Dec 9, 2012 at 3:17 pm #1296905
Franco DarioliBPL Member
Every so often the topic waterhead testing and resulting ratings come up.
I happen to think that the way tent fabrics are usually measured isn't an accurate representation of the way the fabric behaves in real life , that is when the tent is pitched and it rains on it.
The usual way to measure the "waterhead" is to use a Suter tester or something like that.
So a piece of 12x12cm fabric is clamped on to the tester , water is pushed against it at an increased level of pressure.
When you get 3 drops of water passing through the fabric you have achieved the waterhead level of that fabric.
These are the problems I have with that :
For a start tents are not set up with the fabric at an even pressure and that difference is greater in some designs than others, therefore (in my view…) the exact same fabric can give you a different result from tent to tent because of this.
To better understand this, think of the "handkerchief over a glass of water" trick.
(Put an hankie loosely over a glass of water , turn the glass upside down it will drip, now put the fabric of the hankie on tension and it won't leak any longer)
Next is, how does that fabric behave when under pressure from contact with poles underneath it ?
(think of the famous finger against a cotton wall tent..)
But here is another one. Some materials "wet out" . So what happens after a few hours of rain ?
Could it be that a fabric that holds heavy rain for a short time does not do so after a few hours?
Finally , I never seen rain falling evenly (corner to corner without gaps) on a tent.
I suspect that having raindrops over a piece of fabric creates a different surface tension than the Suter type test does.
But of course I could be completely wrong…
(edited to insert photo)Dec 9, 2012 at 4:05 pm #1934347
Jerry AdamsBPL Member
@retiredjerryLocale: Oregon and Washington
Nice self explanatory photo
I have a tarp of DWR fabric that mostly covers just my top half
I tested it for days in the rain and it worked very good
Then I tried it once, and my sleeping bag touched the inside surface, and then it leaked like crazy through that point
So, waterproofness is effected by what happens on the inside surfaceDec 23, 2012 at 8:34 pm #1937826
you are confusing DWR, and waterproof.
Water DWR fabric have a tight weave and a DWR coating is applies to the fibers to prevent water from sticking to the fibers. The DWR coating thickness is only a small fraction of the diameter of the fabric fiber. This means that there is nothing between the fibers only empty space.
While the DWR prevents water from sticking to the fibers, it will not prevent water from getting between the fibers. Once water is between the fibers the fabric has weted out. At this pint simply touching the fabric will pull water all the way through.
For all practical purposes The hydrosatic head of DWR fabric is zero.
Waterproof fabrics are also have a DWR coating but they also have a layer of plastic between you and the fabric. The plastic (polyurethane or expanded PTFE) has no voids big enough for water to get through. So even if the DWR coating fails the water still cannot get through the fabric.
"Next is, how does that fabric behave when under pressure from contact with poles underneath it."
For waterproof fabric water still cannot get through unless the plastic layer is damaged. For DWR fabric the pole will stretch the fabric creating larger voids and it will leak once the fabric wets out.
"But here is another one. Some materials "wet out" . So what happens after a few hours of rain ?
Could it be that a fabric that holds heavy rain for a short time does not do so after a few hours?"
For a DWR fabric once it wets out ((which might take 30 minutes to an hour or longer) it leaks. For a waterproof fabric The DWR coating on the waterproof fabric will wet out but it will still be waterproof because of the plastic layer.
So if you want to stay dry in the worst weather don"t use a DWR or water resistant fabrics. Only use waterproof fabrics.Dec 24, 2012 at 10:01 am #1937940
@nsherry61Locale: Mid-Willamette Valley
And, if I understand it correctly, when waterproof beathables wet out, they retain their waterprooness, but completely loose their beathability. So, they become no better than simple waterproof fabric and trend to get heavy condensation from the inside at that point.
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