(40c) On the Use of Plate-Type Normal Pressure Cells for Silos

Normal pressure cells are usually mounted in experimental silos in order to study the pressure distribution on the silo wall. The obtained information contributes to the understanding of the different phenomena that occur inside the silos. Different types of normal pressure cells can be used (Brown and Nielsen, 1995). Some cells are based on the displacement of a stiff piston, others on the bending of a clamped plate or membrane with or without a fluid behind it (Askegaard, 1961). In all cases, a calibration of the cell is needed to relate the measured signal to the pressure on the surface of the pressure cell. Therefore it is necessary to determine which form of calibration that is suitable for the type of cell that is used. If a cell is going to register the pressure in a liquid, the calibration procedure can be simple, e.g. carried out with a fluid either water or air. However, if the cell is going to register the pressure produced by a granular media, calibration becomes more complicated and the risk of measuring errors increases. The most important principle for a successful calibration is to simulate the actual stiffness relations between deformation of the pressure cell and the granular medium in the silo (Gravesen, 1959). Therefore, it is appropriate to use a sample of the actual granular material for transmitting the pressure to the active surface of the pressure cell during the calibration. However, the redistribution of pressure across the active area of the pressure cell due to its deflection can depend on the design of the calibration chamber and the loading procedure. As one fix-point, it may be useful to have a calibration with a fluid, which gives a uniform distribution of the pressure. For this study, a plate-type cell without fluid behind has been designed (Figure: Normal Pressure Cell). The cell is composed of a 0.6 mm thick and 100 mm diameter plate with strain gauges mounted in a full bridge connection, which makes the cell self-temperature-compensated (Window, 1992). Three forms of calibration procedures were used: Initially weights were used, subsequently a layer of wheat was used between the plate and the weights, and finally air was used to transmit pressure to the membrane. Different results were obtained from each method and valuable information needed in the interpretation of test results was achieved from each of the tests. The conclusions contain recommendations for a more stiff pressure cell, for which the calibration as well as interpretation of test results become simpler.

REFERENCES

[1] Brown C.J. and Nielsen J. (1998). Silos: Fundamentals of Theory, Behaviour and Design. E&F Spon. London. [2] Askegaard, V. (1961). ?Measurement of Pressure between a Rigid Wall and a Compressible Medium by Means of Pressure Cells?. Acta Polytechnica Scandinavica. Ci 11. [3] Gravesen, S. (1959). ?Elastic Semi-Infinite Medium Bounded By a Rigid Wall with a Circular Hole?. Bygningsstatiske Meddelelser. Vol. xxx, no. 3, pp 93-111. [4] Window A.L. (1992). Strain Gauge Technology. Elsevier Science Publishers Ltd. England.