A water column.What’s that?

The water column (or hydrostatic head) indicates how waterproof the fabric is. This measurement is especially important for outdoor and rainwear or tents. But what does this specification really mean? How high must it be to be moisture resistant?

How is the hydrostatic head determined?
To test the waterproofness of a fabric, an empty water cylinder is placed on the fabric and filled with 100 mm of water every 60 seconds. The fill volume, i.e. when 3 drops of liquid penetrate the fabric, indicates the maximum water column. This value is the value derived in the hydrostatic pressure test and is a guide value for water repellency.

How high must the hydrostatic head be?

The hydrostatic head indicates the degree of water repellency of the fabric. (© Ward)
The hydrostatic head indicates the degree of water repellency of a fabric. (© Ward) Although fabrics are considered waterproof at a hydrostatic head of 1,500 mm, it should be remembered that the pressure exerted while sitting or kneeling is different. Thus, for example, the floor of a tent will require a higher column of water than the walls of the tent. The same applies to rain jackets used with heavy hiking packs, as the pressure on the shoulder straps is much higher. This results in a hydrostatic head of more than 10,000 mm, often even 30,000 mm for outdoor jackets. Values of 4,000 mm or more are classified as fully waterproof.
What else should I look out for?
In addition to the membrane itself, the coating is also an important factor. In order to maintain the stability of the fabric over time, a high-quality coating and proper care are very important.

Only good workmanship can guarantee absolute watertightness. The seams of a waterproof suit or tent must be specially treated to prevent water from seeping through the stitching holes. At the same time, zippers are either protected with cover tape or welded or glued.