Skin friction balances for large pressure gradients

If floating element balances are used to measure skin friction in a large pressure gradient, the pressure forces on the floating element introduce severe errors. It was shown by Frei (1979) that this deficiency can be overcome by sealing the gap with a liquid, held in place by the surface tension. The forces due to the surface tension can be calculated for a given geometry and pressure field. In the present paper, experiments are described which show that the calculations are correct and that the errors due to surface tension can be made very small compared with the skin friction force.