Definition
The fluorescence dissolved oxygen analyzer is an analytical instrument based on the principle of fluorescence quenching to measure the concentration of dissolved oxygen in liquids. It indirectly calculates the oxygen molecular content by detecting the fluorescence lifetime or intensity changes of a specific fluorescent substance under the action of excitation light. This instrument belongs to the category of optical sensors and is suitable for water quality analysis in various fields such as environmental monitoring, aquaculture, sewage treatment, and industrial process control.
Principle
The core principle of fluorescence dissolved oxygen analyzer is the fluorescence quenching effect. The surface of the sensor probe is covered with a fluorescent membrane containing oxygen-sensitive fluorescent dyes. When a specific wavelength of excitation light hits the fluorescent film, the dye molecules are excited to fluoresce. Oxygen molecules dissolved in water diffuse into the sensitive membrane and collide with the excited dye molecules, resulting in reduced fluorescence intensity or reduced fluorescence lifetime. The higher the oxygen concentration, the more significant the quenching effect and the greater the change in fluorescence signal. The instrument establishes a quantitative model based on the Stern-Vollmer equation by detecting the relationship between the fluorescence signal and the reference signal.
The Stern-Vollmer equation describes the relationship between fluorescence quenching and quenching agent concentration:
I₀/I = τ₀/τ = 1 + Ksv[Q]
I₀ and τ₀ are the fluorescence intensity and lifetime under anaerobic conditions, respectively, and I and τ are the fluorescence intensity and lifetime under the measurement conditions, Ksvis the quenching constant, [Q] is the concentration of the quenching agent (here dissolved oxygen).
Measurement method
Fluorescence dissolved oxygen analyzers typically use phase detection or intensity modulation techniques. The phase detection method calculates the oxygen concentration by measuring the phase difference between the fluorescence lifetime and the excitation light, which is less affected by the fluctuation of the light source and the aging of the film layer. The intensity modulation method is measured by comparing the fluorescence intensity with the reference signal. When the instrument is working, the excitation light source emits a modulated light pulse, the photodetector receives the fluorescent signal, and the signal processing unit converts the signal into a dissolved oxygen concentration value based on the calibration curve, usually expressed in milligrams per liter or percentage of saturation. The measurement process does not require electrolyte and does not consume oxygen, allowing for long-term continuous monitoring.
Influencing factors
Measurement accuracy is influenced by various factors. Temperature changes can affect the quenching efficiency of fluorescent dyes and the solubility of oxygen, so instruments often integrate temperature sensors for automatic compensation. Pressure changes change the partial pressure of oxygen, which in turn affects the dissolved oxygen concentration, and pressure compensation needs to be considered in deep-water measurements. Chemicals in water samples, such as hydrogen sulfide, chlorine, and other strong redox agents, may interfere with fluorescent film properties. Too fast a flow rate can cause mechanical damage to the sensitive membrane, and too slow a measurement lag. In addition, the aging of the fluorescent film, contamination, and the cleanliness of the optical window can also affect measurement stability. Regular calibration and maintenance can help maintain instrument performance.
Application:
Fluorescence dissolved oxygen analyzers have a wide range of uses in several industries. In environmental monitoring, it is used for dissolved oxygen census and long-term ecological research of surface water, groundwater, and ocean. In the field of sewage treatment, it is used to monitor the oxygen content of aeration tanks and aerobic digestion processes to optimize aeration control and energy consumption. In aquaculture, the living environment of aquatic organisms is ensured by continuous monitoring of dissolved oxygen in the aquaculture water. The food and beverage industry is used for fermentation process monitoring. Industrial circulating water systems control corrosion and microbial growth through dissolved oxygen measurements. Its non-consumable measurement feature is suitable for long-term deployments and automated monitoring networks.
Selection
Measurement needs and environmental conditions should be considered when selecting. The measurement range should cover the expected change in oxygen concentration, with common instruments covering zero to tens of milligrams per liter. Accuracy and resolution need to meet application standards, such as environmental monitoring, which often requires an error of less than 0.1%. The output signal type such as analog current and digital bus needs to be matched with the data acquisition system. The protection level should be adapted to the installation environment, and outdoor or underwater applications should have high waterproof and dustproof capabilities. Corrosion resistance should be considered for the material of the probe, such as seawater corrosion resistant materials should be selected for seawater applications. In addition, calibration ease, maintenance intervals, consumables costs, and compliance with relevant industry standards are also important reference factors for selection.