A pneumatic gauging sensor for measuring the thickness of soft films

Abstract

The use of the pneumatic gauging technique for measuring the thickness of soft films of food materials on hard surfaces was investigated using devices with 45° converging nozzles and 1 mm throat i.d., at air pressures of 250–1960 Pa g and different nozzle rim widths. The quasi-linear pressure response in the gauging region (clearance of less than 300 μm) and the discharge coefficient for the nozzle at low clearances were found to be independent of the upstream pressure, nozzle Reynolds number and nozzle rim width. Draughts made measurements at low pressures difficult; therefore the results reported here are principally for gauging pressures greater than 700 Pa g. The force exerted on the surface proved to be strongly affected by the nozzle shape. The flow from narrow-rimmed nozzles behaved as impinging laminar jets, whereas wide-rimmed devices exhibited lift forces caused by inertial effects in radial flows. The former would seem to be more suitable for gauging soft films and tests on a range of foodstuffs indicated that reasonable accuracy (±20 μm) could be readily achieved on materials which present a quasi-rigid surface with low permeability. Experiments established desirable gauge design and operating procedures, namely (a) narrow-rimmed nozzles, (b) low air supply pressures and (c) performing measurements at the maximum linear clearance, at h/d t ≍ 0.25. The gauging technique failed with viscous materials lacking a finite yield stress and on soft sponges.