flawed silnylon testing methodology

Feedback for the article published June 4, 2005.

flawed silnylon testing methodology

Postby happy_camper » Wed Jan 14, 2009 8:53 pm

Hey Jim,

Wow! Huge amount of information about waterproofness of silnylon and staying dry! Great stuff!

Though, one thing kept bothering me about your silnylon testing methodology -- turning the stuff sacks inside out and filling them with water to simulate water pressure from outside the stuff sack. It seems that doing so creates stresses in the fabric which are completely different than those present in real world conditions, even in immersion.

If you consider the fabric's normal use, the only force stretching the warp and weft apart is the tightness of the material packed inside it. The water pressing against the outside tends not do so under normal conditions. I think this is very significant since silnylon has no real "coating" per-se. The silicone impregnated fibers tend to repel water based on hydrophobic forces, which means stretching those fibers is creating holes for the water to leak through. You can see the effects of this in your photos as the quite large beads which form on the outside of your stuff sacks.

I think a better testing methodology would be to place a dry brick or other heavy object inside a stuff sack, and then submerge it in a bucket of water. You'd need to consider your closure, of course, potentially leaving the opening of the sack out of the water to only test the permeability of the fabric. You might check for leakage after varying periods of time to determine how long it takes for the sack to leak. If you remove the submersion requirement for the stuff sack, which you might not be inclined to do, but which others might find very interesting, submersion could be replaced with a shower or mist.

Thanks for an interesting read!
happy_camper
 

RE: Testing methodology

Postby Jim Wood » Sat Jan 17, 2009 1:06 pm


Hi and thanks for your feedback. I would agree that my testing methodology probably puts more stress on the fabric than might some real world applications. On the other hand, if you consider perhaps a sleeping bag stuffed into a silnylon sack, the forces on the fabric can be very substantial, maybe even greater than the water test.

Likewise, the test on the Sea to Summit Ultra-Sil sack didn't account for wear over time since the sack was new when tested (and therefore probably represents best-case performance). As silnylon is subjected to stress with use, I've found that its propensity to leak tends to increase. By the way, I've conducted the same water test on other dry sacks such as the Camp Inn models mentioned in the article and have observed no leakage at all.

I would personally rather pack a couple of extra ounces and be able to feel confident that the dry sack will perform under than worst conditions than to save the small amount of weight and have to worry about leakage. Some ultralighters might see it differently, however, so it's good to have choices.

Thanks again,
Jim Wood.


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