Dissolved Oxygen Sensor (DO): Principle, Faults & NiuBoL Fluorescence Method Application Guide

1. What is Dissolved Oxygen (DO)? The “Barometer” of Water Body Health

We often say “water is the source of life,” and dissolved oxygen (Dissolved Oxygen, abbreviated as DO) is undoubtedly the key “lifeline” maintaining healthy operation of aquatic ecosystems.

1.1 Definition of Dissolved Oxygen

Dissolved oxygen refers to molecular oxygen (O₂) dissolved in water. It mainly comes from two pathways:

• Atmospheric dissolution: Oxygen in air continuously dissolves into water through water surface contact with atmosphere.

• Aquatic plant photosynthesis: Algae and aquatic plants in water release oxygen through photosynthesis under light.

Dissolved oxygen content is usually expressed in milligrams of oxygen per liter of water (mg/L).

1.2 Key Factors Affecting Dissolved Oxygen

Dissolved oxygen content in water is not constant and is closely related to the following factors:

• Water temperature: Negatively correlated with dissolved oxygen. Lower temperature means higher oxygen solubility; higher temperature means lower solubility — the main influencing factor.

• Atmospheric pressure/altitude: Higher pressure means greater oxygen partial pressure and higher solubility.

• Salinity: Higher salinity means lower dissolved oxygen saturation.

• Water disturbance/reoxygenation capacity: Water flow speed, waterfalls, aeration, etc., accelerate oxygen entry from air into water.

1.3 Dissolved Oxygen & Water Self-Purification Capacity

Dissolved oxygen is a core indicator measuring water self-purification capacity.

Water self-purification refers to the process where water body, after receiving certain pollutants, gradually restores water quality to pre-pollution level through physical (dilution, sedimentation), chemical (oxidation, reduction), and microbial (decomposition) actions.

Organic pollutants in water require aerobic microorganisms for decomposition, consuming large amounts of dissolved oxygen.

• Sufficient DO: High dissolved oxygen content means high efficiency of aerobic microorganisms decomposing pollutants, strong water self-purification capacity.

• Insufficient DO (even anaerobic): Excessive pollutants cause sharp DO drop, water enters anaerobic state, decomposition slows, producing harmful gases like hydrogen sulfide (H₂S) and methane (CH₄), worsening water quality.

2. Traditional Measurement Methods & Technological Innovation for Dissolved Oxygen

2.1 Traditional Chemical Method

Traditional dissolved oxygen measurement is represented by iodometric method (e.g., azide modification method, Winkler method).

Principle: Add chemical reagents to fix dissolved oxygen in water, then determine oxygen content via titration.

Limitations: Complex steps, time-consuming, requires precise operation and professional reagents, cannot achieve continuous online real-time monitoring.

2.2 Electrochemical Probe Method (Polarographic/Galvanic Cell Method)

Principle: Oxygen permeates membrane and undergoes electrochemical reaction at electrode, generating current proportional to dissolved oxygen concentration.

Limitations: Requires electrolyte, prone to polarization, needs regular maintenance (replace electrolyte and membrane), measurement requires water flow over membrane (oxygen consumption), easily interfered by sulfides and other chemicals.

2.3 Fluorescence Method Dissolved Oxygen Sensor Technology (NiuBoL NBL-RDO-206)

Fluorescence method adopted by NiuBoL and other enterprises is a major innovation in dissolved oxygen measurement.

Principle: Based on fluorescence quenching principle in physics.

• Excitation light shines on fluorescent substance on fluorescent membrane cap surface.

• Fluorescent substance is excited and emits fluorescence.

• Oxygen molecules in water contact fluorescent substance, “quenching” or shortening fluorescence extinction time (phase difference).

• Detect phase difference between excitation light and fluorescence to calculate oxygen molecule concentration.

NBL-RDO-206 Dissolved Oxygen Sensor Technical Advantages:

• No oxygen consumption: No electrolyte needed, does not consume oxygen, unaffected by water flow speed.

• Low maintenance: No regular electrolyte replacement, simple maintenance, long fluorescent membrane cap lifespan (approximately 1 year under normal use).

• Strong anti-interference: Unaffected by sulfides and other chemicals, low drift, more accurate measurement.

• Integrated: Online real-time monitoring, built-in temperature compensation and salinity compensation.

3. NiuBoL NBL-RDO-206 Dissolved Oxygen Sensor Structure & Performance Analysis

NBL-RDO-206 is an integrated online fluorescence method dissolved oxygen sensor designed for continuous monitoring.

3.1 Core Structure & Features

3.2 NBL-RDO-206 Dissolved Oxygen Sensor Main Technical Performance Specifications

4. Common Faults & Troubleshooting Guide for NBL-RDO-206 Dissolved Oxygen Sensor

5. Dissolved Oxygen Sensor Selection & Usage Guide

5.1 Selection Considerations

• Measurement Principle: Prioritize fluorescence method (e.g., NiuBoL NBL-RDO-206) for low maintenance cost, strong anti-interference, no flow speed requirement, more suitable for long-term online monitoring.

• Accuracy & Range: Choose appropriate accuracy level based on application (e.g., high-precision aquaculture, general sewage treatment).

• Protection Level: Long-term outdoor or underwater use must choose IP68 high protection products.

• Output & Integration: Ensure output signal (e.g., RS-485 Modbus/RTU) compatible with your monitoring system.

• After-Sales & Maintenance Cost: Consider replacement cycle and cost of consumables like membrane caps.

5.2 Installation & Calibration Points

• Installation Location: Avoid dead water zones or bubble concentration areas, ensure probe fully contacts representative water body.

• Calibration: Fluorescence method sensors generally use two-point calibration (zero oxygen calibration and saturated oxygen calibration). Saturated calibration usually performed in air or saturated water sample, simple operation.

• Maintenance: Regularly check fluorescent membrane cap for dirt attachment, clean when necessary.

6. Important Application Scenarios for Dissolved Oxygen Sensors

Dissolved oxygen sensors have become indispensable tools in environmental monitoring and process control.

• Environmental/Water Quality Monitoring: Real-time monitoring of DO content in rivers, lakes, groundwater, surface water to judge water pollution degree and self-purification capacity — key indicator for water environment quality evaluation.

• Aquaculture: DO is primary factor affecting fish/shrimp growth and survival. Precise monitoring and control (e.g., automatic aeration) can significantly improve breeding efficiency and reduce mortality.

• Sewage Treatment:

• Biochemical treatment stage: DO control in aeration tank is crucial. Too low affects aerobic microbial activity; too high wastes energy. Sensor used for precise aeration control.

• Industrial Wastewater Treatment & Process Control: Used to monitor oxygen content in industrial circulating water, cooling water, pharmaceutical, biochemical, etc., processes.

Common Questions (FAQ)

Q: Does fluorescence method dissolved oxygen sensor require electrolyte replacement?
A: No. Fluorescence method is based on physical principle, no electrochemical reaction involved, no electrolyte needed, greatly reducing maintenance.

Q: Is NBL-RDO-206 affected by water flow speed?
A: No. Since it does not consume oxygen, no requirement for water body flow speed, can accurately measure static or low-flow water DO.

Q: How to calibrate fluorescence method dissolved oxygen sensor?
A: Mainly two-point calibration. Usually includes zero oxygen calibration (using zero oxygen water or solution) and saturated oxygen calibration (in humid air or saturated water).

Q: Why does sensor reading suddenly drop?
A: Possibly real DO drop in water body, e.g., nighttime aquatic plants not photosynthesizing; or sensor membrane cap surface severely contaminated, preventing oxygen effective contact with sensing layer.

Q: How often does dissolved oxygen sensor need cleaning?
A: Depends on water quality. In clean water may be several months once; in sewage or eutrophic water may need weekly or monthly cleaning to remove biofilm or dirt.

Q: How does temperature affect DO measurement? How does sensor handle it?
A: Water temperature significantly affects oxygen solubility. NBL-RDO-206 built-in Pt1000 temperature sensor automatically compensates temperature for DO readings, outputting corrected accurate DO values.

Q: Does salinity affect DO measurement?
A: Yes, higher salinity means lower DO solubility. NBL-RDO-206 supports built-in salinity compensation; users can set salinity parameter for accurate readings in seawater or high-salinity water.

Q: How to read RS-485 Modbus/RTU output data?
A: Need Modbus protocol-supported collector, PLC, or industrial computer to query sensor register addresses for dissolved oxygen and temperature values.

Q: How long can fluorescent membrane cap be used? How to judge replacement needed?
A: NiuBoL membrane cap lifespan approximately 1 year under normal use. When cleaning and recalibration still result in unstable or inaccurate readings, usually need replacement.

Q: Besides dissolved oxygen sensor, what other water quality monitoring products does NiuBoL have?
A: NiuBoL committed to smart environmental monitoring, product line covers wind, solar radiation & illuminance, atmospheric environment & gas, rain/snow/hail, soil moisture & soil quality, water quality (pH, turbidity, conductivity, etc.) sensors and overall solutions.

Q11: What certifications does NiuBoL have?
A11: CE, ISO9001, RoHS, and nationally recognized meteorological calibration certificates.

Summary: Choose NiuBoL for Precise Water Quality Dynamics Mastery

Dissolved oxygen, as core indicator of water self-purification capacity and water environment health, its monitoring precision and stability are crucial. Traditional chemical method is cumbersome, electrochemical method maintenance complex. Fluorescence method dissolved oxygen sensor represented by NiuBoL NBL-RDO-206, with advantages of no oxygen consumption, maintenance-free, strong anti-interference, high precision, and integrated design, has become the preferred tool for modern water quality online monitoring.

Whether in environmental monitoring, smart aquaculture, or fine sewage aeration control, choosing NiuBoL professional sensing technology enables more efficient and precise mastery of water quality dynamics, providing reliable data support for building healthy water ecological environment.

NBL-RDO-206 Online Fluorescence Dissolved Oxygen Sensor Data Sheet

NBL-RDO-206 Online Fluorescence Dissolved Oxygen Sensor.pdf