The basic properties of paper include the dimensions and the mass. In the case of sheet material such as paper, the basis weight mA is determined in accordance with ISO 536 (1995). The value mA is the ratio of the mass m to the area A of a sample.
Another basic property of paper is the caliper. As paper is compressible the caliper must be defined in terms of the measuring instrument. ISO 534 (2003) specifies an instrument with two parallel planar measuring surfaces of 200 mm2 which act on the sample with a surface pressure of 100 kPa. The resulting distance between the surfaces is the caliper D of the paper.
The ratio of the basis weight to the caliper is the density of the paper. The recipro¬cal density is the specific volume.
The moisture expansion of paper, i. e., the change in dimensions with changing ambient climate, is also a basic property. According to DIN 53 130, the moisture expansion (ME) is the ratio of the change of length D L of a sample paper strip, resulting from a change in the ambient climate from 23 °C/45 % relative humidity to 23 °C/83 %relative humidity, to the original length L0. The temperature can also be 20 °C in each case.
The moisture expansion test is standardized in ISO 8226–1 (1994) and ISO 8226–2 (1990).