Cuben – The 422 mm hydrostatic head dirty little secret

Dan,

You are correct. (Edit: – I didn't read James M.'s post before I responded to Dan's question. I defer to James's very well-reasoned analysis)

There is an old forum thread explaining how Franco made one by setting up an opaque vertical PVC pipe 50mm wide and 3m long pipe in his yard. See: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=34234&skip_to_post=292139#292139

A simple DIY unit, like Franco’s, should provide similar test results to any commercial unit if you just test to initial failure (defined as 3 visible drops). The standard procedure also requires that water be added no faster than the prescribed rate. For a low pressure tester like Franco’s it is 600mm pressure rise/minute.

My commercial unit automatically adds water as it leaks out of a fabric sample (electric pump and control valves) in order to maintain an exact pressure for any period of time regardless of the leakage rate. For that type of test, you would need to periodically refill your water column to the prescribed test level and I would expect significant variance for this scenario only.

Thanks Richard, I just wanted to make it known how simple it is for people to test up to at least minimum standards in the 1,000mm – 2,000mm. MMs can be converted easily to inches if that is a stump for people. Anything can be used to create the column; old pipes etc. The most technical part is just sealing the test sample. Multiple clamps or even a final adhesive tape seal would work.

Marco, interesting you should say, "especially with laminates". :>) I should not make it sound that easy to do, but it's something to do if a person has time – a science project for the kids?

I don't know how I missed that article. Thanks for the link guys. I'm surprised it's so short! I'm going to fire it back up since it's so relevant.

I guess that can't be done. Oh well, maybe there will be new interest here. Here's the link again as a starting place;

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=34234&skip_to_post=292139#292139

did such a test with the lightest cuben and an old 4" cardboard tube. Gave the stuff a 5 min wrinkle work out, sure enough some droplets did form at 450-75 ?mm. Wish i had some of the mid weight stuff around still.

"Marco, interesting you should say, "especially with laminates". :>) I should not make it sound that easy to do, but it's something to do if a person has time – a science project for the kids?"

Ha ha, you would think so. No, it is a bit more complex than that. Depending on how it is mounted, you could be stretching one layer more than the other. Laminates are not solid. Rather a series of layers under pressure. When you mount a laminate, chances are, you only mount one layer solidly at any one point. The unevenness could influence stretching, hence, permiability. A flat press would be better. With threads? I don't know. Depends on their compressibility compared to the fabric. I don't know what it is for cuben.

It may not matter. At the low heads that seem to be measured, it seems that strength is not much of a factor.

Ziff, what do you mean you did a test with the lightest Cuben? Which one did you test?

The lightest would be lighter than even the .7 oz Cuben in question.

there is a .33 @ quest outfitters. However this one is the only one available now weighs in @ .66 oz, it may be an oddball its looks lighter thinner than some .7 i used before, that's why i considered it more like the lightest. Quest was/is having trouble getting their regular stuff.

I am just posting here to see if this thread can keep ahead of the "Friendly expertise on camping. Please READ!!" Thread.

Damn you David:-)

just trying to help the ratings

We need the Admin folks to create a "thread velocity chart" – Posts versus Time.

"Friendly expertise on camping" has pulled in 95 posts in 7 hours.

"Dirty little secret" got only 30 in 7 hours out of the blocks, but it did accelerate a bit after that ;-)

I'd love to see a charting function.

It's all fun:-)

That's a great HH Ziff, considering it's about half the weight of the 1/2 weight Cuben.

Well, I was wondering this earlier too! And now I got home to find this post. Good to have a laugh. Now back to our regularly scheduled …….

Certainly not to the bottom of this dirty little secret though. If it even turns out to be "dirty".

Please Click To Read the Final Conclusion

Just wondered if I could get anybody to get in a loop like I do in Excel by mistake (smile).

The recent forum discussion regarding HH measurement correlation with rain is not explained simply enough. The KISS approach (Keep It Simple Stupid)version is:

Source: Clothing test methods By National Research Council (U.S.). Subcommittee on Clothing, Louis Harry Newburgh

“… of the simple routine tests, the hydrostatic pressure tests, group(3) are perhaps the most informative, since they measure a quantity which is influenced by both fabric construction and finish. Within any group of comparable fabrics, the hydrostatic pressure test provides a good indication of rainfall resistance…”

Different HH recommendations are listed from various expert sources primarily because different average rain drop sizes most frequently apply to a geographic market or activity. For the US weather range, REI sets their standard at 3,000 mm. Stop here to adhere to KISS.

The smallest common rain drop size in the US is .6mm and a largest common rain drop size is 6mm. The largest raindrops recorded in multiple countries are 10mm. For the US, there is over a 5,000 fold difference in the kinetic energy between commonly occurring .6mm to 6mm raindrops. There is also a very broad range of shelter force dispersion mechanisms including the vector angle, the tension, the surface texture, the wind, surface water height, etc.

Richard,

A circular reference is much preferable to a "Unrecoverable Application Error" or a "General Protection Fault" (for us old-timers).

Richard – I read your post and I for one appreciate all that you do with respect to these tests. I am of the camp that the cuben you have tested is NOT functionally waterproof. Other batches may be; other types and weights of cuben may be.

Where the discussion took a turn was whether an under 500mm HH meant that using this fabric as a tarp would be prohibative? In this application, I would say no. I would be more concerned if cuben was really weak in terms of tear strength as a tarp (which I believe it isn't). But because of the floorless application with a tarp or tarp hybrid, it makes sense to use cuben. Would I want this fabric used in an enclosed shelter? No – as that might give a new meaning to bathtub floor.

So please don't 'bow down' to anyone. Continue to generate thought – this is what the forums are all about.

A circular reference is much preferable to a "Unrecoverable Application Error" or a "General Protection Fault" (for us old-timers).

Segment violation…sounds sexy…hey, hey…

the science guys are going to end up with '.less than the best

"the science guys are going to end up with '.less than the best"

The phrase "perfect is the enemy of good" comes to mind.

Good is the greatest enemy of Great

A person who argues with his tools is a poor craftsman.

grog

ha!
Dan, actually the stuff in my tent is .5 ,it got a good workout just making the thing, it was good for one rainfall so far , which is good enough for me as i have never punctured a tent or tarp nor do i foresee a force that would make it leak. As long as i don't sleep under 425mm of water!

Good post. That's the spirit!

Ah…to trek where it doesn't rain.

Thanks for chiming in David when you said this; "Where the discussion took a turn was whether an under 500mm HH meant that using this fabric as a tarp would be prohibative? In this application, I would say no. I would be more concerned if cuben was really weak in terms of tear strength as a tarp (which I believe it isn't). But because of the floorless application with a tarp or tarp hybrid, it makes sense to use cuben."

The way I read Ziffs post he meant literally sleeping under a layer of water 422mm thick. I believe that's the point he was making. We could ask him.