Laboratory Tests for Breathability
There are many different laboratory-based tests for breathability. Some are simple methods (cup methods), whereas some are extremely complex.
Cup methods are performed under steady-state conditions and so poorly represent the changing environments of real-life use. One of the most commonly-used methods is the Upright Cup Method, where an aluminium cup is filled with distilled water and covered with the test fabric. After a predetermined period of time the dish weighed and the amount of water that has evaporated is calculated. The inverted cup method is identical to the upright cup method except the apparatus is inverted, which removes the air-gap, thus providing a different testing situation. The Evaporative Dish Method (British Standard 7209) is similar to the upright cup method except a series of samples are prepared and rotated on a turntable, which provides an air current. It shows a good correlation with field trials. The desiccant inverted cup method uses a desiccant such as potassium acetate to draw water vapour through the waterproof-breathable fabric. The technique measures the mass increase that occurs in the cup due to water vapour uptake.
Complex methods of testing breathability continue to be developed, though most do not go on to form international standards. Companies may develop new methods of testing to suit their own fabrics. The DMPC (Dynamic Moisture Permeation Cell) was developed at America’s Natick Army testing laboratory. The test controls both humidity and temperature between flows of gas. Thermal manikins can also be used to determine water vapour permeability. A manikin such as ‘Walter’ is made from water and a ‘skin’ composed of the sample fabric. This sort of testing is exceptionally expensive, but provides results that can be obtained through few other methods, as the manikin not only sweats, but may be able to move as well. Another recent development in testing breathability is the Human-Clothing-Environment (HCE) Simulator, which can be seen as a combination of the sweating manikin model and the, somewhat simpler, sweating skin model. The HCE Simulator separates a cold chamber from a ‘sweat’-distributing hot chamber using a sample fabric. A well-known industry method is the sweating guarded hotplate test, which can perform under dynamic and steady-state conditions. It forms both ISO 11092 and EN 31092. In the test, the sample is held over a pool of water on a heated plate and the amount of energy required to keep the plate at the same temperature, despite evaporating water, is measured. The test takes place in a controlled environmental chamber.