Definition
Optical dissolved oxygen meter is an analytical instrument based on the principle of fluorescence quenching for measuring the concentration of dissolved oxygen in liquids. It indirectly calculates the oxygen molecular content by detecting the fluorescence lifetime or intensity change of a specific substance under the action of excitation light, which is a physical optical detection method. This instrument typically consists of a sensor probe, signal processing unit, and display output module, making it suitable for a variety of water quality monitoring scenarios.
Principle
The core principle of optical dissolved oxygen meters is the fluorescence quenching effect. The surface of the sensor probe is covered with a fluorescent film containing a fluorescent dye that is sensitive to oxygen. When a specific wavelength of excitation light hits the sensitive membrane, the dye molecules are excited to fluoresce. If oxygen molecules are present in the environment, collisions with excited dye molecules will cause a decrease in fluorescence intensity or a shortened lifetime, a process called dynamic quenching. There is a quantitative relationship between dissolved oxygen concentration and fluorescence quenching degree, which can be described by the Stern-Volmer equation:
I0/I = τ0/τ = 1 + KSV· [O2]
Among them I0and τ0The fluorescence intensity and lifetime under oxygen-free conditions, I and τ are the measured fluorescence intensity and lifetime, respectively, KSVis the quenching constant, [O2] is the dissolved oxygen concentration. The instrument calculates the dissolved oxygen concentration value by measuring the changes in fluorescence parameters.
Measurement method
The measurement process of optical dissolved oxygen meter is divided into two stages: signal acquisition and data processing. The sensor probe projects excitation light onto the sensitive membrane, and the photodetector receives the fluorescence signal synchronously and converts it into an electrical signal. The signal processing unit uses phase detection or intensity modulation techniques to calculate the fluorescence lifetime or relative intensity value, and then inverts the dissolved oxygen concentration according to the preset calibration curve. Calibration is carried out at two points before measurement: in a zero-oxygen environment (e.g., sodium sulfite solution) and in a saturated oxygen environment (e.g., air-saturated water). Some instruments support automatic temperature compensation and salinity correction to adapt to different water conditions.
Influencing factors
The measurement accuracy of optical dissolved oxygen meters is influenced by various factors. Changes in ambient temperature can change the quenching properties of fluorescent dyes and the diffusion rate of oxygen molecules, which need to be compensated in real time by built-in temperature sensors. Water pressure has an impact on the oxygen permeability of sensitive membranes, so it is necessary to choose a probe with strong pressure adaptability when measuring deep water. Dirt, bubbles, or mechanical damage to the surface of the sensitive membrane can impede the optical path and oxygen transmission, requiring regular cleaning and maintenance. In addition, certain chemicals in the water (e.g., hydrogen sulfide, chlorine) may react with fluorescent dyes and interfere with the measurement results. The long-term stability of the instrument is also related to the photodegradation properties of the fluorescent dye.
Application:
Optical dissolved oxygen meters have a wide range of applications in environmental monitoring, aquaculture, sewage treatment, industrial process control, and other fields. In the water quality monitoring of rivers, lakes and oceans, the spatio-temporal distribution of dissolved oxygen can be continuously monitored to evaluate the self-purification capacity and ecological health of water bodies. It is used in aquaculture to monitor the dissolved oxygen level of aquaculture ponds in real time to ensure the survival needs of aquatic organisms. The aeration tank and secondary sedimentation tank of the sewage treatment plant optimize the aeration energy consumption and improve the biochemical treatment efficiency through dissolved oxygen data. Cooling water systems in the fermentation industry and power industry also rely on dissolved oxygen monitoring for process control. It does not require electrolyte and has a long maintenance cycle, making it suitable for long-term deployment monitoring scenarios.
Selection
When choosing an optical dissolved oxygen meter, you should consider the measurement needs and environmental conditions. For laboratory intermittent measurements, portable instruments can be selected, focusing on ease of operation and data storage functions. Long-term online monitoring needs to pay attention to the anti-fouling ability, signal stability and protection level of the probe, and the pressure compensation range needs to be confirmed for deep-sea measurement. The measurement range typically covers 0-20 mg/L, and the upper limit of the volume range needs to be confirmed for high concentrations. The response time parameter affects the dynamic monitoring ability, and the model with short response time should be selected for rapidly changing scenarios. The calibration method, communication interface, power configuration, etc. of the instrument also need to match the actual usage scenario. It is recommended to refer to international standards such as ISO 5814, ASTM D888, etc. to verify the performance of the instrument.