Air pressure different from that in the surrounding atmosphere develops in the bulk of granular material flowing out of an open bin through the bottom orifice. When measured on the axis of a small laboratory apparatus the differences have been found to oscillate about a mean changing along the bin and reaching its maximum/negative/well above the orifice. Pressure difference at a given point inside the bed depends upon the orifice size, and upon the size of solid particles. Equalization of the air pressure just above the orifice with that of the air outside the bin influences flow rate of the material. It is suggested that the overlooking of these effects is to some extent responsible for the great variety of flow rate equations so far proposed.

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