There are three main ways to measure nitriding atmosphere: traditional water absorption decomposition rate meter (manual, offline), hydrogen probe (commonly used thermal conductivity method to measure H₂ partial pressure) and ammonia analyzer (infrared and other methods to measure NH₃). When only ammonia or ammonia + known dilution gas is passed, the NH₃ partial pressure can be calculated based on the H₂ content combined with material conservation, and then Kn = p(NH₃)/p(H₂)^1.5. The hydrogen probe has become a mainstream sensor for controlled nitriding due to its simple structure and continuous measurement.
- Decomposition rate meter: manual, offline, can only indirectly reflect nitriding ability
- Hydrogen probe: thermal conductivity method to measure H₂, continuous and closed-loop
- Ammonia analyzer: directly measures NH₃, higher cost
- When only ammonia is passed through: H₂ is measured → NH₃ is calculated from conservation → Kn is calculated
- Low-temperature crystallization of sampling lines is a common fault
Three measurement methods
| Way | Principle | Advantage | Limitations |
|---|---|---|---|
| Decomposition rate meter (glass buret type) | The sample gas passes through water, ammonia is absorbed, and the remaining volume is the proportion of decomposition products | Cheap and intuitive | Manual, offline, accuracy is affected by operation and cannot be closed loop |
| hydrogen probe | Thermal conductivity method measures H₂ partial pressure (H₂ thermal conductivity is much higher than other components) | Continuous measurement, closed loop, simple maintenance | The interference of other gases on thermal conductivity needs to be considered |
| Ammonia analyzer | Direct measurement of NH₃ using infrared and other methods | Measure ammonia directly | Higher cost and need to maintain sampling system |
From H₂ to Kn
When only ammonia is passed through, the ammonia is partially decomposed:
2NH₃ → N₂ + 3H₂
Assuming that the decomposition ratio of ammonia entering the furnace is α, then in the furnace gas:
- H₂ volume fraction = 1.5α / (1 + α)
- NH₃ volume fraction = (1 − α) / (1 + α)
- N₂ volume fraction = 0.5α / (1 + α)
After H₂ is measured, α and NH₃ can be deduced and Kn can be calculated. When nitrogen or predecomposed ammonia is added for dilution, as long as each inlet flow rate is known, the solution can also be solved through material balance. This is the basic structure of controlled nitriding of "hydrogen probe + flow measurement + calculation".
Thermal conductivity principle of hydrogen probe
The thermal conductivity of different gases varies greatly. The thermal conductivity of H₂ is about 7 times that of N₂. The thermal conductivity sensor measures the heat dissipation of the heating element in the gas being measured, and from this the H₂ content is deduced.
On-site FAQ
- Low temperature crystallization blockage: In nitrocarburizing (soft nitriding), the atmosphere contains CO₂. After the temperature of the sampling gas decreases, ammonia and CO₂ will generate hard solids such as ammonium bicarbonate, which will block the sampling tube. According to the official website of Gaosi Controls Nitromate H2 Probe II, its core component design solves the problem of ammonium bicarbonate clogging the pipeline after the temperature drops below about 65 °C;
- sampling location: Should represent the atmosphere in the workpiece area and avoid the air inlet;
- leakage: Air leakage in the sampling pipeline will dilute the sample gas, resulting in low H₂ readings;
- cross interference: Other components in the atmosphere (such as CO₂, CO) will affect the thermal conductivity measurement and require algorithm compensation.
Key points for selection
- Applicable processes (nitriding/nitrocarburizing);
- Installation method and furnace type adaptation (well type, box type, integral-quench furnace);
- Whether it has anti-crystallization design;
- Signal interface and computing power to the controller.
Frequently asked questions
Do I need a decomposition rate meter when I have a hydrogen probe?
It is not required, but a decomposition rate meter can be useful during audits or troubleshooting as a simple means of cross-validation.
How often do hydrogen probes need to be calibrated?
Determine according to the manufacturer's recommendations and usage intensity, and calibrate after replacement, repair, or abnormal readings. Calibrate using a standard gas of known concentration.
Why do the Kn values from different manufacturers not match?
Calculation methods (whether diluent gas is taken into account, pressure units), sampling location, and sensor differences all make a difference. The definitions and calculation methods should be unified when comparing.