In this paper the Flexible Wall Biaxial Tester is introduced. The stresses and strains that can be applied to a sample are aligned along perpendicular axes, which makes the tester very suitable for fundamental investigation of powder flow. The stress state of powders at a free surface is characterized by the flow function. With this tester the unconfined yield strength can be measured in a single experiment, without extrapolation. The tester has been used to measure the flow function of the standard BCR-limestone powder. It is concluded that the flow function measured with the Jenike Tester is too low. A comparison with other measurements reported in the literature is made and the differences explained. A firm recommendation is made for a reliable procedure. (c) 2005 Elsevier B.V. All rights reserved.