Cuben – The 422 mm hydrostatic head dirty little secret

[David Ure]

"This makes me think of Black Diamond's Epic tents. Wonder what the HH is for this?"

David, I e-mailed BD a couple of years ago and they indicated the Epic was water resistant with a HH of 800mm. As you know some claimed no issues in the rain. Others had problems over prolonged, heavy showers. Maybe that is why they were billed as winter shelters (primarily).

Interesting about the dye. Someone try that.

[Steven McAllister]

Hey Dave O,

"As for a column of water on top of a tarp, I have seen plenty of new tarp-ers set up
shelters that would make a bathtub proud."

There is a saying "Practice makes Permanent". I have seen many very experienced(thousand miles or more) backpackers who always seem gravitated to dry ponds, river beds and washes as places to pitch their shelters.

I don't know how many times they have complained about waking in a puddle of water and/or complained about their leaking bathtub floors:-(

[Lawson Kline]

Hey Richard,

The big tent makers use Hydrostatic Head as a marketing hook since most people don't have a clue what its measuring.. STATIC NON-MOVING WATER PRESSURE.

Could you please confirm the hypothetical psi/mm that a 3mm rain drop hitting my cuben fiber tarp at 60 mph would of generated and with this number please explain why my tarp did not mist even though the pressure would of hypothetically been more then 25 times greater then the materials capacity..

I think its time for you to pitch your tarp and take a hose with a high pressure nozzle and see what I am talking about.. After you see the results first hand you will also agree that a suter tester is great for measuring static water pressure but with the complexity of a rain drop it just doesn't work..

Cheers,
Lawson

[Tyson Marshall]

"I think its time for you to pitch your tarp and take a hose with a high pressure nozzle and see what I am talking about.."

+1 ('cuz that's what I'd do)

If someone handed me two pieces of fabric (one silnylon and one cuben fiber) and started spitting out numbers about hydrostatic head, I'd say: "Great! Now, let me pitch these and let me throw some water on these puppies!"

[Aaron Robson]

Related to what Lawson is saying, the one part of this whole HH business that strikes me as a bit suspect is the difference between standard pressure numbers, and pressure with respect to time. The logical argument in my head says that there has to be a practicable difference between 14 psi (~10,000mm equivalent) exerted for a period of 10 milliseconds or ten minutes. Can anyone shed any light on this issue?

[Ross Bleakney]

My cuben tarp has never leaked. But then I do not live in the PNW. But it has been is some serious thunder storms.

This is the real world test that I think is most relevant. I think we are all agreed that Cuben (or silnylon) is just fine in a moderate mist of a rain. However, there is some concern if the rain really starts coming down.

What is more, if you have a nasty thunderstorm, you may not have much condensation. Basically, you have lots of rain hitting you from above, but the humidity (inside the tent) may be pretty low. On the other hand, if you experience typical Pacific Northwest rain, then you may have condensation. Basically, it may not rain hard, but it will rain all day, and all around you. It is essentially like being in a cloud (visibility is very low). Every surface is wet (even underneath). In such weather, it is very difficult to determine if "misting" is caused by condensation or by a weakness in the fabric. In general, I think a hose test (on your tent or tarp) would be sufficient. This would also test the seams of your tent.

There is also another possibility, which I think is unlikely. That is that the material leaks after sitting in rain all day. It seems like that would be pretty easy to test.

My biggest concern is whether the material wears down over time. I haven't heard any reports, but I could see how that could happen. Perhaps a hose test at the beginning of the year for your tent makes sense. Again, if nothing else, it tests whether the seams need resealing.

In general, though, I hope it doesn't stop people from making (and buying) Cuben. The biggest known weakness in Cuben remains its high cost. As worst, it appears that Cuben is no better than silnylon. Well, that sure doesn't bother me. I have no qualms at all about silnylon — the only reason I would pick Cuben over silnylon is because of weight. I would certainly prefer both materials over something heavier.

[Steven McAllister]

Be aware of one gotcha with the hose test.

If you spray the outside of a shelter with cool water on a hot muggy day, you will end up with condensation on the inside. So you still can't assume there is any leakage if you see water inside.

Same effect as if you pour cool water into a glass on a hot muggy day.

[Tyson Marshall]

Hot, muggy day? I have no idea what that is… (at least when I'm here in the PNW) :P

[dan mchale]

Steven, A hose test would have to be performed much like the column test except that you use the column or housing to keep the jet of water from contaminating the other side of the fabric, and of course the housing and jet stream would be firing horizonally so water could drain back. The housing would need a series of offset screens to break up the jet of water that is coming from the hose and nozzle. This would break the jet into raindrops. I would imagine that the rain test machine posted at Backpacker does much the same thing, except that it would have provisions to measure velocity and weight of the rain drops produced. It would even be pretty easy, in an engineering sense, to be able to mount the target pieces of fabric at an angle to simulate various pitch angles, even in the rudimentary homemade yard version.

Perhaps Richard Purchased the wrong machine for our real needs here – which is to subject manifestly different fabric types to a fair test. The test machine link is in the thread at Backpacker that links to this one I think.

Stuart, this actually takes a fair amount of effort and time. Ideas don't fall from the sky like rain. Conjecture like this is the basis of science. Looking around for victims to submit to an HH test, a test that may or may not be appropriate for fabrics developed since these HH types of machines were developed, is not necessarily science. It's easy to get into situations where the fabric is then designed for the test, and that becomes the reality. That type of development can lead to dead ends ultimately.

What really bothers me is the assumptions Richard makes about the viability of his test. He did not even test the tarp in a real sense before returning it and disparaging it and the company that made it. A scientist would have been fascinated by discovering that the tarp's HH results did not jive with standards that have been created, yet people seem to really stand behind the stuff. A scientist would not gone off in a cloud of hysteria and started a post like this – but maybe a journalist/technician would.

[Diana Vann]

"Great! Now, let me pitch these and let me throw some water on these puppies!"

And while you're at it, would some of you who own shelters with cuben floors be willing to pitch your bivys or bug tents and then have someone flood the ground around (and under) the shelter while you're sitting inside?

That may be a lot to ask, but it would be very helpful information for those of us who are currently contemplating placing orders.

[Lawson Kline]

About two years ago I built a rain tester. I used a 2' long piece of 6" diameter clear pipe. The material is attached to the top end and on the bottom end there is a built in adjustable spray nozzle that has been adjusted to evenly hit the material. The end with the spray nozzle is open that way after the water hits the material it can fall back down and drain out the bottom. It works pretty good but its hard for me to calculate the actual pressure of the water stream thats hitting the material due to all the variables.. Maybe some of the scientists on the forum could come up with a formula that can be used.

[dan mchale]

So how did the various fabrics do in your testing Lawson?

[Tyson Marshall]

I remember, distinctly, a week spent in my tent, in Rome, when it rained straight for 5 days (I was in a silnylon floored tent) and I stayed dry… even when my floor turned into a waterbed after (about) night two…

I enjoyed that experience… I shall try and replicate it with Hyperlight's HMG Echo II insert (cuben floor)…

[Lawson Kline]

To be honest with you I have only tested a few varieties of silnylon, tyvek and cuben fiber. Some of the silnylon misted/sprayed thru right away, some misted after a while, and some didn't mist at all. The heavier and the better the finish the less likely chance it was to mist.. All the tyvek I tried misted after some point or another but I can't remember off the top of my head which version was the best.. I want to say it was one of the heavier hard sided versions. The cuben fiber was the only material that didn't wet out or mist. Not sure what kind of hypothetical psi were talking but I think in conditions like my tester made I would be more worried about the materials strength then the material misting..

[dan mchale]

Obviously that's a biased test because you are a Cuben dealer that traffics in nasty secrets. Ha ha (smiles all around). Seriously though, I had my fingers crossed on your outcome. But it does not surprise me really. Even mentioning silnylon in your test in a generic way is not fair to many of the good silnylons out there. I have had some that could not hold 100mm of water for 2 minutes. I think Richard may me suffereing from all the frustration with Silnylon the last few years that had exceptionally poor waterproofness. It did not take much of a machine to see that. There are probably still huge quantities of that stuff floating around, and I salute Richards concern over that.

We don't know the brand or model of Richards HH tester yet I don't think. He said he bought it from a company that went out of business, or it was an older model or something? I would think there is a protocal for these testing machines, where they need to be certified and re-certified, calibrated and recalibrated. Cerifification of any results is of some importance.

[Lawson Kline]

Hey Dan,

I think silnylon is a great material and for alot of applications its much better then cuben fiber.. If you re-read what I wrote, I said that some of the silnylon did not mist at all but facts are facts and the stuff does wet out. I think silicone has some water absorbing properties that cause this to happen. This doesn't mean that the material misted it just means after an extended periods of rain or water contact the material feels a bit wet. Even with the best quality silnylon's you cannot just shake the material dry. Cuben fiber on the other hand just doesn't wet out.. It could rain for a month straight and you can shake the water right off the material and put it away dry..

[Richard Nisley]

Dave O. and Dave U.,

Mark Verber’s blog at http://www.verber.com/mark/outdoors/gear/shelter.html says EPIC: The Black Diamond Epic tents used the Alpine Summit process fabric. It averages a hydrostatic head of 1,300 mm.

I placed an order for the Epic Alpine mini Ripstop Polyester listed below:

On 3/22/11 I tested the OWF fabric they called Epic Alpine. The air permeability (Frazier number) was 2.90 CFM. Please note that this value compares to an ID eVENT Through Hiker jacket that I tested at.53 CFM. This means it would breathe much better than an eVent fabric similar to what is used on the ID thru-hiker jacket.

The HH value for the fabric tested at as 1,124.79 mm H2O. Please note that this is approximately equal to the best silnylon tents currently on the market. If you look back at the old posts on this forum you will find that numerous people reported thet never had a problem and others said that after sustained hard rains the BD Epic tents leaked.

[dan mchale]

It's been pretty well known for quite awhile that these tents are considered a compromise. No big secret there. If I owned one I would make a light fly for it for the more extreme rains – probably out of Cuben Fiber…..or quality Sil. This would fit in with my UL layering concept for dealing with rain. That might be a good after market product for the cottage guys to make – keep in it the tarp design realm rather than full coverage. This could easily be accomplished by pitching a multi-use poncho over the tent as well, but I like the idea of 3 pointed triangular wings to save some weight. I have made awnings like that for my Integral tents to replace ID's heavier vestibules.

I suppose Richard you are looking at these HH numbers to justify your behavior with your tarp. The main difference is that the Cuben has a good reputation, and the Epic tent has been known to not be the best in serious rain.

[Steven McAllister]

Hey Lawson,
Re:
"Even with the best quality silnylon's you cannot just shake the material dry"

I think it is common knowledge that silicone and nylon separately absorb a little water, so I'd assume combined they do as well.

Of course being that the material is so thin it can't hold much water to add a lot of weight. It dries very fast as well.

Think of how heavy the old canvas tents used to get when wet and how moldy they got if you didn't fully dry them:-)

[dan mchale]

Lawson, I was just being devils advocate, joking, and speed blogging at the same time. I did not mean to not take into account the varieties of Sil perspiration you mentioned. Sorry for the error there. (real smile).

[Lawson Kline]

So Richard, Do you think this makes Epic is more rain resistant then cuben fiber?

Also did you happen to work on the calculations on the hypothetical hydrotstatic head of 3mm rain traveling at 60mph and why the psi wasn't strong enough to cause the cuben fiber to mist even though it would of hypothectically been 25+ times more then what the material should be able to handle.

I look forward to your thoughts.

[Dan @ Durston Gear]

I am sending Richard samples of 0.33oz and 1.26oz cuben to get HH numbers for these materials.

I suspect that the reason that 0.7oz cuben seems to be testing low, yet performing excellently in the real world is because the HH test holds the pressure on the fabric for a long period of time, whereas a real rain drop impact lasts for just a split second. Due to capillary action or something else, cuben may be capable of holding a high HH value for split seconds, but if you sustain pressure over a long time then it seeps through… And this doesn't happen in the real world…just some pure speculation on my part.

[David Ure]

"It's been pretty well known for quite awhile that these tents are considered a compromise. No big secret there. "

Yup. Worked so well for Black Diamond that they discontinued using the fabric and made a point of marketing their new 'more waterproof' fabric.

; )

[David Olsen]

So I am wondering what the difference is between the mylar used in space suits and
helium balloons vs mylar used in the cuben fiber? What IS the hydrostatic head of
helium at sea level?

When I first call cubic tech to find out about their product, one of their customers
was using it to make sail kites with the structure to keep the sail open made up of
air filled sections that were inflated with a pump. I wonder how that worked out?

Do the heavier duty space blankets remain waterproof?

Will Aussie wool ever be a good shelter fabric?

Sorry, long day.

[Roger Caffin]

Hi all

There have been some very valid requests for more information about the pressure exerted on a fabric by high-speed rain drops. Would you believe that this is an area where we have very limited information, either theoretical or practical? Yes, I have been scouring the research literature for some time.

One reason for the limited knowledge is that 'rain on a tent fly' is not an area which can attract a huge amount of research funding. Very sad. Some allied areas exist, but they too get little funding, and their test conditions are not the same.

Another reason we have a problem is that the dynamics of what happens when a drop of water hits a rigid surface turn out to be extraordinarily complex. Raindrops are not round for a start, and their shape depends on their size and speed. The bouncing around inside the raindrop of the impact shockwave, and the reflections off the surface inside, are horrible. Validating the assumptions made in creating such a model is also extremely difficult. Supercomputer stuff for sure.

Testing what happens in the real world in a reproducible manner is equally complex. How do you measure the pressure at the surface of a raindrop as it hits that rigid surface? Bear in mind that the dynamics mean that you have to be able to measure the pressure over an area of (say) 0.1 mm x 0.1 mm or smaller, with a time resolution of about one microsecond. We don't have affordable sensors which can do that, as far as I know. And yet, this pressure is what seems to matter.

Now, having thought about all that in relation to a fixed rigid surface, what about the dynamics of the fabric surface? How will it deform? Over what area? and what about the impact angle? How will this affect the behaviour of the raindrop? Basically, dunno.

The final problem is that BPL does not exactly have a NASA-sized research budget. Sigh!

But we will try to get some answers. It just may take a little while.

Cheers
Roger

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