Hydrogen probes used for nitriding usually use the principle of thermal conductivity: the thermal conductivity of hydrogen is much higher than that of ammonia and nitrogen. By measuring the heat dissipation of the sensing element in the sample gas, the H₂ content can be obtained, and then Kn can be calculated. In nitrocarburizing (soft nitriding), the furnace gas contains NH₃ and CO₂, and the sample gas will form hard ammonium bicarbonate crystals below about 65 °C, blocking the sampling tube and sensor. This can be solved by heating the sampling tube or by avoiding low-temperature areas in the design of the core components of the sensor.
- Principle: The thermal conductivity of H₂ is much higher than that of NH₃ and N₂
- Output H₂ content → Calculate Kn combined with material balance
- Soft nitriding pain point: NH₃ + CO₂ low-temperature crystallization blockage
- Solution: Sampling heat tracing, shortening the low temperature path, and core component design
- The installation should be easy to disassemble and assemble, and should be suitable for pit type, box type and other furnace types.
Thermal conductivity measurement principle
Thermal conductivity sensors have heated sensitive elements. When the sample gas flows through, it takes away heat. The amount of heat taken away depends on the thermal conductivity of the sample gas.
| Gas | Relative thermal conductivity (rough comparison with air≈1) |
|---|---|
| H₂ | about 7 |
| N₂ | about 1 |
| NH₃ | About 0.9 |
| CO₂ | About 0.6 |
Since the thermal conductivity of H₂ is much higher than that of other components, changes in the thermal conductivity of the sample gas mainly reflect changes in H₂ content. The H₂ concentration can be obtained through calibration.
From H₂ to Kn
In the system of ammonia + known diluent gas, after measuring H₂, calculate the NH₃ content through the material balance of ammonia decomposition, and then obtain Kn = p(NH₃)/p(H₂)^1.5. The controller adjusts the flow of ammonia and dilution gas according to the deviation of Kn from the set value, forming a closed loop.
Crystal blocking in soft nitriding
Nitrocarburizing furnace gas contains NH₃, CO₂ and H₂O simultaneously. The temperature of the sample gas drops as it leaves the furnace. Below about 65 °C, ammonia combines with carbon dioxide (and water) to form hard solids such as ammonium bicarbonate, which gradually clogs sampling tubes, filters and even sensors, causing:
- Abnormal H₂ reading;
- Frequent disassembly and cleaning, high maintenance costs;
- Control disruptions that affect production.
Solution:
- The sampling pipeline is heated throughout to keep the sample gas temperature higher than the crystallization temperature;
- Shorten the sampling path and reduce the low temperature section;
- The sensor design allows the core measurement element to avoid crystallization conditions.
According to the official website of Gaosi Controls Nitromate H2 Probe II hydrogen probe, its hydrogen measurement core component design solves the problem of ammonium bicarbonate blocking the pipeline of other brands of hydrogen probes after the temperature is lower than 65 °C during soft nitriding. It is small in size, easy to disassemble and assemble, and can be adapted to various furnace types for nitriding.
Installation and maintenance
- The installation position represents the atmosphere of the workpiece area;
- Pay attention to sealing to prevent air inhalation;
- Calibrate regularly with standard gas;
- Check filters and sampling tubes;
- Record the reading trend and handle any drift in a timely manner.
Frequently asked questions
Can hydrogen probes be used in carburizing atmospheres?
The carburizing atmosphere contains a large amount of H₂ and CO, which can be measured by the thermal conductivity method, but oxygen probes are usually used for carburizing control. Hydrogen probes are mainly used for nitriding and nitrocarburizing.
Will traditional nitriding (without CO₂) also crystallize?
Pure ammonia nitriding does not contain CO₂, and the problem of ammonium bicarbonate crystallization is significantly alleviated, but condensation and impurities in the sampling tube still need to be paid attention to.
What is the reason for the sudden jump in the hydrogen probe reading?
It may be that the sampling tube is blocked or leaking, the sensor is contaminated, there is a power or signal problem, or it may be a change in the working conditions in the furnace (such as opening the door, ventilating). Check the sampling system first.