Dissolved oxygen (DO) analysis is essential in different industrial processes, such as water treatment, environmental control, fermentation, pharmaceutical industry and Water-Steam Cycle management.
Therefore, understanding the technologies available for DO measurement becomes essential, ensuring reliable results and more accurate operational decisions.
Currently, the most widely used solutions include sensors with polarographic (electrochemical) technology and optical sensors (based on luminescence or fluorescence quenching). Both options, however, have advantages and limitations, which vary depending on the application and process conditions.
What is dissolved oxygen?
First, it's important to understand the concept of dissolved oxygen in an aqueous medium. When water comes into contact with atmospheric air, it absorbs oxygen molecules and other gases until it reaches saturation point. In this process, oxygen dissolves in a similar way to a solid or liquid.
However, the solubility of gases depends strongly on temperature. While the solubility of solids increases with heat, that of gases decreases. In other words, the higher the temperature of the water, the lower the amount of oxygen it can hold.
Principles of measurement technologies
Polarographic (Electrochemical)
The polarographic sensor is one of the most traditional devices for measuring DO. It performs an electrochemical reaction with dissolved oxygen, generating an electrical signal proportional to its concentration.
Measurements are made using electrodes—cathode and anode—separated by an oxygen-permeable membrane. When an electrical voltage is applied to the cathode, the molecular oxygen is reduced, producing an electrical current. This current is then converted into a measurement signal.
Despite this, this type of sensor requires an internal electrolyte and an initial stabilization period.
Among the common examples, the following stand out: Hamilton sensors and Analyzer Dr. Thiedig.
Main features
- Requires internal electrolyte
- Requires initial stabilization phase
Example: Hamilton Sensors and Dr. Thiedig Analyzer
Optics (Luminescence or Fluorescence Quenching)
On the other hand, optical sensors operate based on the interaction between oxygen and fluorescent materials (luminophores). These materials, when exposed to light, absorb energy and enter an excited state. During this state, they interact with oxygen molecules.
Later, when the electrons return to their original state, they emit light through fluorescence. The presence of oxygen interferes with this process, reducing the intensity or duration of emission. This allows for highly accurate and fast calculations of DO concentration.
Common examples include the Dr. Thiedig Analyzer and Hamilton sensors.
Main features
- Direct and stable measurement
- Does not consume oxygen
- Ideal for environments with the presence of contaminants
Example: Dr. Thiedig Analyzer and HAMILTON Sensors
Technical comparison between technologies
Applications and recommendations
Polarographic technology
This technology is suitable for processes that require continuous measurements in solutions with medium or high DO levels. Furthermore, it's a viable option when the initial budget is limited, even if it requires frequent maintenance.
Optical technology
It is best suited for applications that require high precision, such as fermentation, pharmaceuticals, and water-steam cycle control. Additionally, it offers better performance in environments with extremely low DO levels or aggressive contaminants.
Alutal Expert Perspective
As a leader in industrial solutions, ALUTAL believes that choosing between polarographic and optical sensors should take into account several factors—not just cost and accuracy. Each project presents its own requirements. Therefore, understanding the environment, objectives, and operating conditions is essential before making a decision.
Analysis of polarographic sensors
Polarographic technology remains a reliable alternative, especially due to its competitive initial cost. In general, it is widely used in industries that operate under standard conditions and where periodic maintenance is not a problem.
Advantages of Polarographic Sensors
- Low initial cost
- Wide acceptance in the market
- Measuring range suitable for medium and high concentrations
Challenges of Polarographic Sensors
- Constant exchange of membranes and electrolyte
- Need for frequent calibration
- Lower durability in aggressive environments
- Oxygen consumption, which reduces accuracy in strokes
Analysis of optical sensors
Optical technology represents a breakthrough in the field of DO measurement. Its robust and precise structure favors critical applications with high control standards.
Advantages of Optical Sensors
- High stability and accuracy even at very low concentrations
- Reduced maintenance
- Greater durability in hostile environments
- Measurement without oxygen consumption
Optical Sensor Challenges
- high initial cost
- Excessive technology for simple processes, making the investment less justifiable
Therefore, ALUTAL recommends optical sensors for highly critical operations, while polarographic sensors remain suitable for more conventional applications. The company's experience allows it to analyze each case accurately, proposing solutions aligned with each client's requirements and context.
Finally, in partnership with leading suppliers, ALUTAL guarantees access to the best DO measurement technologies available on the market, offering complete technical support for system installation and maintenance.
Bibliography
- O2 Measurement Guide – Hamilton Company
- Operation Manual Digox optical KS – Dr. Thiedig
- Operation Manual Digox 6.1 KS – Dr. Thiedig
