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A Simple Conductimetric CO₂ Analyzer with Automatic Recalibration: III. Dynamic Response

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

^* Corresponding author.

Conductimetric CO₂ analyzers provide an economical means of monitoring and controlling carbon dioxide concentration ([CO₂]) in plant growth cabinets. When conductimetric CO₂ analyzers are being used to measure canopy CO₂ uptake over a period and compare that with light received, it is essential to know the time between an air sample leaving the canopy and its [CO₂] being registered by the analyzer. A four-way analysis of variance in three replicates of a split split-plot design was used to examine the response time of the conductimetric CO₂ analyzers to successive step changes in [CO₂] ([CO₂]) from 200 to 400, 600, 800, and 1000 µL L^–1, at temperatures of 10, 20, 30, and 40°C, at air flows rates of 540, 1880, and 3700 mm3 s-~, and at water flow rates of 1.0, 0.66, and 0.33 mL s^–1. Response times ranged from 30 to 542 s across all treatments. Significant differences were observed between individual conductimetric CO₂ analyzers, with response time means ranging from 99 to 145 s. Each analyzer, therefore, is unique and must be recalibrated following a change in any one component. A significant water flow rate x temperature interaction was observed (response time means ranged from 86 to 177 s). Comparing these response time means to the 900-s sampling period indicates that an individual conductimettic CO₂ analyzer will adjust to the new steady state following a change in control within 9.5 to 20% of the sampling period. Only 1 data record out of 96 within a diurnal cycle is lost if an alteration in the CO₂ control set point for the plant growth cabinet is made. This data loss is acceptable.

Received for publication January 27, 1993.