In response to these problems associated with measuring fabric permittivity directly, an automated method of permittivity testing has been developed to directly determine either P_L or P_T. The slope  dF/dDP calculated directly for a series of different flow rates would not only give the permittivity but would delineate the range at which fluid flow changes from streamline to turbulent flow, if such a point is present.

According to this proprietary Advanced Permittivity Testing Method for textile permittivity determinations, an essentially constant volume of a fluid from one chamber, with a decreasing volume passes through the fabric being tested to another chamber, with an increasing volume, the sum of the volumes of the chambers being constant.

Figure 2.1 illustrates the principal of the Advanced Permittivity Testing Method. Because the two pistons move in synchronization, a fixed fluid volume is employed during operation. Both the pressure difference across the fabric and the fluid flow velocity can be measured directly and simultaneously.

Figure 2.1a  Principle of the Advanced Permittivity Testing Method

The Advanced Permittivity Testing Method is very flexible, permitting a variety of textile configurations to be tested. Figure 2.1b illustrates specimen holders with two such configurations; for example, a flat and a tubular fabric specimen.

These specimen holders are positioned between the pistons, as shown in Figure 2.1a, thereby, forcing the fluid to pass through the fabric, whatever the configuration employed.

Figure 2.2 delineates the empirical quantities that can be measured directly using the Advanced Permittivity Testing Method. Using these terms, the permittivity can be directly calculated from the slopes of either equation (3) or (4) below.

The velocity F of the fluid passing through the fabric is simply

    (3) F   =   V (A/a)

where V is the linear piston travel speed. The area 'A' is that of the cylinder and 'a' is the effective flow-through area of the specimen.

The required pressure difference DP is measured from the pressure readings at the transducers and is simply

    (4) DP   =   P_u - P_d

where P_u and P_d are the upstream and downstream pressure readings, respectively.

The fabric area 'a' is chosen to keep the DP and piston speed F within the measuring range capability of the apparatus. For fabrics with very high fluid flow capacity a small fabric area (a ® 0) is chosen and for fabrics with very low fluid flow capabilities a large fabric area (a ® A) is used.

Accordingly, from a series of tests using the disclosed permittivity measuring method for a fluid of known viscosity m, either P_L or P_T can be directly calculated from the measured values of F and DP. In this manner, reproducible permittivity determinations can be made for technical fabrics, primarily those used for biotextile and geotextile purposes.

Probably the greatest obstacle in any rigorous analysis of the conductivity of fluids through porous media is the lack of information concerning the deformation of pores under actual service conditions. The Advanced Permittivity Testing Method will permit real time observations of pore deformation. Figure 2.3a shows the testing device equipped for such observations.

The hollow piston rods will permit boroscopic observations of the porous media with the optical conduit transmitting both illumination and images. Alternatively, separate illumination and imaging conduits can be provided.  Probably, the outflow side will provide the most useful information, with the fluid displacing individual fibers. Moreover, the uniformity of flow is an important consideration in eveluating the efficiency of filter media.

Figure 2.4a illustrates an ideally uniform flow through a filter. In contrast, Figure 2.4b shows a highly non-uniform flow. Such observations are, otherwise, considerably difficult to perform. Injecting a dye in the inflow side or a dyed colloidial suspension would allow flow uniformity to be observed. In addition, the Advanced Permittivity Testing Method is amenable to close temperature control, both above and below ambient.

Figure 2.5 Temperature Control

The temperature control jacket can use either electric heating elements for high temperature studies, as shown in Figure 2.5, or a jacket with fluid channels for both high or low temperature studies.