The theory and operation of a Gerdien condenser in a collision-controlled regime is reviewed. Its operation under supersonic flow conditions is then presented and the results of calibration of the instrument in a wind tunnel are discussed. The theory of mobility is reviewed and the importance of a mean-free-path or mean-free-time model between ion-neutral collisions is discussed. Experimental positive ion mobility data are presented and compared with the data of Rose and Widdel  and the extrapolated values of small ion laboratory results. The assumption of chemically frozen flow is then considered concerning the equilibrium distribution of the water cluster ions and their temperature-dependent decomposition rates. It is shown for flow at Mach 3 that when the shock is detached the dominant cluster ions are not frozen. However, when the shock is swallowed the H+(H2O)3 ion is frozen at all altitudes above 40 km and the H+(H2O)4 ion is frozen above 65 km. The predicted mobility of the dominant positive ions, i.e., H+(H2O)3 and H+(H2O)4, based on a mobility dispersion curve for nitrogen, is compared with experimental data. Positive ion density data are presented for four rocket flights during a PCA and one flight during an auroral absorption event. Finally, positive ion loss rates are shown based on the positive ion measurements and proton flux data obtained from rockets and satellites.