Pore Size
Pore size is the rating applied by manufacturers to the membranes they produce. Pore size refers to the largest pore in the membrane through which a particle can pass; larger particles would be excluded.
Pore size is determined by one of several techniques:
- Visual examination using scanning electron microscopy, where a small section of membrane is appropriately treated, put in the microscope, and evaluated, using appropriate imaging software
- Porosimetry is a physical method where liquid is forced into the membrane under pressure and the penetration profile is analyzed mathematically to determine pore size.
- Particle challenge uses particles of defined size to determine the minimum size that can be retained by the filter
Historically, pore size ratings were used to compare filter effectiveness. However, pore size ratings vary from manufacturer to manufacturer and from product to product, making them only marginally useful.
A more relevant method used by Millipore today involves developing functional ratings that look at the function of the filter and evaluate how well the filter performs that function using a “worst-case” scenario. Millipore has established a number of mathematical expressions of inflow particle levels versus outflow particle levels to indicate how well the filters are working:
- Beta Ratio
- LRV
- Fractional Penetration
Beta Ratio
The Beta Ratio is used with non-biological filtration - clarification applications for example - to determine by how much the particulate level in a stream is reduced by the filtering process. The formula is:Number of feed particles (of size X)
Number of effluent particles (size X)
Example:1000 particles in the sample and 10 particles in the filtrate
Beta ratio = 1000/10 = 100
The greater the beta ratio, the greater the retention of the filter.
LRV
Because biological filtration deals with the removal of viable bacteria and viruses that will continue to grow under the right conditions, passage of even a few organisms may be unacceptable. Thus, a more relevant expression of filter efficiency is required. LRV (log reduction value) is used to report microbiological filter performance.The formula is:
Log of(# of particlesa in feed/ # of particles in filtrate)
LRV = Log(1000/10) = Log(100) = 2
aThe particles will be bacteria or viruses.
Fractional Penetration
Fractional penetration is used in microelectronics to measure the number of downstream particles capable of damaging a wafer or reducing the efficiency of the wafer fabrication process. This measure is also used in gas filtration for bioreactors, an application that does not tolerate any particles coming through the filtration device.The formula is:
| %R = | (# of particles in feed - # in filtrate) (# of particles in feed) | X 100 |
Example: 1000 particles in the sample and 10 particles in filtrate
| Fractional Penetration = | (1000 – 10) 1,000 | X 100 = 99% |