Figure shows the impact of the number of harmonics used on the control response of the system with no adsorbent present. While it is possible to collect the data for the entire frequency response control curve with a single square wave at 0.005 Hz (Figure ) or 0.001 Hz (Figure ), with a high number of harmonics, the use of numerous harmonics produces more scatter compared to the use of only a few harmonics as shown in both Figure and. The number of usable harmonics is controlled by the quality of the square-wave data, where cleaner, sharper square waves with less noise allow for more harmonics to be used. The quality of the raw data in Figure was high because this measurement was conducted at 400 ppm of CO2 (bal N2). As shown in Figure , water vapor, measured at a much higher concentration than the CO2 data in Figure , tends to result in lower-quality raw data and, therefore, fewer useful harmonics. The higher concentration gas producing lower quality raw data is an artifact of the mass spectrometer used in this work, which on large step changes results in saturated detected signals, and other detector platforms may not have this issue. In this case, the data in Figure show that 39 harmonics of a 0.001 Hz square wave of water vapor produce more scattered data than 52 harmonics at 0.005 Hz of CO2 in Figure due to decreased quality of the raw square wave data. Also, because the amplitude of the response diminishes at higher frequencies, the Fourier transform of the square wave becomes noisier at higher frequencies (higher noise to signal amplitude), and fewer useful harmonics are obtained. Therefore, as frequency increases, fewer harmonics are used, where the decision of how many harmonics to use is made based on visual inspection of the amplitude spectrum plot obtained via the Fourier transform. Only harmonics that produce a clear peak above the noise on this plot are used in the CSFR analysis.