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
Famous quotes containing the word methods:
“The methods by which a trade union can alone act, are necessarily destructive; its organization is necessarily tyrannical.”
—Henry George (1839–1897)
“The reading public is intellectually adolescent at best, and it is obvious that what is called “significant literature” will only be sold to this public by exactly the same methods as are used to sell it toothpaste, cathartics and automobiles.”
—Raymond Chandler (1888–1959)
“Cold and hunger seem more friendly to my nature than those methods which men have adopted and advise to ward them off.”
—Henry David Thoreau (1817–1862)