Servomex 4900 — Multigas Process Analyzer

Continuous multigas analyzer using paramagnetic oxygen and NDIR infrared measurement technologies. Widely used in hydrogen plants, refineries, SMRs, and air separation units.

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Technician rule: If readings are unstable, always check sample conditioning, pressure stability, and moisture before calibration or sensor replacement.

Working Principle

The Servomex 4900 uses two independent physical principles for continuous gas measurement.

  • Paramagnetic O₂: Oxygen molecules are attracted into a magnetic field, generating a signal proportional to concentration.
  • NDIR (CO / CO₂): Infrared absorption at specific wavelengths determines gas concentration.

Main Components

  • Sample conditioning: filters, regulators, drains.
  • Flow control: capillaries / flow restrictors.
  • Measurement modules: paramagnetic O₂ sensor, IR cell.
  • Electronics: signal conditioning & processing.
  • Controller & comms: display, I/O, Ethernet.

Calibration Procedure

1) Confirm analyzer is stable (no flow alarms).
2) Verify sample & calibration gas pressure.
3) Apply zero gas and allow stabilization.
4) Apply span gas and verify response.
5) Return to process and confirm DCS values.

Major Troubleshooting

  • Unstable readings: pressure hunting, wet sample, leaks.
  • Slow response: blocked filters or restricted flow.
  • High drift: sensor ageing or contamination.
  • No DCS update: comms, scaling, or quality-bit issues.
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Maintenance (Field Practice)

  • Daily: Check sample flow, pressure, and alarms.
  • Weekly: Inspect filters, drains, and fittings.
  • Monthly: Leak test sample system, verify calibration.
  • Annually: Sensor health check and validation.
Poor maintenance causes more analyzer faults than electronics failure.

Discussion

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