Frazier Air Permeability Tester
The TSI Filter Tester sets face velocity (air flow) which is not exactly the same as CFM (cubic feet/min/square foot) in the Frazier Instrument, but which can be converted to CFM with the proper formulas. It reads the resulting resistance to air flow in mm of H2O.
This is essentially how the normal use of the Frazier Instrument works except in reverse. This can also be done with the Frazier Instrument by setting the desired air flow and reading the resulting differential pressure (resistance). This kind of test is often done with the Frazier Instruments, most notably for filters where there is interest in determining the difficulty of moving air through a filter (e.g. face masks when there is a need to know how difficult it is to breathe through the filter).
Flow rates through different types of samples are almost never linear with changes in resistance (differential pressure). The relationship is usually some sort of log function and sometimes there is actually a negative function where flow rates actually decrease with increasing rates of resistance (differential pressure).
This is a direct function of the type of sample tested – each type of sample has its own distinctive (and usually different) graph. It is possible to get a rough correlation between the Frazier Instruments and other designed instruments but the correlation is seldom exact. (The various Frazier Instruments by contrast correlate very closely.) This lack of close correlation appears to be due to differences in design and calibration as well as to the use of less accurate sensors.
Usually a rough correlation can be established through empirical comparisons. However, each type of test material needs to be individually correlated. Even a slight change in the fibers such as size, cross section, or material substance can change the correlation. There is no one correlation for all materials.