The breathability of a textile, a key feature for sporting and outdoor clothing, is determined by the textile's permeability to water vapor. According to the Hohenstein Institute, customers should not rely too much on advertising claims as many products that are called “breathable,” are, in fact, not. This is partly because of difficulties in selecting the most suitable test procedure to measure breathability, on the one hand, and in finding harmonized measurement criteria, on the other. Only a verified statement about the Ret (resistance to evaporative heat loss) value of a textile, says the Institute, indicates its real breathability, and this can be accurately determined by using the “skin model,” on which the Institute's breathability label is based.
Many different methods exist to assess the breathability of a textile but they differ from one another in the test procedure and requirements, the set-up, the ambient conditions, and the units in which the results are expressed, explains the Institute. Therefore, the results are difficult to compare. The methods for measuring the water vapor permeability of a textile can be divided into two groups: the gravimetric methods, also known as “cup methods”, and the “sweating guarded-hotplate” methods, which use a skin model in the measurement process.
Cup methods are all based on determining the amount of water vapor that passes through a textile by gravimetric analysis, i.e. by weight. Cup methods can be used as part of quality control directly at the production site. However, some of them can only be used for specific purposes. For example, some cup methods are suitable for products where only small quantities of water vapor need to be transported through the textile, while others are mainly suitable for textiles with a waterproof coating. The advantage with cup methods is that the test is generally easy to set up, the procedure is simple and it only takes a relatively short time. These tests are therefore low-cost. The main disadvantage lies in the wide range of existing cup methods, based on a variety of national and international standards. Since the test requirements differ from one test to another and are not accurately defined, it can be difficult to reproduce the tests or compare the results.
Another way of determining the water vapor permeability of a textile is the “sweating guarded-hotplate” method. This method calculates the Ret value by using a skin model that simulates the sweating of human skin. The main component of the skin model is a porous sintered metal plate, heated to 35 °C, which simulates the heat and moisture given off by the skin. The test is carried out in standardized conditions in a climate-controlled cabinet in which defined ambient test conditions have been set. Also, the skin model is suitable for textiles of any kind that can be worn by people, both water-repellent textiles and textiles intended to transport large or small volumes of water vapor. Because of the complexity of the test set-up, the Ret test is mainly carried out by well-trained laboratories and entails higher costs. The complexity of the test also means that it takes longer than most cup methods. However, results from different laboratories can be easily reproduced and compared.
The Hohenstein Institute developed a skin model for measuring the heat and water vapor permeability of textiles back in the 80s and has long-standing experience in measuring the Ret value. The Institute was founded by Prof. Dr.-Ing. Otto Mecheels in Bönnigheim, Germany in 1946. Mecheels' priorities were research and teaching directed to practical applications. Today the Hohenstein Institute offers manufacturers, retailers, service providers and consumers worldwide competence primarily on the development, testing and certification of textile products.