Certain Sea to Summit products are made of waterproof materials, and those designed for exposure to really wet environments have seams which are seam-taped or welded. But – just how waterproof are our dry sacks, or Tarp Ponchos or the floor of our Escapist Bug Shelter?
Occasionally, you’ll hear someone make a comment that they assume a thing will be absolutely waterproof. But there’s clearly a difference between “garden hose” and “fire hose” waterproof, and therefore ‘waterproof’ is a scale, not an absolute. 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. (In certain countries, a fabric can only be described as ‘waterproof’ if it has a hydrostatic head greater than a certain clearly defined minimum. Unfortunately, the USA does not have a generally agreed on definition for waterproof)
Here’s what hydrostatic head 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. Five meters of water in the tube and the hydrostatic head would be expressed as 5,000mm. And so on.
How waterproof a fabric needs to be for a particular function will vary depending on conditions. A tent floor which is waterproof enough for high-and-dry Southern California may be inadequate for boggy campsites in New Zealand. A dry sack which works well to keep rainwater off of gear and clothing inside a backpack may be inadequate for whitewater rafting. And so on.
Sea to Summit draws attention to this by describing a dry bag or dry sack on its website as ‘suitable or not suitable for boating/marine use’ (there are other factors apart from waterproofness involved in this definition, most particularly abrasion resistance and tear strength)
The fabric used in Sea to Summit Lightweight Dry Sacks and Big River Dry Bags will support a column of water 10,000mm high – approximately 33 feet. The eVent laminate used in our eVent Compression Dry Sacks and eVac Dry Sacks will support twice this pressure (over 60ft!).
The fabric used in Sea to Summit Ultra-Sil® Dry Sacks will support over 2,000mm, and thus we describe them as ‘not designed for boating/marine use’.
The fabric used in Sea to Summit Ultra-Sil® Nano Dry Sacks will support 1200mm, and thus they have no place in a boating or marine environment.
Of course, even though some of our dry bags have a 10,000 mm hydrostatic head, we’re not suggesting that a Sea to Summit Lightweight Dry Sack or Big River Dry Bag can be taken down to 30 feet or so under water – the roll-top closure will not support anything like this water pressure. With any brand of dry bag, water may begin to seep through the roll-top closure if it is under water even a shallow depth or for a significant period. Depending on how well the roll-top closure was sealed, seepage may occur in a matter of minutes of the top of the dry sack being submerged. This is the reason why Sea to Summit puts advisories on its dry bags and dry sacks explaining that they are not suitable for submersion use, and why ‘double bagging’ of sensitive / electronic devices is essential.
Understanding waterproofness is not limited to dry bags and dry sacks, however. Consumers should inform themselves of the hydrostatic head (or water head) of a fabric prior to making a purchase of any piece of gear designed to protect against wet situations – dry sacks, rain jackets, and (most particularly) tent floors.
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 toSummit products, or more explanation of any of the above points, send me an email via this blog.