Debunking Hydrostatic Head

[Tom Keefe]

So I have to ask the question, how useful is the hydrostatic head rating used to measure tent materials really?

I have several point that I would like to bring up about using this measurement.

1) The testing is done under ideal conditions with new materials that have not been pre-saturated, creased, folded, stretched, walked on, or otherwise subjected to what would be considered "normal use" before measurement.

2) The test only reveals the pressure threshold at which the material allows water to seep through. It does not consider the volume of water that is allowed through after that threshold is reached, nor does it consider whether or not or to what extent the material might degrade after this limit is reached one or more times (how the material performs after subsequent tests).

3) This measurement offer no real indication of the durability of the floor material itself. Consider that a heavy duty plastic garbage bag likely has a high HH rating simply because it is not woven so there are no voids (anyone with a HH testing apparatus, I would be greatly amused to see numbers). Or even take the material used for hydration reservoirs- I would suspect that this material would have a VERY high HH rating, but it would still make a very poor tent material.

I'm not going to try to put any material, method, or manufacturer on a pedestal. I simply think that HH is realistically a very poor measure of the quality of a material. I was wondering if anyone else has thought about this?

edit: For those who don't already know, the hydrostatic head rating is the height of a water column over a sample of fabric at which the fabric begins to allow water to pass through. If they were really being more scientific about it they could simply use the pressure rating rather than the column height e.g., X.XX kPa/bar/psi rather than something like 1500mm. Height is only one part of the equation p = h ρ g where p = pressure, h = height (of the column), ρ = density of the liquid, and g is the acceleration of gravity (9.8 meters/second^2).

[Greg Mihalik]

"So I have to ask the question, how useful is the hydrostatic head rating used to measure tent materials really?"

What I am reading is …

1-a) The testing is done under ideal conditions with new materials that have not been pre-saturated, creased, folded, stretched, walked on, or otherwise subjected to what would be considered "normal use" before measurement…. I simply think that HH is realistically a very poor measure of the quality of a material. [emphasis added]

2-a) The test only reveals the pressure threshold at which the material allows water to seep through. It does not consider the volume of water that is allowed through after that threshold is reached,

2-b) nor does it consider whether or not or to what extent the material might degrade after this limit is reached one or more times (how the material performs after subsequent tests).

3-a) This measurement offer no real indication of the durability of the floor material itself.

3-b) Consider that a heavy duty plastic garbage bag likely has a high HH rating simply because it is not woven so there are no voids (anyone with a HH testing apparatus, I would be greatly amused to see numbers). Or even take the material used for hydration reservoirs- I would suspect that this material would have a VERY high HH rating, but it would still make a very poor tent material.

…which boils down to –
A) HH "..is a poor measure of fabric quality"
B) "Wear and Tear" degrades fabrics
C) HH by itself is meaningless, by itself, for evaluating tent [fly] material

To which I would say –

A) HH is not intended to define "fabric quality", only it's ability to shed water. Abrasion, durability, tear resistance, color fastness,… and HH are all to be considered when looking a "quality".

B) yep.

C) yep.

You are right.
Life is messy.
There is no on-size-fits-all.

HH was never intended to be "the" parameter for fly material. It is a major discriminator simply because all the other factors are minor in comparison, perhaps with the exception of "strength". All the other factors have, to a large degree, been sorted for the lightest and "adequately durable for UL" applications. HH is perhaps the "weakest" factor, hence the one most indicative of function.

If you dont' pay attention to HH you are going to get wet.

So, what do you suggest?

[Tom Keefe]

I'm not sure what could be used as an alternative. I guess that's one of the reasons I put this forward. There are some very smart people on here who might have some ideas.

I just think that there's something wrong with the fact that HH is the only measure of performance of shelter material that we generally see. Even disregarding the wear factors other measures fit into the equation like fabric weight, tensile strength, elongation, weave type, and weave density. Other performance measures would be tear resistance and abrasion resistance (probably the most difficult to measure).

It just seems that all too often people misuse or misrepresent the HH rating when considering different shelters or materials.

[Greg Mihalik]

"I just think that there's something wrong with the fact that HH is the only measure of performance of shelter material that we generally see. "

We pay attention to HH because the other factors are optimized. But if you look back you will see that strength is an issue with .34 cuben (tearing along the seams), and HH quality control for spinnaker was an issue.

Spinnaker is gone.

Cuben in .34 weight for flys is uncommon.

HH is being "tuned" to find the lightest coating that will be accepted by the backpacking community. So it alone is in the spotlight. Not as a "trick", but with the assumption that users understand that the other qualities are "good enough".

Edit: I forgot the most important factor: Weight – it is the driver (here) for all fabric choices, and is taking us "to the edge" on HH.

[James Marco]

Like so many things, HH is simply one measument in a large variety of tests.

With very fine weaves, very fine fibers, the actual surface tension of the water becomes very important. Some of the pictures of the microscopic holes/bubbles may be actual surface tension, but this is only a guess. Is it meaningless? No. In conjuction with a series of other test results, it can tell you a lot about a material used in shelters.

I remember old canvas tents. They generally fail the HH ratings with lots of holes. Yet, they were accepted fabrics for tents/flys for a few hundred years. Oiled/waxed canvas was the standard for raingear, "oilskins". But, I really doubt they would perform well in HH testing.

Newer fabrics are far better. With a siliconized coating they can reach good to excelent levels. But, they fail when placed in contact with something. A silnylon "drybag" really isn't. So, as you say, the rate at which water passes through becomes important. Silnylon makes good flys. Cuben, can have a low HH but still perform well, because it does not pass *much* through.

One test does not make a fabric ideal, nor, eliminate it. HH is just one more piece of data.

[Woubeir (from Europe)]

And even silnylon has a lower HH compared to PU-flys.

[Franco Darioli]

And even silnylon has a lower HH compared to PU-flys.
It can go either way , just depends on how much coating there is.
Some brands have silicone on one side and PU on the other but that reduces fabric tear strength.

A more basic problem with the conventional HH test is that the surface tension on those tests is different to that of rain on a tent in reality.
In a way alluded in the canvas tent comment above.

[David Thomas]

>'If they were really being more scientific about it they could simply use the pressure rating rather than the column height"

Working regularly pressure ratings and data, I've gotten used to the simple conversion. To the point where I don't use any conversion, I just think in mmHg for blood pressure and atmospheric pressure, "w.c. (inches of water column) for low pressure things like HVAC air ducts or residential natural gas service.

There have been times when I'm actually using millimeters of Hg or inches of water. A standard mercury barometer being one case, the sphygmomanometer in my wife's exams rooms is another. For pressures between a 1/4" w.c. and many feet of w.c., I'll sometimes use clear tubing filling with water.

1 g is implied, so it such measurements shouldn't be interpreted literally while on the Moon, on Mars, or during roller coaster rides.

[steven franchuk]

"The test only reveals the pressure threshold at which the material allows water to seep through. It does not consider the volume of water that is allowed through after that threshold is reached,"

The amount of water that get through will depend on the following:
1, size of the hole.
2. Density of the fabric
3. Other additional fabric layers that may exist after the garment is stiched together.
4. The amount of rain falling

If a test existed for this it would be meaningless to any hiker. You cannot know any of that with any certainty and it is not going to help you when a leak is going to occur.

"For those who don't already know, the hydrostatic head rating is the height of a water column over a sample of fabric at which the fabric begins to allow water to pass through. If they were really being more scientific about it they could simply use the pressure rating rather than the column height e.g., X.XX kPa/bar/psi rather than something like 1500mm. Height is only one part of the equation p = h ρ g where p = pressure, h = height (of the column), ρ = density of the liquid, and g is the acceleration of gravity (9.8 meters/second^2)."

The density of water is a constant. It doesn't change. Last I looked water rain was the only type of rain that occurs on earth. Gravity is also a constant. You cannot go anywhere were there will be a noticeable difference. A column of water XXmm high will generate a fixed pressure at the fabric. No mater what your pipe is made of or its inner diameter, as long as the fluid is water the pressure at the fabric will be the same for a specific column height. So as far as people on earth are concerned Hydrostatic head is a scientific measurement.

"This measurement offer no real indication of the durability of the floor material itself. Consider that a heavy duty plastic garbage bag likely has a high HH rating simply because it is not woven so there are no voids (anyone with a HH testing apparatus, I would be greatly amused to see numbers)."

Note it isn't the woven faric of your rain pants (golite tumalo listed in your profile) is not waterproof. Neither is it the DWR coating it has is waterproof. What is waterproof is the solid sheet of plastic that is bonded to the inside surface of the fabric. Rain may saturate the outer fabric but water will still not get through until the plastic layer is damaged. Once the plastic is damaged it will always leak in that location. The only way water will get through is if a hole is punched through, a tear occurs. or abrasion caused a hole in the plastic.

Note fabrics like Event that are air permeable have microscopic holes in the plastic. Water cannot get through those microscopic holes because rain drops cannot exert enough pressure to push the water trough. Your Tumalo pants don't have any microscopic holes in the plastic layer. Your rain pants utilize a different and less effective method to allow them to breath. they are only vapor permeable not air permeable.

If the plastic is the only thing the make the fabric water proof and breathable, why glue fabric to it?

Fabric is inherently stronger and allows a thinner plastic layer to be used which reduces weight and cost. The fabric will also protect the plastic from abrasion and tearing. So in short the fabric may hold up better in the hydrostatic head test than the plastic garbage bag.

[Roger Caffin]

Hi Tom

I think you are setting up HH as a bit of a straw man for debunking. Your focus on HH as the sole measure of fabric quality is not how the real world operates. In addition, many of your claims are either wrong or misleading.

> The testing is done under ideal conditions with new materials that have not been
> pre-saturated, creased, folded, stretched, walked on, or otherwise subjected to
> what would be considered "normal use" before measurement.
Reality is that even here at BPL we have had tests run on fabrics both in the new state and in the 'used' state. Such testing showed that spinnaker fabric was great when new, but leaked a bit at the creases after a while. It also showed that while Cuban was great when new, the lighter grades tended to get pinholes at the creases after a while. We KNOW about this.

I am sure most serious tent and tarp manufacturers are aware of these things and do their own testing and assessment as well.

> The test only reveals the pressure threshold at which the material allows water to
> seep through. It does not consider the volume of water that is allowed through
> after that threshold is reached
True, and false.
As an extreme example, EPIC fabric is great when new and at low pressure, but we KNOW that once it gets dirty or once you exceed the threshold it leaks like a sieve. We KNOW these things.

I think what is happening here is that the vendors have to simplify things so they can make the specs for a tent fit into a finite space for the 'average' customer. HH is used as one obvious and useful parameter in a spec list, but not the only one. Not wishing to be rude to the world, but I suspect that most customers (especially novices) wouldn't know what HH really means anyhow, and further detail would be wasted on them.

> This measurement offer no real indication of the durability of the floor material itself
So? It is a measure of pressure, not of fabric durability. To suggest otherwise seems rather silly to me. Once again, would the average tent buyer know what a Martindale test is? Or the difference between nylon 6,6 and polyester? Or …

> I simply think that HH is realistically a very poor measure of the quality of a material.
I don't think anyone else has ever suggested that HH is a measure of fabric quality. It is however a useful measure of the waterproofness of a COATING. We use other parameters to assess overall fabric quality. Come to think of it, what do YOU mean by fabric quality anyhow? You have not even defined it. And if you haven't defined it in measurable terms, thern you cannot measure it.

I understand what you are trying to say, but I suggest that the problem is that you may have misunderstood what everyone else is talking about. We don't talk much about tensile strength or tear strength as, for the most part, the fabrics used have more than enough strength. I don't know of anyone else who has ever suggested that HH is the sole measure of fabric quality, but it is a useful spec for the coating.

Cheers

[Tom Keefe]

When I said that listing the pressure rather than the height of the water column was more scientific, it was really just to say that the driving factor of the measurement is pressure and not distance, so why would we use a measure of distance to represent this value? I never meant to imply that any variable other than water column height would change.

Think about the person who has no knowledge of what a HH rating is, he sees on the package/advertisement that X tent has a 1500mm waterproof coating. Do you think he is going to think pressure? Probably not.

I think (rather cynically I suppose) that if Joe Somebody goes out looking to buy a tent will see that tent A has a 2000mm coating and tent B has a 2500mm coating, and thinks "I'm not really sure what that means, but tent B has a higher rating, so that must be better right?".

Consider that tent A is a high quality silnylon tent, and tent B is a standard polyester (think Wal Mart tents) with a pu coating. Let's say they both have the same fabric weights listed, the same packed weight, and cost roughly the same. Without knowing the other properties of the materials used in these two tents Joe Somebody could get ripped off.

It just seems to me that if manufacturers are going to list something like HH ratings as a measure of performance, then personally I think that other measures of performance should be used as well. That's my main gripe really. That and the aforementioned issue with not testing the material HH rating under "normal use" conditions.

Also Roger, I never meant to say that people on BPL specifically misuse the HH rating. I've read enough on here to know that's not the case. I brought this up on here because I know we have some people here that are very knowledgeable is this area (most probably more so than myself), and that's why I chose to bring this up here. I can't think of anywhere else where a discussion like this could take place. :)

[Greg Mihalik]

" Without knowing the other properties of the materials used in these two tents Joe Somebody could get ripped off."

I guess I'm dumber than Joe.

If everything else is equivalent in terms of strength, weight, UV etc., but one has a higher waterproof rating, why wouldn't that be the "best buy"?

[Tom Keefe]

"If everything else is equivalent in terms of strength, weight, UV etc., but one has a higher waterproof rating, why wouldn't that be the "best buy"?"

The example was meant to illustrate that you could have two tents made with material of the same carry weight, fabric weight, and cost (all the measures commonly available), but one could still be preferable over the other despite the fact that it may have a lower HH rating.

I never said that the strength would be equal. High quality silnyon would obviously be stronger than standard pu treated poly of the same fabric weight. This illustrating the point I'm trying to make that a having a measure of strength is just as (if not more) important than having a measure of water permeation resistance.

[Ken Thompson]

"High quality silnylon would obviously be stronger than standard pu treated poly of the same fabric weight."

I'm dumber than Joe and this is not obvious to me. Care to elaborate?

[Jeff Jeff]

More pseudoscience on BPL…

HH is a standard measurement that is useful for comparing the relative water resistance between materials. By your logic, measuring down fill power, CLO, etc are all poor measures as well.

[Bradley Danyluk]

Your issues are solved simply by choosing to stop underestimating your Jill Average. Perhaps she doesn't know what HH means, and therefore logically concludes not to use it as a strong measure in a buying decision. Perhaps she doesn't know what it means, and therefore decides to do a small amount of research before making a buying decision.
If she doesn't know what HH is, and STILL makes an impulse buy based solely on that number (which she knows she doesn't understand), then how is that anyone's fault but hers?
In that case she's probably made a lot of other uninformed / arguably-stupid decisions in her life. Would you like to also discuss and try to prevent all of those, too? I'm a socialist and even I would say that's absurd malarkey.

[Anonymous]

"I'm dumber than Joe and this is not obvious to me. Care to elaborate?"

Have no dog in this debate, just clarifying. Might not be obvious, but it's somewhat well known among fabric focused folks (FFF's) that most forms of nylon are stronger per weight than most forms of polyester (especially tensile and tear strength).

It's also somewhat known that silicone coating slightly improves the above strengths of nylon (possibly of polyester too, but for whatever reason(s) it's a less common combo), where as PU coating somewhat weakens the fabric it is applied to (as Roger hinted at earlier), so yes, silnylon would be somewhat significantly stronger per weight than PU coated polyester.

Whether or not that would really matter in actual use in the field, i don't know, perhaps only for extreme 4 season tents for Arctic, high altitude mountain, etc conditions like the Hillebergs? In any case, PU coated polyester will definitely tend to be significantly heavier than silnylon per similar strengths and durability.

[hwc 1954]

>> "High quality silnylon would obviously be stronger than standard pu treated poly of the same fabric weight."

But, at the same price point, the PU coated polyester fabric will probably be stronger and more durable and more waterproof, albeit heavier, than a silnylon.

Everything is a tradeoff among weight, durability, and price.

[Tom Keefe]

"HH is a standard measurement that is useful for comparing the relative water resistance between materials. By your logic, measuring down fill power, CLO, etc are all poor measures as well."

That's just the issue though. It's not a measure. It's a test of an aspect of performance. If it were actually a measure they would give something like the thickness of the layer of PU that was applied, or the weight of silicone/pu that was applied per yard of fabric. Either of these would be measures and not test results.

[Woubeir (from Europe)]

Like already said, HH is just a way to present certain data. Is it perfect ? No. But it allows to test something in a controlled environment with controlled fabrics. Once you take them outside for real use, you loose that control. Then the chaos begins and everybody can start to manipulate those data so that they come out as the best.
So instead of allowing to manipulate those data, I would educate the consumer first.

Then, there are testing methods and standards for other properties, but it remains up to the manufacturer to publicise the results. Some do, others don't (if they already have those results).

Then, about tensile strength and Si vs PU: I could explain but why would I if somebody already has done the effort to do that:

"Tear Strength. Unlike tensile strength, tear strength is not a simple function of the weight of the fabric. Weave construction details and coatings play crucial roles. The simple wing-rip test begins with cutting a slit partly in from one edge of a fabric test piece. Both the warp and weft axes are tested. The fabric edges each side of the cut are gripped and then pulled in opposite directions. The force needed to smoothly tear and extend the slit in the fabric is the tear strength.

To understand tear strength it helps to focus on what's going on at the very end of the rip, where the first unbroken yarn is now at the front-line. With an uncoated fabric yarns are relatively free to shuffle around. As the stress increases the weave distorts, resulting in the load being shared over a number of yarns immediately behind the first unbroken yarn. The more the weave is 'glued-up' with a low-elasticity, deeply-embedded coating (like PU-coatings) the less the weave can distort and the more the stress concentrates on that single, front-line yarn. The result is a substantially reduced tear strength. You can think of the coating as making the fabric more 'paper-like' and less pure 'textile-like'. If the coating is very elastic – as is the case with silicone elastomer, or the fabric has a relatively elastic, laminated membrane which sits up more on its surface, then the yarns are not so constrained. The sharing of the load can still be taken by a number of unbroken yarns because the silicone coating or TPU membrane can stretch as the yarns reposition themselves. In this case the coating material's own strength may actually add a little to the tear strength. Some tent manufacturers (particularly those using nylon fabrics that are coated both sides with silicone elastomer) make a feature of the high tear strength of their outer tent fabric."

[edvin mellergård]

Yes, but then you got even less useable information, the same amount of PU/sil doesn't always give the same waterresistance. If it's added as a layer on top of the fabric you'll get other results than if it's "pushed into" the fabric.

You also get different results depending on if you apply it in layers or as a single coating, if you coat both sides or just one.

[Tipi Walter]

Let's get back down to Earth and focus on the realities of hydrostatic head. One of the biggest peeves of the backpacker is having a tent floor which allows water to seep thru when that person is sitting up with his butt on the floor and the tent is atop wet snow or in a small "lake" or on saturated ground. No decent tent floor should leak in a single drop of water in these conditions. Why do they?

Because often the deniers are way too low (30 denier) with substandard coatings. This goes for ground cloths too.

BACKYARD TEST
Here's my recommendation—take your tent floor or your ground cloth out to the backyard today and get the water hose (hopefully it's not 0F) and create a soaked puddle on the grass. Place floor/cloth atop puddle and sit on it for 5 minutes. Look to see if any water comes thru. If so—failure.

SECOND STEP
Find a material which will not allow this and which has good puncture resistance. The point of the test is to sort thru all the different materials until you come up with one that works, especially relevant when looking for a ground cloth which can be placed under (or above) a substandard tent floor.

[Ken Thompson]

"As a side note, they also state that materials with fire retardants added to them significantly weaken the tear strength. In their testing they state that adding a fire retardant can reduce tear strength by up to 90% in common tent making materials."

This is what I have always heard.

[Jeff Jeff]

"That's just the issue though. It's not a measure. It's a test of an aspect of performance. If it were actually a measure they would give something like the thickness of the layer of PU that was applied, or the weight of silicone/pu that was applied per yard of fabric. Either of these would be measures and not test results."

It's a measure of a property of the material. Not all measurements have to be a geometrical property of the material.

[Tipi Walter]

"As a side note, they also state that materials with fire retardants added to them significantly weaken the tear strength. In their testing they state that adding a fire retardant can reduce tear strength by up to 90% in common tent making materials."

So, which popular tents use flame retardant? I know Hilleberg does not, just wondering about Big Agnes or MSR or TarpTent? How about cuben fiber tents?? I myself can't stand the stink of the chemicals used in Mt Hardwear tents. Why should something we sleep inside for months at a time outgas like a FEMA trailer?

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