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A Simple Conductimetric CO₂ Analyzer with Automatic Recalibration: I. Design, Implementation, and Functionality

USDA-ARS Systems Res. Lab., Natural Resources Inst., BARC-W, Baltsville, MD 20705
USDA-ARS, U.S. Water Conservation Lab., 4331 E. Broadway, Phoenix, AZ 85040

^* Corresponding author.

Controlled-environment plant growth cabinets may be used to investigate the long-term effect of elevated carbon dioxide concentration ([CO₂]) on plant growth. Infrared gas analyzers (IRGAs) are normally used to monitor and control [CO₂] in plant cabinets. With many cabinets in use, however, it soon becomes impractical to purchase an individual IRGA for each cabinet. A more economical method of monitoring and controlling [CO₂] relies on the change in electrical conductivity when CO₂ is dissolved in demineralized water. This work describes the design, implementation, and functionality of an inexpensive conductimetric system for controlling [CO₂] in plant growth cabinets. Regressing electrical conductivity against [CO₂] over the range 0 to 1000 µL L ^–l yields a quadratic response. Calibration drift inherent in the conductimetric CO₂ analyzer requires that each analyzer be recalibrated periodically. Automatically recalibrating with an IRGA every 900 s gave control of the [CO₂] within the plant enclosures to within 10 to 15 üL L^–1 of the set point. The [CO₂] control system is robust enough to maintain this accuracy regardless of the desired [CO₂] set point or the mass of plant material within the plant growth cabinet. In this approach, only one IRGA is required to control [CO₂] in many plant growth cabinets if each cabinet has a dedicated conductimetric CO₂ analyzer.

Received for publication January 27, 1993.