1、 Industry background and equipment importance
1.1 Significance of dissolved oxygen (DO) monitoring
Dissolved oxygen (DO) refers to the content of free oxygen in water, which directly affects:
Self purification capacity of water bodiesAdequate DO can promote aerobic microbial degradation of organic matter.
Survival of aquatic organismsFish and other aerobic organisms have low requirements for dissolved oxygen (DO) (generally ≥ 5 mg/L).
Industrial water treatmentSewage treatment aeration control, oxygen supply during fermentation process, boiler water anti-corrosion control, etc.
1.2 Application Fields of Online DO Analyzer
sewage treatment plantReal time monitoring of aeration tank, anaerobic tank, and outlet to optimize aeration energy consumption.
Drinking water plantMonitoring of dissolved oxygen (DO) in source water and factory water to prevent iron and manganese oxidation and microbial growth.
industrial wastewaterControl of high oxygen consuming processes in chemical, pharmaceutical, food and beverage industries.
aquacultureMonitoring of ponds and circulating water systems to ensure the health of aquaculture water bodies.
environmental monitoringLong term ecological monitoring of rivers, lakes, and ocean stations.
1.3 Common Measurement Techniques
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| Electrochemical method (membrane electrode) | Oxygen is reduced at the cathode through the breathable membrane, generating a current signal | Widely applicable, requiring regular replacement of membranes/electrolytes |
| Optical method (fluorescence quenching) | The luminescence intensity of fluorescent substances quenched by oxygen molecules is inversely proportional to DO | Long maintenance free period, not affected by H ₂ S interference, suitable for harsh environments |
| Similar to electrochemical method, but using polarization voltage | High precision, with certain requirements for flow rate |
2、 Overview of Working Principle (Taking Electrochemical Membrane Electrode as an Example)
The water sample enters the electrode chamber, and oxygen passes through the breathable membrane into the internal electrolyte.
Reduction reaction occurs on the working electrode (cathode):
O2+4 H++4e− →2H2O
The magnitude of the generated current is proportional to the oxygen content passing through the membrane, and is converted into DO concentration (mg/L or ppm) through the circuit.
Temperature and atmospheric pressure can affect oxygen solubility, and the instrument has built-in temperature and pressure compensation algorithms.
3、 Key points of daily maintenance
3.1 Daily Inspection
Reading checkObserve whether the DO value is reasonable (compared with process expectations or historical trends), and promptly identify any abnormal fluctuations.
Electrode appearanceCheck if the membrane surface is damaged, contaminated, or if bubbles are attached.
Flow pool/immersion sleeveConfirm that there is no blockage, biofilm or sediment accumulation.
Temperature/pressure compensationConfirm that the sensor temperature is normal and the air pressure value is entered correctly.
3.2 Weekly maintenance
Clean the surface of the membraneGently wipe the film with a soft sponge or cloth dipped in clean water to avoid scratches.
Check the electrolyte level(Electrochemical method): Supplement the electrolyte specified by the manufacturer when insufficient.
Check cables and connectorsPrevent water ingress or corrosion to ensure stable signal transmission.
Flow system flushingRinse the sampling tube and circulation pool with clean water to prevent microbial growth.
3.3 Monthly maintenance
Replace the membrane cap/membrane component(Electrochemical method): Membrane fouling or aging can reduce response speed and accuracy.
Calibrate zero and range(See Part Four for details).
Check the flow rateEnsure that the water sample flows through the electrode surface at a speed within the manufacturer's recommended range (to avoid errors caused by stagnant water).
System disinfection (optional)Food grade fungicides can be used for short-term cyclic cleaning in environments prone to bacterial growth.
3.4 Quarterly/Annual Maintenance
Thoroughly inspect sensors and transmittersIncluding circuit boards, wiring terminals, and seals.
Replace consumablesElectrolyte, membrane cap, O-ring, etc.
performance verificationCompare with laboratory standard methods (such as iodometric method), and the deviation should be within the allowable range (± 0.3 mg/L or ± 5%).
Software/firmware backupSave configuration parameters for easy fault recovery.
4、 Calibration methods and steps
4.1 Calibration Types
Zero CalibrationDetermine the signal value of DO=0 in an anaerobic environment.
Span CalibrationCalibrate the full-scale response in a standard environment with known oxygen concentration.
4.2 Electrochemical Calibration Procedure (Example)
preparation
Perform zero calibration using an anaerobic solution (such as a 5% Na ₂ SO3 solution).
Use saturated air-water (approximately 8.24 mg/L at 25 ℃ under normal pressure) or a standard oxygen solution of known concentration for range calibration.
Ensure that the water temperature and air pressure sensors are functioning properly.
zero point calibration
Immerse the electrode in an anaerobic solution and gently stir to ensure uniformity of the solution.
Wait for the reading to stabilize (usually 1-3 minutes) and execute the zero calibration command.
Record the zero signal value and check for drift.
Range calibration
Place the electrode in saturated air-water and perform range calibration after the reading stabilizes.
If using standard oxygen solution, it is necessary to ensure that the oxygen concentration of the solution is known and the temperature has been compensated.
verify
After calibration, verify with another water sample of known oxygen concentration, and the error should meet the accuracy requirements of the instrument.
Optical sensorMost sensors use single point air calibration (automatic calibration by exposing the sensor to air), which generally does not require zero point calibration and only requires regular comparison and verification.
4.3 Precautions
Preheat and stabilize the sensor in the test medium before calibration (usually 10-15 minutes).
Avoid using electrochemical methods in water containing high concentrations of H ₂ S, SO ₂, and organic solvents, as this can lead to membrane poisoning.
The anaerobic solution and saturated air-water used for calibration must be freshly prepared to avoid failure.
Record the time, operator, and environmental conditions of each calibration for traceability purposes.
5、 Common faults and troubleshooting
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| The reading is abnormally high/low | Membrane fouling or aging, insufficient electrolyte, calibration failure | Clean or replace the membrane cap, replenish the fluid, and recalibrate |
| The reading fluctuates greatly | Bubble adhesion film surface, unstable flow rate, and temperature compensation failure | Remove bubbles, adjust flow rate, and check temperature sensor |
| Electrode breakage, cable damage, transmitter malfunction | Check wiring, replace electrodes or transmitters |
| Decreased membrane permeability and electrolyte degradation | Replace membrane components and electrolyte |
| Fluorescent cap contamination, light source attenuation | Clean the fluorescent cap and contact the manufacturer to replace the optical components |
6、 Management and optimization suggestions
Establish SOPDevelop detailed daily inspection, maintenance, calibration steps, and acceptance standards.
Preventive Maintenance PlanDevelop a cycle table based on usage frequency to avoid sudden failures affecting process control.
Data recording and analysisUse SCADA or LIMS systems to save historical data and promptly detect trend drift.
Spare Parts ManagementMembrane caps, electrolytes, fluorescent caps, etc. should have safety stock.
Regular comparisonQuarterly or semi annual validation with laboratory standard methods (such as Winkler titration) to ensure data reliability.
Train operators: Master the principles, calibration methods and safety precautions of different sensors.
7、 Conclusion
Online dissolved oxygen analyzer is an important equipment for ensuring accurate data in water treatment, industrial production, and ecological environment monitoring. passStandardized daily maintenance, regular calibration, and troubleshootingIt can ensure its long-term stable operation and measurement accuracy. Reasonably select based on the characteristics of different measurement techniques and integrate them into managementPreventive maintenance and data traceabilityIt can significantly improve the level of process control and the reliability of monitoring data.