Nozzles are simple devices often used in chemical engineering, mainly either as flow rate sensors or as various jet generating actuators. This paper discusses the question of characterising their steady-state behaviour by a numerical value-or perhaps by a few values. in spite of overall simplicity of nozzles, their behaviour is not straightforward and the characterisations used so far specify it imprecisely. The present author introduced in 1985 a characterisation that specified the dependence of properties on Reynolds number sufficiently well, universally for all nozzle shapes so far tested, and had the advantage of being physically substantiated. The losses inside a nozzle are assumed to be entirely specified by the exit displacement effect of wall boundary layer. This effect, in turn, is assumed to be governed by laminar layer growth law. So far, all evidence for this theory was favourable but only indirect. This paper presents recent direct experimental verification obtained by measuring the boundary layers in a large-scale nozzle model across a wide range of Re. The predictions of the 1985 model are fully supported. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.