BD to use Nanotech in upcoming tents
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Mark,
All falling objects have a so-called terminal velocity, a speed they can’t surpass due to air resistance. Therefore, larger rain drops generally fall faster because their heftier size helps them power through air resistance more easily than little drops.
Droplets that reach 0.2 mm, the threshold for calling a drop rain, are usually heavy enough to overcome the force of ascending air that exists in every cloud. Rain can be either "cold rain", originating from melting snowflakes, or "warm rain", which evolves without the snowflakes. Cold rain has very large drops, but fewer drops overall. Warm rain contains very many, but small drops. Raindrops can range in size from 0.2 mm in diameter, to around 6 mm. Those larger than 6 mm are inevitably broken up in their fall to the earth. 8 mm is only common in a typhoon.
The size of the rain drop will determine its terminal velocity and its hydrostatic pressure. The temperature will also vary the terminal velocity with colder temperatures providing higher velocity; this spreadsheet is for 50F. Every resource uses different names to define rain drop sizes but the following is an overview.
The static water pressure of rainfall is calculated according to rain drop speed except for the last row (typhoon). Wind also influences the water pressure of rainfall. When subjected to wind velocity of 20m/sec (44.7mph-comparable to a typhoon), the static pressure of rainfall is estimated to reach 35,677 mm.
Thanks to Carter Young I had to go back and review an assumption which I determined was incorrect. Carter’s post said, “It seems to me that the flies used for expedition double-walled tents (MH Trango, TNF VE-25) are far less waterproof than you suggest. For example, Mountain Hardwear says on its website that the Trango 3.1 rates 1,500mm for the fly and 3,000mm for the floor.
MEC, which has always done a fine job listing tent specifications, says its Nunatak has a floor rated to 10,000mm and a fly to 2,000. When MEC previously sold Bibler and Moss tents, I believe that the Bibler Todd-Tex canopy was rated at 10,000mm and the Moss flies at about the same–Moss floors were I believe 15,000mm.”
Many vendors (Marmot, Sierra Designs, Eureka, The North Face (historically) list their tent fly hydrostatic head in PSI. Since they didn’t specify their test method, I assumed they used the industry standard ISO 811 or JIS 1092 test; so, I simply converted their PSI values to mm values and used the average converted number in my original post. What I discovered today is that many years ago The North Face listed the hydrostatic head of their tent based on the Mullen Burst (MB) test. For those of you not familiar with this test, it was originally designed to determine at what point cardboard boxes would catastrophically fail. Historically other tent vendors felt they had to use this MB rating system to be competitive with the North Face. Recently the North Face switched to the industry standard testing approach. Some of the other tent vendors still only use the old Mullen Burst number, some use both (without telling you which test is for which number) That is why my hydrostatic head value for PU coated 70D nylon was too high as Carter Young correctly pointed out. The correct value is approximately 5,000 mm. I updated my original post to reflect this number.
Conventional double wall tents commonly use outer fabrics with hydrostatic head rating from 1,200 mm up to 5,000 mm. As a matter of practice they don’t usually test above 5,000 mm for double wall tent outer material.
Kelty was the first tent manufacturer to use light tent flies. They found that the force dispersion angles and the separate inner tent allowed the occupant to be satisfied using a lighter coating even though water passed through the fly. What happens in a heavy rainfall is that water does go through the taut fly layer and runs down to the ground along the inside surface. A little will drip onto the inner wall but since it was not under pressure it will barely dampen it. Because the inner tent is breathable the moisture from humid air and from the exhalations of the humans inside would not condense on the inner tent walls but would pass out to the outer fly and leave the inner wall dry. They found that they didn't get double wall customer complaints with as low as a 1,500 mm fly rating.
You can do a Google search on any mm number such as “tent xxxx mm” and see many products in every category. The following is a list with just one example for each common hydrostatic head rating used in double wall tent flies.
1200 mm Big Agnes Fly Creek UL 2
1500 mm Sierra Designs Clip Flashlight
1800 mm SD Clip Flashlight
2000 mm Kelty Ridge 2
3000 mm Mountain Hardwear Viperine 2
4000 mm Terra Nova Laser Competition
5000 mm Coleman Helios XÂł
10000mm Bibler Todd-Tex (single wall)
25000mm RAB Summit Extreme Tent (single wall)Single wall tents with hydrostatic heads less than the thresholds shown in the following table are vulnerable to misting whereas double wall tents inner layer effectively mask this phenomenon. The only light weight material that will avoid misting is Cuben.
Materials such as silnylon, Epic, and Nanotech, heavily rely on surface tension to achieve their protection. Smoke, dust, and aging cause a significant reduction in their functionality.
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