A dew point meter (DP) and an infrared (IR) CO
2 analyzer were assembled in a humid CO
2 adsorption/desorption system in series for simultaneous online measurements of H
2O and CO
2, respectively. The humidifier, by using surface-flushing on a saturated brine solution was self-made for the generation of humid air flow. It was found that by this method it became relatively easy to obtain a low H
2O content in air flow and that its fluctuation could be reduced compared to the bubbling method. Water calibration for the DP-IR detector is necessary to be conducted for minimizing the measurement error of H
2O. It demonstrated that the relative error (RA) for simultaneous online measurements H
2O and CO
2 in the desorption process is lower than 0.1%. The high RA in the adsorption of H
2O is attributed to H
2O adsorption on the transfer pipe and amplification of the measurement error. The high accuracy of simultaneous online measurements of H
2O and CO
2 is promising for investigating their co-adsorption/desorption behaviors, especially for direct CO
2 capture from ambient air.
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