Definition
A fluorescence dissolved oxygen meter is an analytical instrument based on the principle of fluorescence quenching for measuring the concentration of dissolved oxygen in liquids. It accurately calculates the concentration of oxygen molecules by detecting changes in fluorescence lifetime or intensity of a specific fluorescent substance under the action of excitation light. Compared to traditional electrochemical methods, this method eliminates the need for electrolyte and oxygen-permeable membranes, reduces maintenance requirements, and has a faster response time.
Principle
At the heart of the instrument is an oxygen-sensitive fluorescent substance, usually a metal-organic complex or a specific dye, which is cured in the matrix of the sensor probe. When a specific wavelength of excitation light irradiates a fluorescent material, its electrons absorb energy transition to the excited state, and then fluorescence is released during the return to the ground state. Dissolved oxygen, as a fluorescence quencher, collides with excited molecules and absorbs their energy, resulting in reduced fluorescence intensity or shortened fluorescence lifetime. The relationship between oxygen concentration and fluorescence lifetime τ follows the Stern-Volmer equation:
τ0/τ = 1 + KSV· [O2]
where τ0is the fluorescence lifetime under oxygen-free conditions, τ is the fluorescence lifetime at the time of measurement, KSVis the sudden extinction constant, [O2] is the dissolved oxygen concentration. The instrument calculates the real-time concentration of dissolved oxygen by measuring fluorescence lifetime changes, combined with temperature compensation.
Measurement method
Fluorescence dissolved oxygen meters are usually measured using phase detection or time-resolved techniques. The phase detection method estimates the fluorescence lifetime by comparing the phase difference between the excited light and the fluorescence signal. The time-resolved method directly measures the time it takes for fluorescence to decay to a specific intensity. Both methods effectively eliminate the interference of light source fluctuations and probe coating aging. When measuring, the probe is immersed in the liquid to be measured and maintains an appropriate flow rate to ensure adequate contact between the sensor surface and the medium. The built-in temperature sensor synchronously measures the sample temperature and automatically compensates for oxygen solubility to improve measurement accuracy.
Influencing factors
Measurement results are influenced by a variety of factors. Temperature changes change the solubility of oxygen and the quenching efficiency of fluorescent substances, so real-time temperature compensation is required. Ambient pressure affects the partial pressure of oxygen and pressure correction is considered in closed systems or deep water measurements. Certain chemicals in the media, such as hydrogen sulfide or strong oxidizing agents, can have irreversible effects on fluorescent coatings. Too low a flow rate can lead to oxygen depletion on the sensor surface, while too high a flow rate can generate frictional heat. In addition, dirt or bubbles on the probe surface can hinder oxygen diffusion and require regular cleaning and maintenance.
Applications
The instrument is suitable for a variety of industrial and scientific research scenarios. In environmental monitoring, it is used for online monitoring of dissolved oxygen in surface water and sewage treatment plants. The aquaculture field relies on it for oxygen concentration control in ponds and circulating water systems. Process monitoring for bioreactors in the fermentation industry. It is used in the power industry to detect the oxygen content of boiler feedwater and condensate. It is also used in food and beverage production for oxygen-free environmental verification of the filling process. These applications benefit from the fact that they do not consume electrodes and have long maintenance intervals.
Selection considerations
When choosing an instrument, consider your measurement needs. The measurement range should cover the actual application concentration, and common instruments cover the measurement of zero to saturated concentration. Resolution and accuracy need to meet process control or monitoring standard requirements. Response time is critical in process control, often ranging from a few seconds to tens of seconds. The protection level should be adapted to the installation environment, such as outdoor or wet places, a higher waterproof level. The output interface needs to be compatible with the existing data system. In addition, corrosion resistance should be considered for the construction of the probe, and the calibration interval and consumables replacement cost for long-term use should also be evaluated.