A simple omegatron, without guard rings and side plates, for measuring partial pressures has been studied. The factors influencing the collected ion and electron currents are discussed. Deviations and variations in omegatron collection efficiency are eliminated by using partial pressure ratios and the ionization gauge reading corrected for the gas mixture. Curves of ion versus electron currents are obtained. Possible effects from a hot tungsten filament are investigated. Fragmentation pattern and principal ion production are measured and discussed. The omegatron sensitivity is a function of secondary electron emission from the electron collector. Only one characteristic peak of each gas is necessary for residual gas analysis, when the fragmentation pattern is known.