Lea and Nurse apparatus

The Lea and Nurse apparatus, shown schematically in Fig. 5 as an example of a permeability method, was the first equipment designed for routine service. It is now quoted in some national standards2021 including an International Standard now in preparation (ISO/TC 119/SC/2 N 279). The sample is compressed to a known porosity in the permeability cell and maintained within it by means of porous paper discs at each end, and supported by a rigid perforated plate. The standard cell size of cross-sectional area of 1.207 cm2 is usually used. 

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... 

Gooden and Smith [29] modified the Lea and Nurse apparatus by incorporating a self-calculating chart which enabled surface areas to be read off directly (Figure 1.3). The equation used is a simple transform... 

The Carman-Kozeny equation relates the drop in pressure through a bed to the specific surface of the material and can therefore be used as a means of calculating S from measurements of the drop in pressure. This method is strictly only suitable for beds of uniformly packed particles and it is not a suitable method for measuring the size distribution of particles in the subsieve range. A convenient form of apparatus developed by Lea and Nurse 22 1 is shown diagrammatically in Figure 4.4. In this apparatus, air or another suitable gas flows through the bed contained in a cell (25 mm diameter, 87 mm deep), and the pressure drop is obtained from hi and the gas flowrate from h2. 

The specific surface area of eement is eommonly determined directly by air. permeability methods. In the Lea and Nurse method (LI 5). a bed of cement / of porosity 0.475 is eontained in a cell through which a stream of air is f passed, and steady flow established. The specific surface area is caleulated ( from the density of the eement, the porosity and dimensions of the bed of j powder, the pressure differenee aeross the bed, and the rate of flow and ] kinematie viscosity of the air. In the Blaine method (B36), a fixed volume of I air passes through the bed at a steadily deereasing rate, whieh is controlled / and measured by the movement of oil in a manometer, the time required i being measured. The apparatus is ealibrated empirically, most obviously / using a cement that has also been examined by the Lea and Nurse method. The two methods gave elosely similar results. The Blaine method, though not absolute, is simpler to operate and automated variants of it have been devised. 

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