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Two tests were run for each of three fabric samples; i.e., with and without
application of a vacuum to the fabric specimens. Constant heads varied from 0.05-0.6 m.
5.2.2 Results of Permeameter Tests
The observed flow rates and a calculated permittivities at each head for the three
fabric samples are presented in Appendix E. The permittivities (Ψ) observed in the tests
at heads of 0.05 m were the same order of magnitude but slightly greater than the
permittivities reported by the manufacturers (see Table 5.7). This observation may be
due to systematic differences between the permeameter test and ASTM D 4491.
Table 5.7 Comparison of Ψ for 3 Fabrics Using ASTM D 4491 and a Permeameter
(h = 0.05 m)
Fabrics (not vacuumed) ASTM D 4491 Ψ, sec -1 Permeameter Ψ, sec -1
Mirafi non-woven 1.5 2.1
Amoco woven 0.2 0.36
Belton woven 0.4 0.67
Eliminating entrapped air appeared to have no consistent effect on flow rates
through the fabrics. The flow rates for the non-woven Mirafi and the woven Belton were
greater without eliminating entrapped air. The lower fabric permittivities observed in the
flume do not appear to be caused by air trapped in the fabric.
A typical plot of flow rate versus head is presented in Figure 5.9. These data
demonstrate a nonlinear relationship between flow and head. The best-fit equations
relating flow to head show that the flow is a power function of the head with exponents
ranging from 0.46 to 0.71.
The ASTM method assumes that flow is laminar (i.e. viscous effects dominate);
therefore, the flow rate should be a linear function of the head (exponent equal to 1). If
inertial forces dominate (turbulent flow), the flow rate should be a function of the square
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