When performing water quality testing using the COD rapid digestion spectrophotometry method, frequent fluctuations or abnormal variations in instrument readings can directly compromise the accuracy of results, making it impossible to correctly determine the COD content of the water sample. To rapidly identify the cause of the fault and resolve the issue of reading fluctuations, targeted troubleshooting and remediation methods have been compiled based on practical testing experience; these are suitable for routine COD testing operations and maintenance.
I. Reading Fluctuations Caused by Instrument Hardware Faults (No-Load Anomaly)
Abnormal Phenomenon
After the instrument has warmed up, the absorbance reading fluctuates continuously and unstably while the colorimetric cell is empty (no-load/air blank state).
Troubleshooting and Remediation Steps
1. Strictly follow equipment specifications to fully warm up the instrument for 15 minutes. This ensures the light source and optical path system reach a stable operating state, preventing light source fluctuations caused by insufficient warm-up.
2. Visually inspect the interior of the colorimetric cell. Clean away any residual water stains, dirt, dust, or debris from the digestion process to ensure the cell is clean, dry, and unobstructed.
3. Empty the colorimetric cell to maintain a no-load state, perform the “air blank” (zeroing) operation, and continuously monitor the no-load absorbance data for changes.
4. If the absorbance reading continues to fluctuate and fails to stabilize after zeroing, the issue is likely a hardware fault within the instrument itself—most probably due to light source aging, optical path misalignment, or sensor failure. Testing must be stopped immediately, and the equipment’s after-sales engineer should be contacted for repair and calibration.
II. Continuous Upward Fluctuations Caused by Excessive Chloride Ions
Abnormal Phenomenon
When the treated water sample is poured into the cuvette for measurement, the instrument reading rises continuously without stabilizing, exhibiting significant fluctuations.
Cause of Fault
Excessive chloride ion content in the water sample is the primary cause. In COD… In rapid digestion reaction systems, high concentrations of chloride ions cause interference by continuously consuming the oxidizing agent and generating colorimetric artifacts. This destabilizes the detection system, leading to fluctuating absorbance values and erratic readings—a common issue when testing sewage, seawater, or saline industrial wastewater.
Troubleshooting and Remediation Steps
1. Assess the water sample type; prioritize checking for chloride interference in saline wastewater, chemical wastewater, seawater, and similar samples.
2. Perform a low-ratio dilution (selecting the ratio based on the sample’s contamination level) to reduce the chloride ion concentration and eliminate interference with the digestion and color-development reactions.
3. After dilution, repeat the digestion, cooling, and measurement processes to verify if the readings have stabilized.
III. Fluctuating readings caused by inadequate mixing or insufficient cooling of the water sample
Anomalous Phenomenon
After the sample is placed in the instrument for measurement, the reading fluctuates repeatedly within a narrow range—neither continuously rising nor falling—preventing a stable reading.
Causes
On one hand, uneven distribution of substances following digestion—combined with failure to mix the sample thoroughly—leads to inconsistent concentrations within the colorimetric cell. On the other hand, incomplete cooling results in excessively high sample temperatures; thermal convection destabilizes the optical path, while insufficient water levels in the cooling bath compromise overall cooling efficiency.
Troubleshooting and Remediation Steps
1. Check the water level in the instrument’s cooling bath and ensure it reaches the standard fill line; low water levels reduce cooling efficiency and slow down sample cooling, so top up with purified water to the mark as needed.
2. Ensure that digested samples have been allowed to cool to room temperature in the cooling bath; never place high-temperature samples directly into the instrument for measurement.
3. After performing the steps above, re-measure the sample; the reading will stabilize quickly, eliminating the issue of fluctuating values.
IV. Continuous downward fluctuation of readings caused by the inclusion of settled bottom residue
Symptom
During measurement, the reading continuously and slowly drops, failing to stabilize at a fixed value.
Cause
After high-temperature digestion, small amounts of flocculent precipitate, metal oxides, or reaction residues form at the bottom of the digestion tube. If the sample containing this sediment is poured directly into the cuvette, the suspended particles continue to settle, causing the light transmittance within the cuvette to change constantly; this results in a continuous downward fluctuation of the absorbance reading.
Troubleshooting and Remediation Steps
1. After the digestion reaction is complete, do not shake the sample vigorously while it is cooling; allow sufficient time for solid residues to settle completely to the bottom of the tube.
2. Pour only the clear supernatant into the cuvette, ensuring that no sediment or residue from the bottom is transferred.
3. Proceed with the measurement immediately after loading the sample to resolve the issue of fluctuating readings and obtain stable, accurate COD data.
Post time: Sep-17-2026
