Definition
A dissolved oxygen detector is an analytical instrument used to measure the concentration of dissolved oxygen in liquids. Dissolved oxygen refers to molecular oxygen dissolved in water or other liquids, and its concentration is a key parameter for evaluating water quality, biological activity, and chemical processes. The instrument converts oxygen content into a readable signal through specific sensing technology, which is widely used in environmental monitoring, aquaculture, industrial process control and scientific research.
Principle
The core of the dissolved oxygen detector is based on the principles of electrochemical or optical sensing. The electrochemical principle typically employs a Clark electrode, which consists of a cathode, an anode, and an electrolyte, covered with a breathable membrane. When oxygen diffuses through the thin film into the electrode, a reduction reaction occurs at the cathode, generating a current signal proportional to the oxygen concentration, and the dissolved oxygen concentration can be calculated by measuring the current. The reaction process can be expressed as: O₂ + 2H₂O + 4e⁻ → 4OH⁻.
The optical principle is based on the fluorescence quenching effect. When oxygen comes into contact with the fluorescent substance, it will quench the fluorescence intensity or change the fluorescence life, and the degree of quenching is related to the oxygen concentration, and the dissolved oxygen value can be determined by detecting optical changes. This method does not consume electrolyte and requires less maintenance.
Measurement method
The measurement methods of dissolved oxygen are mainly divided into two categories: in-situ measurement and laboratory measurement. In-situ measurement immerses the sensor directly into the liquid under test for real-time continuous monitoring for on-site or process control. Laboratory measurements are usually analyzed using portable or benchtop instruments under controlled conditions after sample collection, and attention should be taken to avoid bubble interference with air contact during sample collection.
Calibration is a key step to ensure accurate measurement, and generally uses zero-point calibration and full-scale calibration. Zero point calibration uses an oxygen-free solution (e.g., sodium sulfite solution), and full-scale calibration criteria use air-saturated water or a standard solution with a known oxygen concentration. Some instruments support automatic temperature and salinity compensation to correct for deviations caused by environmental factors.
Influencing factors
Dissolved oxygen measurements are influenced by a variety of factors. Temperature changes change the solubility of oxygen and sensor response characteristics, usually compensated by built-in temperature sensors. Liquid pressure affects the rate of oxygen diffusion and pressure correction needs to be considered in deepwater measurements. Salinity or ionic strength can change oxygen activity, and the calibration mode should be selected for high-salt environments.
The water flow velocity has an effect on the membrane electrode, and a low flow rate may result in low measurements. In addition, sensor membrane contamination or aging, electrolyte consumption, optical window fouling, etc. can also cause reading drift and require regular maintenance and verification.
Applications
In environmental monitoring, dissolved oxygen detectors are used to assess the eutrophication status and ecological health of surface water, groundwater and oceans. The aquaculture field relies on it to monitor the oxygen content of water bodies to ensure the survival and growth of farmed organisms. In the process of sewage treatment, dissolved oxygen is the core parameter controlling the efficiency of biochemical reactions, which is related to the treatment effect and energy consumption.
The food and beverage industry uses dissolved oxygen detectors to monitor the oxygen content in production water and products to maintain quality and shelf life. In industrial applications, such as boiler feedwater, pharmaceutical processes, and chemical synthesis, dissolved oxygen monitoring helps prevent corrosion and optimize reaction conditions.
Selection considerations
When selecting, the measurement principle should be determined according to the application scenario. The cost of electrochemical instruments is relatively low, suitable for routine monitoring; The optical instrument has good stability and is suitable for long-term continuous measurement or liquids containing interfering substances. The measurement range should cover the expected oxygen concentration, and common instruments range from 0 to 20 mg/L or 0 to 200% saturation.
Consider whether the accuracy and resolution of the instrument meet standard requirements, while focusing on response time, calibration intervals, and protection levels. For wild or humid environments, choose a model with corresponding protection design. Maintenance needs such as membrane replacement, electrolyte filling, or cleaning frequency are also important factors in selection, which should be comprehensively evaluated in combination with the conditions of use and the ability of the operator.