What are the Differences between Optical, Galvanic, and Polarographic DO Measurement?

Dissolved oxygen (DO) is one of the most critical parameters in liquid analysis. It directly affects biological activity, chemical reactions, corrosion rates, and process stability across industries such as water and wastewater treatment, aquaculture, food & beverage, pharmaceuticals, and industrial process control.

Inaccurate DO measurement can lead to poor treatment efficiency, product quality risks, excessive energy consumption, or regulatory non-compliance. As process automation advances, people no longer focus only on “whether oxygen is present,” but on how precisely and reliably it is measured over time.

So far, there are three mainstream technologies used to measure dissolved oxygen concentration:

• Optical (luminescent) measurement

• Galvanic electrochemical measurement

• Polarographic electrochemical measurement

Each method has its own working principle, performance characteristics, and ideal application scenarios. Understanding their differences is essential when selecting the right DO sensor or controller for a specific process. Let’s dive into the main topic!

What Is the Optical Measurement Method of DO?

Optical dissolved oxygen measurement is an accurate DO monitoring method based on luminescence quenching technology instead of the electrochemical reaction.

An optical DO sensor or controller contains a sensing cap coated with an aluminous dye. When getting excited by a light source (usually an LED), the dye emits light. Then, the dissolved oxygen molecules reduce (quench) the intensity or lifetime of this emitted light. The sensor ultimately converts this change into a DO concentration value, which is what the operator requires for further process management.

The optical measurement method does not consume oxygen during operation, ensuring accurate readings even in low-flow or static conditions. Since most optical DO sensors operate without electrolytes or membranes, routine replacement is typically unnecessary, significantly reducing maintenance effort.

Additionally, optical measurement exhibits minimal dependency on flow velocity and offers high long-term stability, making it well-suited for continuous and reliable online monitoring.

Thanks to these characteristics, the optical DO measurements are widely valued in applications where maintenance access is limited or frequent recalibration is impractical. Typical use cases include municipal and industrial wastewater treatment, environmental water quality monitoring, aquaculture and fish farming, and long-term online monitoring systems that require stable and reliable performance over extended periods.

What is the Galvanic Measurement Method of DO?

Galvanic DO measurement is an electrochemical method that generates its own electrical signal through a chemical reaction.

A galvanic sensor consists of a cathode, an anode, an electrolyte, and an oxygen-permeable membrane. Once the dissolved oxygen diffuses through the membrane, it reacts at the cathode, which produces a current. This current is proportional to the oxygen concentration.

Galvanic dissolved oxygen measurement is based on an electrochemical reaction in which oxygen is consumed during the measurement process. Unlike polarographic sensors, galvanic sensors are self-polarizing, meaning they do not require a warm-up time and can begin measuring immediately once immersed in the sample. This allows for a faster initial response, which is especially useful in applications that demand a quick startup.

However, because oxygen is consumed during measurement, galvanic sensors require a stable flow condition to maintain accuracy. They also rely on membranes and electrolytes, which must be replaced periodically as part of routine maintenance. Despite these requirements, galvanic DO sensors are widely used due to their simplicity and immediate readiness.

As a result, galvanic measurement is commonly applied in field measurements and portable dissolved oxygen meters, aquaculture systems, and water treatment processes where flow conditions are relatively stable and a rapid response is important.

What is the Polarographic Measurement Method of DO?

Polarographic dissolved oxygen measurement is one of the earliest and most traditional electrochemical methods used for dissolved oxygen analysis.

A polarographic sensor has a structure similar to a galvanic sensor, consisting of an anode, a cathode, an electrolyte, and a gas-permeable membrane. However, it differs in that an external polarization voltage must be applied between the anode and cathode before measurement can begin.

After the polarization (warm-up) period, dissolved oxygen diffuses through the membrane and is reduced at the cathode, generating an electrical current proportional to the oxygen concentration in the sample. Because oxygen is consumed during this electrochemical reaction, the measurement is influenced by sample flow rate, membrane condition, and maintenance status.

Polarographic sensors are a mature and well-understood technology with decades of application in both laboratory and industrial environments. When operated under controlled conditions and supported by regular maintenance, they are capable of delivering reliable and accurate dissolved oxygen measurements.

Due to these characteristics, polarographic DO sensors are commonly used in laboratory analysis, controlled industrial processes, and applications where maintenance schedules are well managed and operating conditions remain stable.

What’s the Difference Between Optical, Galvanic, and Polarographic DO Measurement?

Although optical, galvanic, and polarographic methods all measure dissolved oxygen concentration, they differ significantly in measurement principles, operational behavior, and maintenance requirements.

From a measurement standpoint, optical sensors rely on physical luminescence quenching and do not involve any electrochemical reaction. Galvanic sensors generate a measurement signal through an electrochemical reaction that produces its own current, while polarographic sensors also use an electrochemical reaction but require an external polarization voltage to operate.

These differences directly affect oxygen consumption during measurement. Optical sensors do not consume oxygen, whereas both galvanic and polarographic sensors consume a small amount of oxygen as part of the electrochemical process.

Maintenance requirements also vary by technology. Optical sensors typically require minimal maintenance, as most designs do not involve electrolyte or membrane replacement. Galvanic sensors demand moderate maintenance due to periodic replacement of electrolytes and membranes, while polarographic sensors generally require higher maintenance, including polarization management, membrane care, and regular calibration.

In terms of response behavior, optical sensors provide stable readings without any warm-up time. Galvanic sensors offer fast startup because they are self-polarizing, whereas polarographic sensors require a warm-up period before accurate measurement can begin.

Flow dependency is another important consideration. Optical sensors show minimal sensitivity to flow conditions, galvanic sensors exhibit moderate flow dependency, and polarographic sensors are more sensitive to sample flow rate and membrane condition.

In practice, no single dissolved oxygen measurement method is universally superior. The optimal choice depends on specific process conditions, available maintenance resources, accuracy expectations, and the total cost of ownership over the instrument’s lifecycle.

Optical, Galvanic, and Polarographic DO Controllers and Sensors

Modern process automation systems commonly integrate dissolved oxygen sensors into controllers, transmitters, and multi-parameter analyzers to enable continuous monitoring and closed-loop control.

On platforms like this, dissolved oxygen solutions are designed to support multiple DO measurement technologies while seamlessly combining with pH, conductivity, turbidity, and ORP measurement in a single system. Standard industrial communication outputs and robust signal processing ensure reliable operation in demanding industrial and environmental conditions.

In practice, optical DO sensors are typically selected for long-term and low-maintenance applications, while galvanic and polarographic sensors remain practical options for cost-sensitive installations or portable measurement where routine electrochemical maintenance is acceptable. Choosing the appropriate DO measurement method based on process conditions helps ensure stable data, efficient operation, and dependable automation performance.