Just how waterproof is waterproof?
Certain Sea to Summit products are made of waterproof materials, and those designed for exposure to really wet environments are made of waterproof fabrics with the seams either seam-taped* or welded.
In the case of our dry sack range, we use different fabrics depending on the expected use: A backpacker will want the lowest weight whereas a white-water river rafter will need the highest possible abrasion resistance. These fabrics will have varying levels of waterproofness. But – just how waterproof are they?
Occasionally, you’ll hear someone make a comment that they assume a thing will be absolutely waterproof. But clearly there’s a difference between “garden hose” and “fire hose” waterproof. Fortunately, there’s a simple way of quantifying this: How much pressure will a fabric sustain before water forces its way through the weave/waterproofing layer?
The most common way of expressing this ability to resist pressure is called hydrostatic head. Here’s what it means:
Imagine stretching a piece of fabric across the base of a tube (so tightly that water cannot leak out around the seal). Now fill the tube with water. If the water forces its way through the fabric when the tube is filled to 1 meter, the hydrostatic head for that fabric is 1,000mm. 5 meters of water in the tube and the hydrostatic head would be expressed as 5,000mm. And so on.
Unfortunately, there is no universally accepted standard for the hydrostatic head which constitutes ‘waterproof’. How waterproof a piece of gear needs to be is often a case of the conditions under which it will be used: a tent rain-fly used in the Californian Sierras will not be exposed to the same level of wet weather as a rain-fly used in British Columbia. (Wind-driven moderately heavy rain has a hydrostatic pressure of about 2000mm).
Sea to Summit has selected fabrics for its range of dry sacks based on their hydrostatic head performance as well as on how rugged and lightweight the finished product needs to be for its intended use.
The fabric used in Sea to Summit Lightweight Dry Sacks, Stopper Dry Bags and Big River Dry Bags will support a column of water 10,000mm high – approximately 33 feet. They vary in ruggedness from moderate to very tough.
The fabric used in Sea to Summit Ultra-Sil® Dry Sacks will support over 2,000mm. These products are very light weight and are designed for use inside a backpack – the fabric is not suitable for abrasion/puncture risk environments such as boating.
However, when we say a dry sack uses 10,000mm hydrostatic head fabric, we’re not suggesting that it can be taken down to 30 feet or so under water – the roll-top closure will not support anything like this water pressure. This is the reason why Sea to Summit Dry Sacks carry an advisory stating that they are not intended for submersion use. It is not because the fabric is in some way inferior to other products on the market; quite the opposite. It is because a roll-top closure will only support a modest amount of hydrostatic pressure, even if it is carefully closed. This is true for all brands of dry sacks, not just those from Sea to Summit. A wave washing over your boat? No problem for a Lightweight, Stopper, Big River or Hydraulic Dry Bag. The same dry sacks under several feet of moving water for minutes at a time after capsizing? Probably more water pressure than the roll-top closure can keep out.
If you’re going to be using any piece of ‘waterproof’ gear in very wet conditions, you should inform yourself of the hydrostatic head of its fabrics prior to making a purchase – this includes dry sacks, rain jackets, and (most particularly) tent floors. Balance this against how much weight you’re prepared to carry, and how rugged you need the item to be.
Bear in mind that a product can be made of waterproof fabric (Ultra-Sil® Compression Sacks, for instance), but not have tape-sealed seams – and therefore will only be water resistant, not waterproof.
If you’d like any more information about the hydrostatic head of any Sea to Summit products, or more explanation of any of the above points, send me an email via this blog.
* Sea to Summit uses seam tape from the industry leader, Beamis – its part of the superior construction that makes our gear perform the way it does.