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 CO(2) analysers in which CO(2) partial pressure (P (CO2)) is calculated as a function of the outputs of CO(2) and O(2) analysers.

METHODS

Three mass flow controllers were used to generate 25 different reference mixtures of O(2), N(2) and CO(2). Reference gas mixtures were combinations of P (CO2) = 2, 4, 6, 8, 10 kPa and O(2) partial pressure (P (O2)) = 10, 20, 40, 60, 80 kPa (balance N(2)). CO(2) and O(2) analyser data were fitted by a calibration equation which took into account the effects of oxygen partial pressure and nonlinearity of the CO(2) analyser. The calibration coefficients were tested in a separate validation data set with a variety of combinations of CO(2) and O(2).

RESULTS

Our new calibration method yields a standard deviation of CO(2) measurement error that is significantly lower than a CO(2)-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 CO(2) cannot eliminate the error related to the collision broadening effect of O(2), which in our CO(2) 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 CO(2) analyser readings can be substantially affected by background oxygen. This effect can be corrected for by calibrating the CO(2) analyser with gases containing known proportions of both CO(2) and O(2).