The neatest method of measuring moisture in the paper is by defining the water content, known as the absolute moisture. Even though we have absolute values, a percentage is used as measuring unit, i.e. weight percentage related to the paper weight. An absolute moisture or water content level of 4% means that 100 g of paper contains
4 g of water.
Another option is defining the relative humidity: the relative air humidity between the paper sheets is measured using a sword-type hygrometer. The measured air layer is in equilibrium with the surrounding paper sheets. This value depends on the temperature and refers to the maximum volume of water the ambient atmosphere can absorb at a certain temperature.