Fixed-pressure permeametry

When the fluid used in a permeameter is a gas, the humidity of the gas can affect the value of the measured surface area. It is usual to pass the gas through a drying tower before passing it through the powder bed. If this is done, however, it should be realized that, if the powder being tested is in equilibrium with a humid atmosphere, the dry gas flow through the plug will start to dry the powder bed and it may be necessary to wait some time for equilibrium rates to be established. 

One universal rule which has to he followed in permeametry is that the flowing fluid must move downwards through the powder plug if the plug is not restrained mechanically at both ends. If the fluid is passed the opposite way, the powder plug will expand owing to the movement of the fluid, and false readings will be obtained. 

Where and Hp are the pressure measured in the manometers as shown. It should be noted that in the above equation there is no need to know the value of the viscosity of the fluid nor the exact value of the acceleration due to gravity in the laboratory where the experiments are being carried out. For a given flow meter and for a specific powder, the above equation reduces to the form 

The last two factors of equation 9.3 are constants so that using this equation surface areas can be calculated very rapidly. It can be seen that one advantage of calibrating the permeameter, using a standard powder, when all the powders to be measured are of the same material, is that it is not necessary to assume a value for the correlation factor B. 

By always carrying out measurements simultaneously on the powder bed and the flow meter, it is not necessary to know the absolute pressure in the permeameter system. However, the total pressure drop through the permeameter system should always be kept as small as possible because, for large pressure changes, it is necessary to allow for compression effects in the gas. 

---

As in the case of fixed pressure permeametry, if a series of measurements is going to be performed on one substance then the equipment can be calibrated using a powder of known surface area and then there is no need to assume a value for the factor B nor is it necessary to measure quantities Hp, Hp a, T], r, A, g,, provided they are all fixed. When all these quantities are fixed and calibration is achieved using a powder of known surface area, the simple relationship for calculation the unknown surface... 

The settling of particles, constrained to fixed positions, in a stagnant liquid is analogous to the permeametry situation where the liquid is moving and the bed is fixed. For a sedimenting suspension, the pressure head may be replaced by the gravitational minus the buoyant force on the particles ... 

---