygen probe are different. Which one is correct?
There may be deviations. Use fixed carbon sheets for third-party comparison to find out the source of the deviation, and then decide which side to correct.
How long should verification records be kept?
According to customer requirements and enterprise quality system regulations. The automotive industry usually requires a longer storage period, and it is recommended to keep it at the same time as parts traceability records.