The second of these was developed by Lea and Nurse.  The bed is 25 mm in diameter and 10 mm thick.  The desired porosity (which may vary in the range 0.4 to 0.6) is obtained by using a calculated weight of sample, pressed to precisely these dimensions.  The required weight is given by:
A flowmeter consisting of a long capillary is connected in series with the powder bed.  The pressure drop across the flowmeter (measured by a manometer) is proportional to the flowrate, and the proportionality constant can be measured by direct calibration.  The pressure drop across the bed is measured by a similar manometer.  Thus the required pressure/flow ratio can be obtained from the ratio of the two manometer readings, and when fed into the Carman equation, yields an "absolute" value of the air permeability surface area.  The apparatus is maintained at a constant temperature, and dry air is used so that the air viscosity can be obtained from tables.