A convenient way of measuring normal thrusts on fluid‐immersed surfaces is by use of a pressure gauge attached to a hole in the surface. With viscoelastic fluids exhibiting normal stress effects, it is shown that a systematic error exists, independent of hole size, but dependent on wall shear stress. Experiments were made in an open channel with various holes from
in. to ¼
in. diameter, and also with an ⅛
in. wide slot. Errors were small with 30P silicone fluid, but with two viscoelastic solutions errors were always negative
and of the order of 25% of the first normal stress difference. No significant difference has been seen yet between sharp‐edged holes of different sizes or between holes and slots. A theoretical treatment for plane creeping flow over a deep slot predicts that the error is negative, independent of slot width, and 25% of the first normal stress difference. Some implications for normal stress measurements are considered.