Weighing at low gas pressures is a technique which is highly susceptible to temperature inhomogeneities. These temperature inhomogeneities cause spurious mass changes, the effect being usually called the thermomolecular flow effect. In a former paper by Poulis, Pelupessy, Massen and Thomas these effects were explained in the low pressure region (below 10 mtorr) on the basis of transverse and longitudinal Knudsen forces, which are both related to the Knudsen pressure difference, which at low pressures accompanies a temperature inhomogeneity. Though in nature a low pressure effect, the thermomolecular flow effect appears to be present at pressures as high as 100 torr. In the present paper the theory, originally set up for the low pressure region, is extended to higher pressures. Here, laminar gas flow can arise, accompanied by shearing forces, which have been allowed for in the theory. The theoretical results are compared with experimental data.

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