Introduction to the Principles of Dissolved Oxygen Sensors
Introduction to the Principles of Dissolved Oxygen Sensors
I. Basic Concept of Dissolved Oxygen Sensors
Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water, expressed in milligrams of oxygen per liter of water (mg/L). DO exists in water as individual oxygen molecules. The concentration of dissolved oxygen is a crucial indicator of water quality and plays a significant role in water purification processes.
The level of dissolved oxygen in water is influenced by factors such as atmospheric pressure, water temperature, and salinity. Water bodies that are not contaminated by oxygen-consuming substances (typically organic matter) are generally saturated with dissolved oxygen. For instance, the DO levels in clean surface water are close to saturation. When the organic matter content in water is high, the rate of oxygen consumption surpasses the rate of oxygen replenishment, leading to a decrease in DO levels, potentially approaching zero. Under hypoxic conditions, organic matter decomposes, resulting in putrefaction and fermentation, which severely deteriorates water quality. Therefore, DO is used as an index of water pollution in water quality assessments.
II. Applications Requiring Dissolved Oxygen Measurement
- Sewage Treatment:
- Monitoring and managing DO levels are critical in the treatment of wastewater to ensure effective biological processes.
- Environmental Monitoring of Rivers and Lakes:
- DO sensors help in assessing the health and quality of aquatic ecosystems by monitoring DO levels.
- Aquaculture:
- Maintaining optimal DO levels is essential for the health and growth of fish and other aquatic organisms in aquaculture settings.
III. Principles and Types of Dissolved Oxygen Sensors
Current market offerings for dissolved oxygen sensors primarily utilize three detection methods: polarography, galvanic cell method, and fluorescence method.
- Polarography:
- Principle: This method employs a gold or platinum ring as the cathode and a silver-silver chloride (or mercury-mercurous chloride) as the anode, with potassium chloride as the electrolyte. The cathode is covered with an oxygen-permeable membrane made from materials like PTFE, PVC, polyethylene, or silicone rubber. A polarization voltage of 0.5~1.5V is applied between the electrodes. When dissolved oxygen reaches the gold cathode through the membrane, the following reactions occur:
- Cathode reduction: O2+2H2O+4e−→4OH−\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-
- Anode oxidation: 4Cl−+4Ag−4e−→4AgCl4\text{Cl}^- + 4\text{Ag} – 4e^- \rightarrow 4\text{AgCl}
- By measuring the diffusion current, the solubility of dissolved oxygen is determined. Compensation for temperature, salinity, and air pressure is implemented to ensure accuracy.
- Principle: This method employs a gold or platinum ring as the cathode and a silver-silver chloride (or mercury-mercurous chloride) as the anode, with potassium chloride as the electrolyte. The cathode is covered with an oxygen-permeable membrane made from materials like PTFE, PVC, polyethylene, or silicone rubber. A polarization voltage of 0.5~1.5V is applied between the electrodes. When dissolved oxygen reaches the gold cathode through the membrane, the following reactions occur:
- Galvanic Cell Method:
- Principle: In this method, when external oxygen molecules penetrate the membrane, the following reactions take place:
- Silver cathode reduction: O2+2H2O+4e−→4OH−\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-
- Lead anode oxidation: 2Pb+4KOH+4OH−→2KHPbO2+2H2O2\text{Pb} + 4\text{KOH} + 4\text{OH}^- \rightarrow 2\text{KHPbO}_2 + 2\text{H}_2\text{O}
- Oxygen is reduced to hydroxide ions at the silver cathode, generating electrons from the external circuit; the lead anode is corroded by potassium hydroxide solution, producing potassium hydrogen lead acid and releasing electrons to the external circuit. The signal current generated is proportional to the dissolved oxygen concentration.
- Principle: In this method, when external oxygen molecules penetrate the membrane, the following reactions take place:
- Fluorescence Method:
- Principle: This method is based on the quenching effect of specific substances on active fluorescence. Blue light from an LED excites the fluorescent material on the inner surface of a fluorescent cap, causing it to emit red light. By detecting the phase difference between the red and blue light and comparing it to an internal calibration value, the concentration of oxygen molecules is calculated. The final value is adjusted for temperature and air pressure.
- Advantages: The fluorescence method features no membrane, no electrolyte, no polarization, and a long service life. It does not consume oxygen and is unaffected by flow rate. It includes a built-in temperature sensor for automatic compensation and is resistant to interference from chemicals like sulfides. It offers minimal annual drift, rapid response, and accurate measurements.
By leveraging these advanced principles, dissolved oxygen sensors provide essential data for maintaining water quality in various applications, ensuring safe and healthy water for both human consumption and aquatic life.