Decision Tree: Calibration Failure
Symptom: zero/span won’t stabilize, cal fails, or cal deviation out of limit.
Rule: Verify gas + flow + manifold integrity before adjusting calibration values.
Isolation steps
1) Confirm correct calibration gases:
- Correct cylinder label and concentration
- Certificate valid (not expired)
- Regulator not leaking
2) Confirm stable flow on calibration gas (same as normal sample flow stability).
3) Check calibration manifold/valves:
- Leaking solenoid valve or manual valve causes mixing → unstable cal
- Check for incorrect valve lineup (sample + cal mixing)
4) Confirm warm-up time and temperature stability (if heated sensor).
5) If still fails:
- Inspect sensor condition / age
- Check wiring and analog scaling (wrong range can look like cal fail)
Field tip: A leaking cal valve often causes slow drifting during cal. Fix leakage and repeat before changing span factors.