Tent material question?

Question

When tent material is tested for water proofness, the hydrostatic pressure it can stand before it leaks is given in either pounds per square inch or millimeters. Is this millimeters of mercury or water? I'm assuming mercury so correct me if I'm wrong. I have one
tent where the floor material is rated at 165 psi and another which is
rated at 3,000 mm. The 165 psi converts to 8,533 mm and the 3,000 mm converts to 58 psi. Am I doing this conversion right or am I comparing apples to oranges?
I'm comparing a Sierra Designs Stretch Prelude to a Eureka K2.

PS. I already own both of these tents and am just trying to compare the specs.

Answer 1

Your calculations seem correct.
http://www.onlineconversion.com/pressure.htm
Using millimeters of mercury would make the Sierra Designs 2.8 times more waterproof which is believable. Using millimeters of water would make it 38.7 times more waterproof which is hard to believe. If you end up in a puddle both tents should keep you dry since the floor is where most tents leak.

Answer 2

"Waterproof Ratings

Even though most fabric manufactures calculate waterproof figures in their own labs, they generally use similar testing methods, so you can take their waterproof ratings at face value: the higher the number, the better the garment resists water.

According to government standards, the water-force a material must withstand in order to be deemed 100% waterproof is 25 PSI. You can zip up a Marmot Essence Jacket, whose PreCip Plus material has a rating of 45 PSI, sit in the rain (7 PSI) and stay dry as a bone. Step in front of a fire hose for an hour and if you can stay on your feet, you'll get wet. Simple.

Yet outdoor companies don't always use PSI to highlight waterproof quality. Instead they tout large figures that have a more tangible meaning than pounds per square inch. For example, the Cloudveil Zorro Jacket has a waterproof rating of 10000mm (10K). This figure represents the amount of rainfall the fabric can take in a 24-hour period before the wearer gets wet. In Imperial terms, the Zorro Jacket can withstand nearly 33 feet of rain in 24 hours. Seattle, by comparison, gets 36 inches of rain a year.

In a closed system, a 20k rated jacket will retain its waterproofness longer than a 10k jacket. However, out in the field any jacket's waterproofness can degrade over time until the garment eventually becomes a clammy, wet mess. Oils, dirt, sweat, and other contaminates find their way into the fabric, weakening the hydrophobic forces until the water repellency becomes so poor that water can just break through to the other side. To combat this, Gore-Tex and the like coat their hydrophobic material with additional impermeable membranes. While this prevents contamination and ensures waterproofing, it has detrimental effects on that other magic word—breathability."

Answer 3

Schnurrbart gave you some great technical data. My advice is more basic.

I've owned and used countless tents, including one I made myself, year round in all conditions (I also sold gear for a number of years) and I've never had a case where a reasonably new or well-cared-for older tent leaked directly through the fabric. Most moisture from the outside comes in either through seams that are not sealed, poor design (window and door placement, fly arrangement) or pitching the tent incorrectly. But the major source of wetness inside a tent is something that many people new to tent camping fail to account for: condensation. This occurs because moist air on the inside of the tent, both environmental humidity and the water vapor in the breath and perspiration from the human occupants, condenses on the cooler tent floor and walls.

In fact, the dryest tents I've ever used are the old models that Cannondale made in the 1970's which had a double wall (fly built into the tent structure with an air gap between the fly and inner wall) both of which were made of NONwaterproofed regular nylon! Think of an umbrella -- they are made of plain, nonwaterproof nylon. What would happen in a heavy rainfall is that water would seep through the taut fly layer and run down to the ground along the inside surface. A little would drip onto the inner wall but since it was not under pressure it would barely dampen it. But, because the whole tent was 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. When I would winter camp with these tents I would get no frost on the inside at all, unlike fellow campers with urethane coated tent flies. In the summer, I never had the drippy walls and damp floor that I would get with my conventionally waterproofed tents.

So, my advice is, don't worry in the least about the water permeability of tent fabric -- as Schnurrbart explains, such data make sense with jackets, but tents are a whole other matter. In my opinion, less is better in this case. If you want to stay really dry in a tent in pouring rain in warm weather, pitch it WITHOUT the waterproof fly underneath a plastic tarp pitched high enough above the tent that there is good air flow between the two. Of course, that is not always practical (plus it is dangerous to be camped beneath a tree in an electrical storm.)

Look for a well made tent with tight stitching, good flaps over all zips, good ventilation that you can keep open but sheltered during a storm, sealed seams and a taut fly with a large space above the tent wall. Beyond that, just choose the size and shape that suits you at a weight that is manageable. And, when you set it up, don't let your protective groundsheet extend beyond the tent floor (it will just channel rain under the floor.)

The sound of gentle rain on a tent roof is the sweetest lullaby I know.

PS The Sierra Designs will greatly outlast the Eureka -- if you can afford it, it's the better choice. I have friends who are still using battered 20 to 30 year old Sierra Designs tents whereas Eureka's materials usually start to deteriorate after 10 years of light use and 5 years of heavy use (stitching fails, waterproofing peels off floors, etc.).

Answer 4

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Answer 5

The testing is based on Water..Not Mercury. It doesn't rain Mercury..And the psi ratings are done on the tent material BEFORE any chemical waterproofing is applied......