Oxygen Probe Selection, Installation, Burnout, and Service Life

• Selection: temperature range, insertion length, zirconia structure, whether built-in thermocouple
• Installation: represent the workpiece area, avoid air inlets and flames, and have good sealing
• Reference air: clean, dry, stable flow
• Charcoal burning: Carry out regularly according to the condition of carbon deposits
• Establish probe files: entry and exit, inspection, verification, replacement records

Key points for selection

• Operating temperature range: Confirm that it matches the process temperature;
• Insertion length and mounting interface: Ensure that the probe tip is in a representative atmosphere;
• Structure type: Integral zirconia, zirconium ball, welded type, zirconia head, etc., with different thermal shock resistance and sealing methods;
• Whether there is a built-in thermocouple: Built-in thermocouple measures tip temperature, Nernst calculation is more accurate;
• Applicable atmosphere: The carbon deposition tendencies are different under working conditions such as carburizing, quenching, and generator outlet.

Installation location

1. Represents the atmosphere and temperature of the area where the workpiece is located;
2. Avoid direct injection ports of enriched gas and diluted air;
3. Avoid direct radiation from combustion tube flames and radiant tubes;
4. Keep away from places prone to air leakage such as furnace doors;
5. The mounting flange and sealing are reliable to prevent air from penetrating along the outer wall of the probe.

Reference air

The reference air is the "known quantity" in Nernst's equation and must be:

• Clean (no oil, no dust), dry;
• The flow rate is stable, set according to the probe manual;
• Check the air pump and pipeline regularly, and call the police when the pump stops.

Burn charcoal

Carbon will accumulate on the outer surface of the probe in a carburizing atmosphere, causing signal distortion or even short circuit. Charcoal burning is to pass a small amount of air to the outside of the probe to burn off the carbon deposits.

• The frequency is set according to the carbon potential of the atmosphere and the carbon deposition. High carbon potential conditions require more frequency;
• The signal will be temporarily abnormal during charcoal burning, and the controller should suspend adjustment or maintain output;
• Observe the signal recovery after burning the charcoal. Slow recovery may mean that the probe is aging.

Calibration and diagnostics

• comparison method: Use the results of a three-gas analyzer or a carbon-fixing chip to back-deduct the mV, and compare it with the probe output;
• Impedance test: An increase in probe internal resistance usually indicates aging or poor electrode contact;
• response test: Observe the response speed after burning or when working conditions change.

The output error of a new probe usually needs to be controlled within ±5 mV; when carburizing at 930 °C, a 5 mV deviation can cause a carbon potential deviation of about ±0.10, so trend management is more important than "replacing it if it breaks".

Life management

Create a probe file:

These records are also important evidence in CQI-9 audits that carbon potential is controlled.