There have been numerous attempts to measure the pressure that granular materials exert on storage and processing equipment, but no completely satisfactory method has yet been developed. The present work examines two membrane transducers immersed in a bedding of granular food materials. Pressure generated by the particles deformed a 27-mm diameter duralumin membrane equipped with a strain gauge. Ten types of granular materials, with equivalent particle diameters ranging from 0.1 (coarse flour) to 7 mm (peas), were tested. The procedures and equipment were set up to calibrate the transducer under air pressure and through a layer of granular materials with several levels of moisture content. Our results showed that to assure a standard error of calibration constants of < 1%, the diameter of the transducer membrane should be >= 12 times the particle dimension. Transducer readouts depended on the position of the membrane relative to the flat surface in which the transducer was installed. When the membrane was located 0.45 mm above the surface, the measured signal was 8-11% higher, while for a position 0.3 mm below the surface, the signal was 3-8% lower, than when the membrane was fitted directly onto the surface. Experiments performed with seeds demonstrated that their higher moisture content increased the measured signal. The transducer should therefore be calibrated in a bedding of material with a moisture content similar to that in which it will be used. (C) 2008 Elsevier B.V. All rights reserved.