At low concentrations, dissolved oxygen measurement becomes highly sensitive to sample handling, pressure changes, trapped gas, and flow-cell behaviour. The analyser may be healthy while the measurement environment is not.
That is why apparent instrument instability is often really a sample presentation problem.
Symptoms operators often see
Typical symptoms include drifting values, spikes after flow interruption, unstable readings after maintenance, or poor agreement with a downstream grab sample or laboratory instrument.
These symptoms do not all point to the same cause, so the sample path must be reviewed carefully.
Common root causes
Frequent causes include gas release during pressure reduction, trapped bubbles in the flow cell, poor flow pattern, contaminated wetted parts, or incorrect sensor conditioning. Sample stoppages can make these effects worse by allowing gas pockets to form.
Where gas collects, readings often rise sharply when flow restarts.
What to check first
Check cell orientation, venting, flow stability, pressure drop across the system, and whether the sample conditions encourage degassing. Then confirm sensor calibration and compare with an independent reference where possible.
The physical sample environment should be checked before assuming the sensor has failed.
What stable operation depends on
Stable low-level oxygen measurement depends on continuous flow, bubble-free presentation, controlled pressure reduction, clean wetted parts, and correct calibration practice.
When these conditions are controlled well, the analyser signal usually becomes stable as well.
