In gas chromatographic (GC) practice, straightforward splitting of the column effluent over a mass spectrometry (MS) system and a flame-ionization detection (FID) system leads to unpredictable split ratios and, thus, to poor quantification. We therefore decided to implement a Deans' switch, which should allow for quantitative transfer of the column effluent to either FID or MS. Since FID works under atmospheric pressure and an MS under vacuum conditions, it is difficult to establish suitable dimensions (lengths and diameters) of the capillaries needed for a 'balanced' switch. Generally, the Poiseuilie equation, which describes the flow of fluids through tubes of circular cross-section, is used to this end. However, the motion of gases in small capillaries at low pressures, as is the case in GC-MS interfaces, is fundamentally different from that at near atmospheric pressures. This becomes manifest from a substantial drop of the dynamic viscosity of the gas, η, to an effective viscosity, η/F, or, in other words, the Poiseuille equation is no longer valid. Adapting the Poiseuille equation by the introduction of the correction factor, F, has been reported. In this paper F and the use of an equation for F expressed in terms of readily accessible parameters will be discussed. In addition, a successful design of a balanced FID/MS Deans' switch will be demonstrated.