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A Simple Conductimetric CO₂ Analyzer with Automatic Recalibration: II. Factors Affecting Calibration

USDA-ARS, U.S. Water Conservation Lab., 4331 E. Broadway Rd., Phoenix, AZ
USDA-ARS, Systems Res. Lab., Natural Resources Inst., BARC-W, Beltsville, MD 20705
Dep. of Statistics, Kansas State Univ., Manhattan, KS 66502

^* Corresponding author.

Conductimetric CO₂ analyzers provide an economical means of monitoring and controlling carbon dioxide concentration ([CO₂]) in enclosed plaut growth cabinets. They rely on dissolving CO₂ from an air samplein demineralized water and measuring the resulting change in electrical conductivity. Regressing the analog output from the conductivity meter against [CO₂] values over the range 200 to 1000 µL L^–1 yields a quadratic response. Calibration curves for conductimetric CO₂ analyzers are affected primarily by temperature and air and water flow rates. Air and water flow rates are relatively fixed, whereas temperature varies over time in normal operation. A covariate analysis for a three-way treatment structure in a randomized complete block design determined how the intercept (B₀) and the linear and quadratic parameter estimates (B₁ and B₂) varied with temperatures of 10, 20, 30, and 40°C, air ltow rates of 540, 1880, and 3700 mm³ s^–1 , and water flow rates of 1.0, 0.66, and 0.33 mL s^–1. Temperature effects were greatest, with air flow rate causing minor changes, while water flow rate had no effect. Temperature and air flow rate significantly altered B₀, with only minor changes in B₁ and B₂. Variations in B₀ were sufficient enough to alter the [CO₂] around the desired set point by 70 µL L^–1 for all treatments evaluated. Deviations in [CO₂] control due to variation in BI and B~ were within 10 to 15 µL L^–1; only the adjustment in B₀ must be made every sampiing period (every 900 s) to mainlain [CO₂] control within 10 to 15 µL L^–1 of the set point.

Received for publication January 27, 1993.