Conversion Factors for the air Permeability Measured by Different Instruments.

The air permeability values found by measurements with the Bekk, Schopper and Gurley instruments are interchangeable with the aid of conversion factors. The theoretical factors are derived. Experiments with widely different papers have shown that for most papers the deviations from the theoretical values fall well within the limits of accuracy offered by the Schopper and Bekk tests. The conversion factors are not reliable if very dense papers are measured with the Schopper apparatus, whereas some difficulties are encountered if very loose papers are tested in the Bekk instrument.