Yokogawa FLXA402 — pH / ORP Analyzer
Modular liquid analyzer for pH and ORP measurement. Used in utilities, refineries, chemical plants, water treatment, and critical process applications where electrode health and sample condition dominate measurement reliability.
Technician rule: 90% of pH problems are caused by
electrode condition, fouling, reference poisoning, or sample issues,
not the transmitter.
Working Principle
pH measurement is based on the potential difference between a measuring electrode and a reference electrode. The FLXA402 converts this millivolt signal into pH after temperature compensation.
- Glass electrode develops voltage proportional to hydrogen ion activity.
- Reference electrode provides stable comparison potential.
- Temperature sensor compensates slope changes.
- Transmitter converts mV → pH using calibration data.
Main Components
- pH / ORP sensor: glass bulb, reference junction, temperature element.
- Sensor cable: high impedance, moisture-sensitive.
- Process fitting: immersion, insertion, or flow assembly.
- FLXA402 transmitter: input module, CPU, display.
- Power & I/O: 4–20 mA, digital comms, alarm relays.
Calibration Procedure
1) Inspect sensor for coating, cracks, or dry bulb.
2) Rinse electrode with clean water (never wipe glass).
3) Perform 2-point calibration using fresh buffer solutions.
4) Allow readings to stabilize fully before accepting points.
5) Check slope and zero values; replace sensor if out of limits.
6) Return to process and monitor stability.
Major Troubleshooting
- Slow response: fouled electrode, aged glass, low temperature.
- Drifting pH: reference poisoning, junction blockage.
- Unstable reading: air bubbles, grounding issues, dry sensor.
- No change with process: broken glass bulb or cable fault.
Discussion
Share FLXA402 pH issues, electrode life problems, calibration drift, and real plant experiences. (Requires GitHub login.)