It is crucial to achieve precise control of dissolved carbon dioxide (dCO ₂) in processes such as fermentation, cell culture, aquaculture, and water treatment that have strict requirements. Traditional single parameter control suffers from lag and inaccuracy. This scheme proposes an advanced control strategy based on the linkage of dissolved CO ₂ sensor and pH electrode, aiming to improve the stability and efficiency of the process.
1、 System composition and principles
The system core consists of three parts:
Dissolved CO ₂ sensor: directly and continuously measures the partial pressure or concentration of CO ₂ in the medium as the main control variable of the control system. It responds directly and is the ultimate goal of control.
PH electrode: as a key auxiliary adjusting variable. Due to the dissolution of CO ₂ in water, it forms carbonic acid, causing a decrease in pH value, and there exists a definite mathematical relationship between the two in specific systems (such as carbonate buffer systems) (Henry's law, carbonic acid equilibrium).
Intelligent controller: receives signals from two sensors, executes core control algorithms, and outputs instructions to executing agencies (such as valves or peristaltic pumps that regulate ventilation).
2、 Linkage control strategy
The core of linkage control lies in utilizing the complementarity of two parameters to form a "feedforward feedback" composite control mode.
Feedforward control (based on pH): The response of pH value to changes in CO ₂ is usually faster than that of dedicated dCO ₂ sensors. When there is a sudden change in CO ₂ production rate due to microbial metabolism or other reasons during the process, there will be a small but detectable shift in pH. The controller can predict the trend of dCO ₂ changes in advance based on this, and fine tune the actuator in advance (such as increasing aeration) to achieve "early intervention" and suppress large fluctuations.
Feedback control (based on dCO ₂): dCO ₂ sensors provide the most direct and final feedback on control objectives. The controller compares the measured value of dCO ₂ with the set value, and uses PID and other algorithms for precise correction to eliminate steady-state errors. This ensures the long-term accuracy of control.
Data mutual verification and security assurance: The readings of the two sensors should be within the theoretically calculated reliable range. If there is a sharp decrease in pH without a corresponding increase in dCO ₂ reading, the system can determine a sensor malfunction (such as pH electrode drift or dCO ₂ membrane damage) and trigger an alarm to switch to safe mode or rely on more reliable sensors to prevent misoperation.
3、 Advantages of the plan
Rapid response: By utilizing the rapid response characteristics of pH, the inherent response delay of dCO ₂ sensors is overcome.
Accurate control: Combining feedforward and feedback, significantly reducing overshoot and fluctuations, maintaining a highly stable process environment.
High reliability: The dual sensor redundancy design provides data verification function, enhancing the system's fault tolerance and safety.
Optimizing energy consumption: Smooth control reduces the frequent start stop and large movements of actuators (such as air pumps), which helps to reduce energy consumption.
Summary: The linkage control scheme between dissolved CO ₂ sensor and pH electrode, through the organic combination of soft sensing and direct measurement, has constructed a more intelligent and robust control system, which is particularly suitable for occasions with strict requirements for biochemical reaction environment. It is an effective means to achieve automation of fine chemical and biological processes.