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

Abstract

AbstractConductimetric CO2 analyzers provide an economical means of monitoring and controlling carbon dioxide concentration ([CO2]) in enclosed plant growth cabinets. They rely on dissolving CO2 from an air sample in 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 flow 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 B1 and B2 were within 10 to 15 μL L−1; only the adjustment in B0 must be made every sampling period (every 900 s) to maintain [CO2] control within 10 to 15 μL L−1 of the set point.