Methods
Measurement consists of packing the powder into a cylindrical "bed" having a known porosity (i.e. volume of air-space between particles divided by total bed volume). A pressure drop is set up along the length of the bed cylinder. The resulting flow-rate of air through the bed yields the specific surface by the Kozeny–Carman equation:
where:
- S is specific surface, m2·kg-1
- d is the cylinder diameter, m
- ρ is the sample particle density, kg·m-3
- ε is the volume porosity of the bed (dimensionless)
- δP is the pressure drop across the bed, Pa
- l is the cylinder length, m
- η is the air dynamic viscosity, Pa·s
- Q is the flowrate, m3·s-1
It can be seen that the specific surface is proportional to the square root of the ratio of pressure to flow. Various standard methods have been proposed:
- Maintain a constant flowrate, and measure the pressure drop
- Maintain a constant pressure drop, and measure the flowrate
- Allow both to vary, deriving the ratio from the characteristics of the apparatus.
Read more about this topic: Air Permeability Specific Surface
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