Moisture equilibrium: It is the condition reached by a material when it no longer takes up moisture from or gives up moisture to, the surrounding atmosphere.
Pre-conditioning: To bring a sample or specimen of textile to relatively low moisture content (approximate equilibrium atmosphere with relative humidity between 5% and 25%) before conditioning in a controlled atmosphere for testing.
Conditioning: To bring a material to moisture equilibrium with a specified atmosphere. Before a textile is tested, it is conditioned by placing it in the atmosphere for testing in such a way that the air flows freely through the textile and keeping it there for the time required to bring it into equilibrium with the atmosphere.
Atmospheric conditions and relative humidity: The dampness of the atmosphere can be calculated in terms of humidity.
Absolute humidity: The weight of water present in a unit volume of moist air, that is, gm/m3.
Relative humidity: The ratio of the absolute humidity of the air to that of air saturated with water vapour at the same temperature and pressure, expressed as a percentage.
RH% = (Absolute humidity of air / Humidity air saturated with water vapour) × 100
Reference: A Practical Guide to Textile Testing by K. Amutha (Woodhead Publishing)