This test method is used in the trade for acceptance testing of commercial shipments of nonwoven fabrics; however, caution is advised since information about between-laboratory precision is incomplete. Comparative tests as directed in 5.1.1 may be advisable.

5.1.1 In case of a dispute arising from differences in reported test results when using this test method 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. Test specimens should then be randomly assigned in equal numbers to each laboratory for testing. The average results from the two laboratories should be compared using the appropriate Student't-test and an acceptable probability level chosen by the two parties before testing is begun. If a bias is found, either is cause must be found and corrected or the purchaser and the supplier must agree to interpret future test results with consideration to the known bias.

Thickness is one of the basic physical properties of nonwoven fabrics. In certain industrial applications, the thickness may require rigid control within specified limits. Bulk and warmth properties of nonwoven fabrics are often estimated from their thickness values, and thickness is also useful in measuring performance characteristics, such as, before and after abrasion and shrinkage.

The thickness values of most nonwoven fabrics will vary considerably depending on the pressure applied to the specimen at the time the thickness measurement is taken. In all cases, the apparent thickness varies inversely with the pressure applied. For this reason, it is essential that the pressure be specified when discussing or listing any thickness value.