Dynamic gauging is demonstrated experimentally to work well in duct flows, thereby being suitable for monitoring the development of soft films in flowing systems, as illustrated using the growth of a freezing fouling layer viz. an ice film. For low duct velocities 1(440 < R-D < ca. 2000) we identify an undesirable multiple steady state phenomenon and show how to eliminate it. Dynamic gauging then proves useful up to Re-D = ca. 10 000. Above that, we infer that the throat of the gauge nozzle would no longer be within the viscous sublayer and recommend further investigation of this regime.