Fluid dynamic gauging has been applied for the first time to measurements on a curved surface, specifically the inner convex surface of an annulus. Two hydraulic cases were examined: (i) the quasi-static case, where there is no flow in the annulus, other than the flow imposed by the working action of the gauge and (ii) the turbulent case, where there is additionally a forced advective flow in the turbulent regime (14,000 < Re-annulus < 32,000). The nozzle clearance-flow rate characteristics resembled those reported previously for flat geometries, with one exception; at small clearance values (0 < h/d, < 0.1), flow rate was found to be independent of clearance because of a leakage flow arising from the curvature of the surface. The experimental results for the quasi-static case showed very good agreement with simulations of the configuration using computational fluid dynamics (CFD). The agreement indicates that the technique may be used not only for measuring the thickness of deposits on curved surfaces, but also for measuring their strength. CFD simulation of the turbulent case was not attempted, but the experimental results imply that the technique could be used reliably as a real-time in situ thickness sensor for this scenario, which is often employed in laboratory fouling studies. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1937-1947, 2009