A new calibration method that compensates for the effects of O₂ and N₂ on infrared CO₂ analysers

Objective. This study evaluates a method for calibrating mainstream CO₂ analysers in which CO₂ partial pressure (P_CO2) is calculated as a function of the outputs of CO₂ and O₂ analysers. Methods. Three mass flow controllers were used to generate 25 different reference mixtures of O₂, N₂ and CO₂. Reference gas mixtures were combinations of P_CO2 = 2, 4, 6, 8, 10 kPa and O₂ partial pressure (P_O2) = 10, 20, 40, 60, 80 kPa (balance N₂). CO₂ and O₂ analyser data were fitted by a calibration equation which took into account the effects of oxygen partial pressure and nonlinearity of the CO₂ analyser. The calibration coefficients were tested in a separate validation data set with a variety of combinations of CO₂ and O₂. Results. Our new calibration method yields a standard deviation of CO₂ measurement error that is significantly lower than a CO₂-only calibration method in the validation data set (0.54% versus 2.72%, P < 0.05). P_CO2 measurement errors produced by the single gas calibration equation are significantly correlated with P_O2 in both the calibration (R = - 0.9906, P < 0.05) and validation data sets (R = -0.9642, P < 0.05), but the errors given by our new calibration equation are independent of P_O2 (R = -0.0364, NS, and R = -0.0305, NS, for calibration and validation data sets respectively). Calibration with only CO₂ cannot eliminate the error related to the collision broadening effect of O₂, which in our CO₂ analyser is approximately a 1% underestimation of P_CO2 for every 10 kPa (75 mmHg) increase in P_O2. Conclusions. This study shows that non-dispersive infrared CO₂ analyser readings can be substantially affected by background oxygen. This effect can be corrected for by calibrating the CO₂ analyser with gases containing known proportions of both CO₂ and O₂.