In an ion-drag flowmeter, an improvement is provided for alleviating the problem that the deflection of the ion stream may be altered dependent upon the concentration of polar molecules due to changes in the mobility of the ions caused by the clustering of polar molecules to the ions as result of collisions therebetween in which the polar molecules adhere to the ions. The improvement involves keeping the ambient electric field sufficiently weak that the clustering of polar molecules to the ions approaches a dynamic equilibrium in which thermal effects dominate electric field effects in promoting collisions between polar molecules and ions such that the mobility of the ions is substantially stabilized independent of the concentration of polar molecules.
