I. Overview

　　Online dissolved oxygen analyzerIt is a type used forReal time and continuous monitoring of dissolved oxygen (DO) concentration in liquids (usually water or wastewater)The instrument is widely used inWastewater treatment, industrial process control, aquaculture, environmental monitoring, surface water/groundwater assessmentIn other fields. Dissolved oxygen (DO) is a key parameter for measuring the self purification capacity, biochemical reactivity, and water quality health status of water bodies. Accurate monitoring of its concentration is of great significance for ensuring stable process operation, optimizing resource utilization, and environmental protection.

　　Online dissolved oxygen analyzerBy directly contacting the measured medium through sensors, the dissolved oxygen concentration is converted into an electrical signal, which is processed by a transmitter and output as a digital or analog signal. It supports data recording, alarm, and remote monitoring, meeting the requirements of industrial processesHigh precision, high reliability, and continuous monitoringThe demand.

2、 Definition and Importance of Dissolved Oxygen

1. Definition of dissolved oxygen

Dissolved oxygen (DO) refers toMolecular oxygen (O ₂) dissolved in water or other liquidsUsually, inMg/L (milligrams per liter) or ppm (parts per million)expression Its content is affected by factors such as water temperature, pressure, salinity, and atmospheric pressure:

　　water temperatureThe higher the temperature, the lower the saturation concentration of dissolved oxygen in water (for example, the saturation DO is about 9.1 mg/L at 20 ° C and drops to 7.6 mg/L at 30 ° C);

　　pressureThe higher the altitude (lower the air pressure), the weaker the water's ability to dissolve oxygen;

　　salinityAn increase in salt content will reduce the solubility of oxygen (saturated dissolved oxygen in seawater is about 20% lower than in freshwater);

　　Atmospheric contactThe more complete the contact between water and air (such as aeration tanks and rivers), the closer the dissolved oxygen is to saturation.

The Importance of Dissolved Oxygen

　　sewage treatmentIn the activated sludge process, aerobic microorganisms rely on dissolved oxygen to decompose organic matter (such as COD/BOD), and DO concentration directly affects treatment efficiency and sludge activity;

　　industrial processIf the boiler feedwater requires extremely low DO (<0.01 mg/L) to prevent oxygen corrosion of metal pipelines; Chemical reactions such as fermentation and oxidation-reduction require precise control of DO concentration;

　　aquacultureFish and aquatic organisms rely on dissolved oxygen for respiration. Low dissolved oxygen (<3 mg/L) can lead to hypoxia induced death, while high dissolved oxygen promotes growth;

　　environmental monitoringThe DO level of rivers and lakes reflects the self purification capacity of water bodies (such as DO<2 mg/L indicating "black and odorous water body"), and is one of the core indicators for water quality evaluation.

3、 Working principle of online dissolved oxygen analyzer

The core of an online dissolved oxygen analyzer is toConversion of dissolved oxygen concentration into measurable electrical signalsThe mainstream technological routes includeElectrochemical method (primary battery method/polarographic method)andOptical method (fluorescence method)The two methods have significant differences in their principles and are suitable for different scenarios.

（1） Electrochemical Method

Electrochemical methodThe chemical reaction between electrodes and dissolved oxygen generates an electric current or potential difference, divided intoGalvanic Cell MethodandPolarographic MethodThe two have similar structures, but different driving methods.

1. Basic structure

Electrochemical sensors mainly consist of the following parts:

　　Breathable film (thin film)Covering the electrode surface, allowing oxygen molecules to pass through but blocking liquids and impurities (such as suspended solids and oils), protecting the internal electrolyte;

　　Electrode systemIncluding:Cathode (usually gold/platinum)andAnode (usually silver/lead), as well as internalElectrolyte (such as KCl solution)；

　　Signal processing moduleConvert the weak current/potential output from the electrode into a standard signal (such as 4-20 mA, 0-10 V).

2. Working principle subdivision

(1) Polarographic method

　　Drive Type: External application required0.6~0.8 V DC voltage(Polarization voltage), forcing the oxygen reduction reaction;

　　reaction process：

　　Cathode (gold/platinum)Oxygen obtains electrons at the cathode and is reduced to hydroxide ions (alkaline conditions) or water (acidic conditions). The reaction equation is:

O2+2H2O+4e − → 4OH − (alkaline)

Or $$O2+4H ^+4e ^ - \ \ rightarrow 2H2O \ \ quad (\ \ text {acidic})

Anode (silver): Silver is oxidized to silver chloride (Ag+Cl − → AgCl+e −), providing an electronic circuit; Signal output: The diffusion rate of oxygen is proportional to the concentration of dissolved oxygen in water, and the concentration of dissolved oxygen is reflected by the reduction current at the cathode (usually nA μ A).

(2) Galvanic Cell Method

　　Drive TypeNo external voltage required, relying onSpontaneous oxidation-reduction potential difference of electrode materialsDrive reaction (similar to battery principle);

　　reaction process：

　　Cathode (gold)Reduction of oxygen to hydroxide ions (same as polarography);

　　Anode (lead)Lead is oxidized to lead oxide (Pb → Pb ² ⁺+2e ⁻), which reacts with ions in the electrolyte to form insoluble substances (such as PbCl ₂);

　　signal outputThe spontaneously generated current is linearly related to the concentration of dissolved oxygen and does not require additional power supply, but the response speed is slightly slower than that of polarography.

3. Advantages and disadvantages of electrochemical method

advantage	shortcoming
Mature technology, low cost, suitable for most conventional scenarios	Breathable membranes are prone to contamination (such as oil and algae) and require regular maintenance or replacement
Wide measurement range (0-20 mg/L or higher)	Electrolyte needs to be regularly replenished (primary battery method) or electrodes are prone to aging (polarographic method)
Fast response time (usually 30-60 seconds)	Due to the significant influence of temperature and air pressure, compensation circuits are required

（2） Optical Method (Fluorescence Method)

Optical method based onSpecific interaction between fluorescent substances and dissolved oxygenIndirectly measuring DO concentration by detecting changes in fluorescence intensity is a new technology developed in recent years, representing the mainstream direction of online dissolved oxygen meters.

1. Basic principle (fluorescence quenching effect)

The surface of the sensor probe is coated withFluorescent dyes (such as ruthenium complex Ru (bpy) ∝² ⁺)When excited by a specific wavelength of blue light (about 470 nm), the dye emits red fluorescence (about 600 nm);

When dissolved oxygen molecules diffuse to the vicinity of the fluorescent layer, they will interact with the excited fluorescent dyeNon radiative energy transfer (fluorescence quenching)Leading to a decrease in fluorescence intensity or a shortened fluorescence lifetime;

　　The degree of decay in fluorescence intensity or lifetime is directly proportional to the concentration of dissolved oxygenThe more oxygen molecules there are, the stronger the quenching effect. By detecting changes in the fluorescence signal, the DO value can be calculated.

2. Sensor structure

　　Light source (LED): emits excitation light of a specific wavelength (usually blue light);

　　Fluorescent sensing filmA sensitive layer coated with fluorescent dye, in direct contact with the measured liquid;

　　Photodetector (photodiode)Receive fluorescent signals (red light) and convert them into electrical signals;

　　Compensation moduleCorrect the effects of temperature, air pressure, and light interference (such as real-time compensation with built-in temperature sensors).

3. Advantages and disadvantages of optical methods

4、 Typical application scenarios of online dissolved oxygen analyzer

　　1. Wastewater treatment (core application)

　　activated sludge processMonitor the DO concentration in the aeration tank (usually controlled at 2-4 mg/L) to ensure aerobic microbial activity and avoid "over aeration" (energy waste) or "hypoxia" (decreased degradation efficiency);

　　Anaerobic/anoxic unitControl denitrification, anaerobic digestion and other processes through low DO (<0.5 mg/L) or anaerobic environment;

　　effluent monitoringEnsure that the DO of the discharged water meets the standard (such as the requirement of ≥ 2 mg/L in the "Pollutant Discharge Standards for Urban Sewage Treatment Plants").

　　2. industrial process control

　　Boiler feedwaterMonitor the DO at the outlet of the deaerator (with a requirement of<0.01 mg/L) to prevent oxygen corrosion of the boiler pipeline;

　　Chemical reactionsSuch as fermentation process (DO needs to be maintained at 0.5-3 mg/L to balance bacterial activity), redox reaction (precise control of oxygen concentration);

　　Food and BeverageMonitor dissolved oxygen (DO) in the production of purified water and fruit juice to avoid product oxidation and deterioration.

　　3. Aquaculture and ecological monitoring

　　aquaculture waterReal time monitoring of dissolved oxygen (DO) in fish ponds and shrimp ponds (ideal range of 5-8 mg/L) to prevent oxygen deficiency and floating heads;

　　Surface water/groundwaterEvaluate the self purification capacity of rivers and lakes (DO ≥ 5 mg/L is considered a healthy water body), and monitor the risk of eutrophication (low DO may be accompanied by algal blooms).

　　4. other scenarios

　　Swimming Pool/SPAControl DO to inhibit microbial growth (usually ≥ 4 mg/L);

　　Marine explorationMonitor the distribution of dissolved oxygen (DO) in deep-sea or polar water bodies and study their ecological adaptability.

5、 Selection and usage precautions

　　1. Key parameters for selection

　　measurement rangeConventional wastewater (0-20 mg/L), high-precision scenarios (0-2 mg/L or 0-0.1 mg/L);

　　Technical TypeElectrochemical method (economically practical, frequently maintained) vs optical method (high-precision, low maintenance, suitable for long-term online monitoring);

　　output signal4~20 mA (industrial standard), RS485 (digital communication), HART protocol (remote configuration);

　　Protection levelIP65 (splash proof), IP68 (submersible type, used for deep wells/aeration tanks);

　　Temperature/pressure adaptabilitySpecial design is required for high temperature (>60 ° C) or high pressure (>1 bar) scenarios.

　　2. Key points for use and maintenance

　　electrochemical methodRegularly clean the breathable membrane (wipe with a soft cloth to avoid scratching), replenish electrolyte (original battery method) or replace electrodes (after aging);

　　Optical methodAvoid direct exposure to strong light on the probe and calibrate it regularly (usually every 3-6 months with air saturated water or zero oxygen solution);

　　Environmental interferenceAvoid strong electromagnetic fields and turbulent areas of bubbles during installation to ensure stable immersion depth of the sensor;

　　calibration: UseAir saturated water (close to 100% saturation)andZero oxygen solution (such as sodium sulfite solution)Perform two-point calibration to improve accuracy.

VI. Summary

Online dissolved oxygen analyzer is a core tool for water quality monitoring and industrial process controlElectrochemical method (primary battery/polarography)andOptical method (fluorescence method)Two types of technologies are used to achieve real-time and accurate measurement of dissolved oxygen concentration. among which

　　electrochemical methodWith mature technology and low cost, it is suitable for conventional sewage treatment and industrial scenarios, but attention should be paid to membrane fouling and maintenance costs;

　　Optical methodWith membrane free design, high anti-interference ability, and long-term stability, it has become an application (such as pharmaceutical and semiconductor pure water monitoring). Although the initial investment is relatively high, long-term operation and maintenance are more economical.

In the future, withMicro nano sensor technology, Internet of Things (IoT), and artificial intelligenceThe development of online dissolved oxygen analyzers will further advance towardsMiniaturization, intelligence (automatic diagnosis, predictive maintenance), multi parameter integration (such as real-time measurement of DO/pH/turbidity)The direction evolution provides stronger technical support for water resource management, industrial green production, and ecological protection.