ASTM D4491/D4491M-17

5.1.1 In case of a dispute arising from differences in reported test results when using these test methods for acceptance testing of commercial shipments, the purchaser and the supplier should conduct comparative tests to determine if there is a statistical bias between their laboratories. Competent statistical assistance is recommended for the investigation of bias. As a minimum, the two parties should take a group of test specimens that are as homogeneous as possible and that are from a lot of material of the type in question. The test specimens should then be randomly assigned in numbers to each laboratory for testing. The average results from the two laboratories should be compared using Student's t-test for unpaired data and an acceptable probability level chosen by the two parties before the start of testing. If a bias is found, either its cause must be found and corrected, or the purchaser and the supplier must agree to interpret future test results in light of the known bias.

5.1.2 When the dispute involves test results produced with either the Method B falling head test or the Method C air flow test, the Method A constant head test, performed with a 50-mm head should be used as the referee method.

5.1.3 When the dispute involves Method C, the actual water temperature used for the water flow tests must be recorded and the viscosity of water at the test temperature must be used in the conversion from the air flow to water flow as described in Section 16, without the application of the temperature correction.

5.1.4 Permittivity is an indicator of the quantity of water that can pass through a geotextile in an isolated condition.

5.1.5 As there are many applications and environmental conditions under which a geotextile may be used, care should be taken when attempting to apply the results of these test methods to the field performance of a geotextile.