Decision Tree: High O₂ Bias (Air Ingress / Dilution)
Symptom: O₂ reads higher than expected; process stable but analyzer high.
Key idea: Most high bias is from air leaks into a negative-pressure or low-pressure sample system.
Isolation steps
1) Compare reading on calibration span gas. If span is correct, the sensor is likely OK → suspect sample leak/dilution.
2) Leak check sample system:
- Check fittings from probe/sample point to analyzer
- Check filter bowls, drain points, unions
- Check any flexible tubing or cracked lines
3) Check sample pressure regime:
- If sample line is under suction (pump pulling), small leaks pull air in → high O₂.
- Stabilize with correct regulator/flow and minimize suction leakage points.
4) Confirm sample integrity:
- Ensure sample point not drawing ambient air (bad probe sealing)
- Confirm purge/vent lines not open to atmosphere incorrectly
5) If no leak found:
- Verify calibration gas values and regulator accuracy
- Check sensor drift/aging if both process and cal are high
Field tip: If the analyzer is high only when sample flow is low, suspect a leak at a fitting or drain point. Fix flow first and re-check bias.