Who's heard of water column ratings? Probably most outdoor enthusiasts. A scientific explanation of the water column test might read as follows: The water entry pressure test, or hydrostatic pressure test, is a simple procedure to determine resistance to water penetration under hydrostatic pressure. The test is defined by an ISO standard. Is that clear? Or to put it more simply: water is applied to one side of a fabric sample and the pressure steadily increased until water penetrates the material in three places. The result shows the resistance of the fabric to the penetration of water. The rating is expressed in metres of water pressure and abbreviated to mm H2O. Retailers typically refer to the number of mm of “water column” or even mm of “waterproofness”. However, this method of testing the waterproofness of a garment is not relevant to real-life situations. For example, when you wear a jacket, any pressure that is exerted is not static. Even when you put on waterproof trousers and kneel in a puddle, the pressure exerted is not particularly high. Your knee will quite simply push aside the water in the puddle. When a raindrop falls on a garment, the process is dynamic, but this is not simulated in the water column test. Even fabrics with very low water column ratings have turned out to be waterproof under real life conditions. The Swiss Federal Laboratories for Materials Science and Technology (EMPA) analysed several garments made with fabrics whose water column ratings were below 120 mm and established that the garments remained fully waterproof when rainfall was simulated. These were closely woven fabrics with a water-repellent fabric treatment. Compared with ratings of 10,000 mm or even 20,000 mm, it is hard to believe that these fabrics could be used to produce waterproof garments.

Partly waterproof?

Nevertheless, in the retail trade water column test methods and ratings are the generally accepted way of determining the degree of “waterproofness”. “The higher, the better” is a common misconception. However, a garment cannot be “a bit waterproof” or “a bit more waterproof”. Waterproofness is an absolute measure: a fabric is either waterproof, which means that it does not let water through, or it isn't. Even a tiny hole in the fabric, measuring no more than 10 μm (0.01 mm), would result in a water column rating of 0 mm and amount to a leak. Equally, the test cannot predict how long the garment will remain waterproof. Fabrics that achieve ratings higher than 20,000 mm H2O can start showing signs of leakage within a few uses or after home laundering. To cut a long story short: the test is totally unsuitable for determining the waterproofness of a garment, let alone its durability. It's rather like judging your state of health by weighing yourself.

The solution: a test in the rain

So what should we do? To determine whether or not a garment is waterproof, the finished garment needs to be tested in the rain. At Gore we can simulate rain in a sophisticated testing facility specifically designed for this purpose. Light drizzle in Scotland or the kind of wind-driven rain encountered in the Himalayas – we can simulate both conditions in our standardised rain room facility. Garment prototypes not only have to survive this test when new but also after a simulated ageing process. This is the only way in which we can ensure that the garment is durably waterproof. If a prototype fails to pass this test, the necessary changes have to be made before it can be brought to market. Only when the garment has withstood rain can it go into production and carry the GORE-TEX GUARANTEED TO KEEP YOU DRY™ promise. Needless to say, the rain test is just one of the many laboratory and field tests that we carry out. In order to gain style approval, new jacket prototypes are put through their paces in as many as 100 rigorous tests in our testing facilities to ensure that the garments conform to our high quality standards - and one of these tests is the rain room test.