The dissolved CO ₂ sensor converts the concentration of dissolved CO ₂ in water into a measurable signal through physical and chemical processes. Its core mechanism can be divided into two categories: electrochemical and optical methods, both of which are based on the physical and chemical changes caused by the interaction between CO ₂ and specific media.
Electrochemical method: ion reaction and potential/current signal conversion
Electrochemical sensors are based on the detection of changes in ion concentration generated by the reaction between CO ₂ and electrolyte. Taking the potentiometric method as an example, its structure includes a pH sensitive electrode, a reference electrode, and a CO ₂ selective membrane. When CO ₂ molecules penetrate the selective membrane and enter the electrolyte, a reaction occurs:
CO₂+H₂O⇌H₂CO₃⇌H⁺+HCO₃⁻
The concentration of H ⁺ generated by the reaction is logarithmically related to the concentration of CO ₂, and the pH sensitive electrode outputs a potential signal by detecting changes in H ⁺ concentration (i.e. pH value changes). The reference electrode provides a stable reference potential to ensure detection accuracy. The built-in chip of the sensor converts the potential signal into a dissolved CO ₂ concentration value, with an accuracy of ± 1ppm, suitable for low to medium precision scenarios such as sewage treatment and aquaculture.
The current method sensor generates a current signal proportional to the concentration through the electrolytic reaction of CO ₂ on the surface of the working electrode. For example, CO ₂ is reduced to CO at the working electrode:
CO₂+2H H2O
The current intensity generated by the reaction directly reflects the concentration of CO ₂, with higher accuracy (± 0.5ppm), but requires regular replacement of the electrolyte, which is mostly used in high-precision demand scenarios such as biomedicine.
Optical method: light absorption and fluorescence quenching effect
Optical sensors utilize the absorption or fluorescence quenching properties of CO ₂ molecules towards specific wavelengths of light for detection. The Non Dispersive Infrared (NDIR) sensor calculates the concentration of CO ₂ by measuring the absorption intensity of 4.26 μ m infrared light. The attenuation of light intensity is linearly related to the concentration of CO ₂, with an accuracy of ± 0.5ppm and a response time of about 60 seconds. It is suitable for scenarios such as food and beverage, environmental monitoring, etc.
The fluorescence quenching method sensor relies on the quenching effect of CO ₂ on fluorescent substances. The fluorescent reagent (such as ruthenium complex) coated on the surface of the fluorescent probe emits red light under blue light excitation, and CO ₂ molecules bind to the reagent to shorten the fluorescence lifetime. The sensor calculates the concentration by detecting the degree of fluorescence intensity attenuation. This method has a fast response time (<30 seconds), no maintenance required, but slightly lower accuracy (± 1ppm), and is susceptible to strong light interference. It is suitable for dynamic scenarios such as portable detection and soilless cultivation.
Environmental factors and optimization strategies
Temperature, pressure, and water composition all affect the performance of sensors. An increase in temperature will accelerate the dissolution and reaction rate of CO ₂, and most sensors have built-in temperature compensation modules to correct errors; The increase in water pressure enhances the solubility of CO ₂, and some models require pressure sensor compensation; Turbidity or oil stains may obstruct the optical path of the optical sensor or contaminate the electrochemical film, and the probe needs to be cleaned regularly.
Electrochemical and optical methods each have their own advantages: electrochemical methods have low cost, small size, and are suitable for low-cost scenarios; Optical methods have high stability and long lifespan, suitable for high-precision or long-term monitoring. In practical applications, adaptation technologies need to be selected based on the requirements of the scenario (such as accuracy, cost, and maintenance complexity) to achieve precise monitoring and regulation of dissolved CO ₂.