Sampling System Failures
The sampling system is the most neglected part of an analyzer installation. More than 70% of analyzer issues originate upstream of the analyzer — not inside it.
Field rule:
If the sample is wrong, the analyzer can only report the wrong value accurately.
Typical Symptoms
- Unstable or noisy readings
- Slow analyzer response
- Frequent calibration failures
- Analyzer works with calibration gas but not process gas
- Random alarms with no process change
Common Root Causes
- Plugged or partially blocked sample probes
- Condensation in sample lines
- Leaks allowing ambient air ingress
- Incorrect sample line material
- Long sample transport time
- Incorrect probe location or orientation
Frequent Sampling Failure Modes
- Water carryover: floods filters, sensors, and valves
- Dust carryover: blocks probes and filters
- Adsorption: reactive gases lost in tubing
- Air dilution: leaks reduce gas concentration
- Dead volume: causes slow and misleading readings
Correct Troubleshooting Order
1) Inspect probe condition and insertion depth
2) Check sample line routing, slope, and insulation
3) Verify all fittings for leaks
4) Inspect filters, knock-out pots, and drains
5) Measure actual sample flow and response time
6) Only then inspect analyzer internals
Common Design Mistakes
- Using tubing incompatible with H₂S or NH₃
- No provision for moisture removal
- Overly long sample lines
- No sample flow indication
- Ignoring ambient temperature effects
Best Practices
- Design sampling systems before selecting analyzers
- Always assume moisture and dust are present
- Keep sample paths short and simple
- Install visible flow indication
- Treat sampling as critical instrumentation