Definition
An in-line dissolved oxygen meter is an analytical instrument used to continuously monitor the concentration of dissolved oxygen in liquids. Dissolved oxygen refers to oxygen dissolved in water or other liquids in a molecular state, and its content is one of the key parameters for evaluating water quality, controlling biochemical processes and ensuring industrial safety. The instrument measures in real time through sensors and transmits data to the control system, making it suitable for applications where continuous monitoring of dissolved oxygen is required.
Measurement principle
The measurement of an in-line dissolved oxygen meter is mainly based on electrochemical or optical principles. The principle of electrochemistry is usually the Clark electrode method, and at its core is a primary cell or polar sensor composed of a cathode, anode, and electrolyte. Oxygen diffuses into the sensor through a selective membrane, where a reduction reaction occurs at the cathode, producing a current signal proportional to the oxygen concentration. The optical principle is based on the fluorescence quenching effect, the fluorescent substance on the surface of the sensor fluoresces under the excitation of a specific wavelength of light, and its fluorescence life or intensity is affected by the concentration of dissolved oxygen, and the oxygen content can be calculated by detecting optical changes.
For the principle of electrochemistry, the relationship between current output and dissolved oxygen concentration can be approximated as:
I = k · C · P
where I is the output current of the sensor, k is the sensitivity coefficient, C is the dissolved oxygen concentration, and P is the permeability coefficient. This formula reflects the basic relationship between diffusion current and oxygen partial pressure and membrane properties.
Measurement method
The measurement methods of online dissolved oxygen meters can be divided into in-situ measurement and flow-through measurement. In-situ measurement immerses the sensor directly into the liquid to be tested and is suitable for stationary scenarios such as reaction tanks, channels or tanks. Flow-through measurement uses a sampling system to direct liquid to the sensor chamber for applications containing particulate matter or requiring pre-treatment. Both methods require regular calibration, typically zero-point calibration versus full-scale calibration to ensure measurement accuracy. The calibration process is based on standard methods such as slope calibration in air-saturated water and zero point calibration in an oxygen-free environment.
Influencing factors
The measurement of an online dissolved oxygen meter is influenced by a variety of factors. Temperature changes affect oxygen solubility and sensor response speed, and instruments often have built-in temperature compensation. Liquid pressure and flow velocity can change the rate of oxygen diffusion, especially in flow systems, hydrodynamic conditions need to be considered. Salinity or ionic concentration affects oxygen activity and requires salinity correction in high-salt solutions. In addition, aging, contamination or damage to the sensor membrane can lead to a decrease in sensitivity and require maintenance to ensure performance. Environmental factors such as atmospheric pressure fluctuations can also have an impact on the measured values.
Applications
Online dissolved oxygen meters are widely used in many fields. In environmental monitoring, it is used for continuous monitoring of dissolved oxygen in rivers, lakes and sewage treatment plants to evaluate the ecological health and treatment efficiency of water bodies. In aquaculture, the dissolved oxygen content of the aquaculture water is monitored to provide a basis for oxygenation control. In the food and beverage industry, it is used to monitor the fermentation process and ensure process stability. In industrial circulating water systems, monitoring dissolved oxygen helps control corrosion and microbial growth. In addition, dissolved oxygen data supports reaction kinetics studies in scientific research and chemical processes.
Selection considerations
Measurement needs and environmental conditions should be considered when selecting. According to the measurement range and accuracy requirements, choose the appropriate sensor type, such as electrochemical sensors are suitable for conventional monitoring, and optical sensors have certain advantages in low concentration or easily polluted environments. Consider installation options, such as immersion, plug-in, or flow pools, to accommodate on-site piping or vessel structures. The protection level and material of the instrument should match the site environment, such as corrosion-resistant housings suitable for chemical applications. At the same time, it is necessary to evaluate the maintenance needs, calibration intervals, and data output interfaces of the instrument to ensure compatibility with existing control systems. The stability and reliability of long-term use are also important reference factors for selection.