1、 What is an online dissolved oxygen analyzer?

　　Online dissolved oxygen analyzerThe Online Dissolved Oxygen Analyzer is an automated instrument capable of real-time, continuous, and online monitoring of dissolved oxygen (DO) concentration in water bodies.

Dissolved oxygen (DO) refers to the molecular oxygen (O ₂) dissolved in water, which is an important indicator for measuring the self purification capacity, biochemical activity, and ecological health status of water bodies.

The analyzer is usually installed directly on water pipelines, aeration tanks, rivers, reservoirs, sewage treatment facilities, etc. It collects real-time data through sensors and transmits signals to digital display instruments, PLC, DCS or upper computer systems to achieve remote monitoring and data management.

2、 Why is dissolved oxygen a key indicator of water quality health?

Dissolved oxygen (DO) is of great significance for the ecology and process operation of various types of water bodies

Water type	The Importance of Dissolved Oxygen
Natural water bodies (rivers, lakes, oceans)​	Dissolved oxygen is a necessary condition for the survival of aquatic organisms such as fish, shrimp, and microorganisms; Low dissolved oxygen (DO) can cause suffocation of fish, eutrophication of water bodies, and black and odorous odors
Wastewater treatment (aeration tank, activated sludge process)​	Microorganisms require sufficient dissolved oxygen (DO) for aerobic decomposition of organic matter. Insufficient DO can affect treatment efficiency, while excessive DO can waste energy
Industrial cooling water system​	Excessive dissolved oxygen can accelerate the flow of pipelines and equipmentOxygen corrosion, affecting the lifespan of the system
Boiler and heating system​	Dissolved oxygen is causingBoiler corrosion and scalingOne of the main factors that needs to be strictly controlled
aquaculture​	The amount of dissolved oxygen in water directly affects the growth, feeding, and immunity of aquatic products such as fish and shrimp. Insufficient dissolved oxygen can easily lead to diseases and even death
Drinking water source area​	Dissolved oxygen is one of the important parameters for evaluating water quality and self purification capacity of water sources

3、 Working principle of online dissolved oxygen analyzer

The current mainstream online dissolved oxygen analyzers are mainly based on the following two technical principles:

1. Electrochemical method (polarographic method/primary cell method) - commonly used

✅ Principle:

Using dissolved oxygen to diffuse through the breathable membrane to the electrode surface, a reduction reaction occurs on the electrode, generating a current signal proportional to the concentration of dissolved oxygen. After amplification and calculation, the DO concentration value is obtained.

🧩 Main components:

• Electrode (sensor): including cathode (gold or platinum), anode (silver), electrolyte (such as gel) and oxygen permeable membrane (such as polytetrafluoroethylene membrane);

• Transmitter/transmission module: converts electrical signals into dissolved oxygen concentration values and performs temperature and pressure compensation;

Signal output: 4-20mA, RS485, Modbus, etc., can be connected to PLC and SCADA systems.

🌟 Features:

Fast response speed and accurate measurement;

Suitable for most water quality environments;

Regular maintenance is required (such as membrane replacement, electrolyte replenishment, calibration);

It can be divided into polarographic formula (requiring polarization voltage) and galvanic formula (without external voltage).

2. Optical method (fluorescence method/fluorescence quenching method) - emerging mainstream technology

✅ Principle:

By using blue light of a specific wavelength to excite dissolved oxygen sensitive fluorescent substances (such as ruthenium complexes), the fluorescence intensity or lifetime is affected by the "quenching effect" of dissolved oxygen in water. The dissolved oxygen concentration is calculated by detecting changes in the fluorescence signal.

🧩 Main components:

Optical sensor: including blue LED light source, photodetector, and fluorescent sensitive film;

Intelligent transmitter: processes fluorescence lifetime/intensity signals, calculates DO values, with temperature compensation;

• Film free and electrolyte free design, more stable and durable.

🌟 Features:

No need for electrolyte, no consumable film, and extremely low maintenance;

Strong anti pollution ability, suitable for long-term online monitoring;

High measurement accuracy, good stability, and fast response;

Gradually becoming a field for sewage treatment plants, industrial water systems, and online environmental monitoring.

4、 Typical structural composition of online dissolved oxygen analyzer

component	Functional Description
Sensor (probe)​	Direct contact with water samples for detecting dissolved oxygen, divided into two categories: electrochemical and optical
Transmitter/Control Unit​	Receive sensor signals, amplify, linearize, temperature compensate, and output signals
Signal output interface​	Such as 4-20mA analog signal, RS485, Modbus, HART protocol, used for remote transmission
Install components​	Including immersion, flow, insertion, pipeline installation brackets or fittings
Calibration and maintenance tools​	Such as air calibration kits, standard solutions, cleaning tools, etc. (especially electrochemical probes)

5、 Main performance parameters

parameter	Instructions
measurement range​	0~20 mg/L (common); Partially measurable 0~50 mg/L or higher
resolution​	0.01 mg/L or 0.1 mg/L
precision​	± 0.1~± 0.5 mg/L, depending on the model and calibration status
temperature compensation​	Automatic compensation (usually with built-in temperature sensor)
Pressure/salinity compensation​	Advanced model support to improve measurement accuracy in complex water bodies
response time​	Usually T90<60 seconds (optical method is faster)
Protection level​	IP65/IP68 (suitable for immersion installation)
output signal​	4-20mA、RS485、Modbus、 Relay alarm, etc
material​	Sensor: 316L stainless steel, titanium alloy (corrosion-resistant); Membrane material: polytetrafluoroethylene, etc

6、 Typical application scenarios of online dissolved oxygen analyzer

Application field	specific purpose
sewage treatment plant​	DO monitoring of aeration tank, optimizing aeration rate, energy saving and consumption reduction, ensuring microbial activity
Industrial cooling circulating water​	Monitor dissolved oxygen to prevent oxygen corrosion and extend equipment life
Boiler feedwater system​	Strictly control DO<7 ppb (μ g/L) level to prevent internal corrosion of the boiler
Surface water/river/lake monitoring​	Assess the risk of eutrophication and ecological health status of water bodies
aquaculture​	Real time monitoring of dissolved oxygen in aquaculture water to ensure the survival and growth environment of fish and shrimp
Waterworks/drinking water sources​	As one of the comprehensive evaluation indicators for water quality
Environmental Protection Online Monitoring Station​	As one of the core probes of online automatic water quality monitoring systems (such as surface water stations)

7、 Advantages of Online Dissolved Oxygen Analyzer

advantage	Instructions
✅real-time monitoring​	24-hour continuous online monitoring to timely grasp the trend of DO changes in water bodies
✅data accuracy​	High precision sensors and intelligent compensation algorithms ensure reliable measurement results
✅Remote transmission&automation control​	Can be connected to PLC/DCS/cloud platforms to achieve remote monitoring and intelligent control
✅Save manpower and costs​	Reduce the frequency of manual sampling and laboratory analysis
✅Warning and Control​	Can set upper and lower limit alarms, linked aeration equipment, dosing system, etc
✅Adaptable​	Suitable sensors can be selected according to different water bodies (fresh water, seawater, sewage, high temperature, etc.)

8、 Maintenance and Precautions

Electrochemical sensors require regular:

Replace the oxygen permeable membrane

• Supplement or replace electrolyte

• Perform zero oxygen and saturation calibration (air saturated water or Na ₂ SO3 solution)

• Optical sensors are easier to maintain:

Regularly clean the surface of the sensor

Generally, there is no need to replace the membrane and electrolyte, resulting in a longer lifespan

• Prevent sensor scaling, contamination, and bubble adhesion, which may affect measurement accuracy

Choose the appropriate installation method and location to avoid dead corners or strong impacts of water flow

9、 Summary

✅ Online dissolved oxygen analyzer is the "eye" that ensures the health and process stability of various water bodies. Through real-time, accurate, and continuous dissolved oxygen monitoring, it provides scientific decision-making basis and automatic control foundation for fields such as sewage treatment, industrial control, environmental supervision, aquaculture, and ecological protection.

With the development of optical dissolved oxygen sensing technology, modern online dissolved oxygen analyzers are moving towards more stable, intelligent, lower maintenance, and more adaptable to complex environments. They are important tools for building smart water management, intelligent environmental protection, and green industry.