The Lea and Nurse permeameter

6 Constant flow rate permeameters 1.6.1 The Lea and Nurse permeameter 

In constant flow rate permeameters the flow is maintained constant by using a constant pressure drop across the powder bed. With the Lea and Nurse apparatus [27.28] gure 1.2) the powder is compressed to a known porosity in a special peimeahility cell of cross-sectional area A. Air flows through the bed and the pressure drop across it is measured on a manometer as g and the flow rate by means of a 

The volume flow rate of air through the flowmeter is given by  

Since the terms on the right hand side of the equation are known Sy, may be determined. 

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Figure 9.3. Pioneering designs of permeameters used to measure the Bneness of powders led to a widely used commercial instrument known as the Fisher Subsieve Sizer, a) The Lea and Nurse permeameter. b) The Gooden and Smith permeametcr used a column of fine sand as a standard resistance, c) The Fisher Subsieve Sizer uses a standpipe in a column of water to provide a constant pressure head.

The use of a capillary flow meter which can be calibrated theoretically is a useful feature of the Lea and Nurse permeameter but the presence of the 280 cm of capillary tube makes the equipment bulky for many laboratories. In later permeameters the capillary tube is replaced by a standard pneumatic resistance which has been calibrated using a flow meter for known characteristics. It should also be noted that if the use of the permeameter is restricted to samples of the same type of powder and equation 9.3... 

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