Hydrostatic Head (HH) is a way of measuring how waterproof a fabric is. The test is completed using a long tube that will hold water – the piece of fabric under test is fastened across the open end at the bottom. The tube is then filled with water until water can be seen to penetrate the fabric – at which point the length of water in the tube is read off – for the best fabrics it can be as high as 30,000mm i.e. 3metres high.
Applying this to Garments
In the UK manufacturers are allowed to claim a fabric is waterproof if the HH is 1500mm but most jackets exceed this comfortably and figures of 10,000 to 30,000mm are not uncommon.
Fabric for use in garments require higher levels of waterproofing because garments are subjected not only to driving rain but also to pressure applied by straps and hipbelts of rucksacks.
Gore-Tex and eVent fabrics typically achieve figures of HH upto 30,000mm making them the most waterproof garments around, however some fabric manufacturers, most notably Polartec, aregue that these figures are not necessary, and by accepting a figure of 20,000mm or thereabouts you can make a fabric considerably more breathable – they introduced ‘Neo Shell’ which is becoming more popular and is a proven technology.
A higher HH does not guarantee a better garment since breathability is usually a compromise when water proof performance is maximised.
Softshell garments come in a range of different types from lightweight highly breathable that won’t put up with much rain, to heavier more water resistant garments that are often made from waterproof fabrics – the garment is not called ‘waterproof’ because the seams aren’t ‘taped’.
Applying this to Tents
In order for a tent to resist light showers the HH needs to be around 1000mm, heavy rain and driving wind will create more pressure on the fabric and require a higher HH – around 2000mm. Anything above these figures and the tent will keep out water being pushed through by something physical, like a person leaning on it.
A groundsheet need to have a higher HH figure because of the pressure of people pressing down on it, and should be around 3000mm or higher ideally.
Here are the three facts you should know about the Hydrostatic Head rating system:
- What is it?
Hydrostatic Head is a method of measuring how waterproof an article of fabric is. Manufacturers take a clear cylinder and securely clamp their material on the bottom end. The cylinder is then slowly filled with water and observed to determine the maximum height of the water within the cylinder before water seeps through the material. The process kind of looks like those paper towel absorbency tests you see in commercials where they dump water over the towels to see which holds up longer – just with all the water contained in a tube rather than over a bowl.
- What do those numbers mean?
The numbers corresponding with a Hydrostatic Head rating let you know how waterproof or water resistant a material is. If something has a Hydrostatic Head rating of 5,000 mm, that means it could hold a cylinder full of 5,000 mm of water before it would leak out through the weave. For any fabric to be considered “fully waterproof” it needs a Hydrostatic Head rating of at least 1,000 mm.
- How do I use this new info?
Generally speaking, the bigger the number, the better. For something like a tent, you’ll find a rating of 1,000mm will resist light showers and 2,000mm will stand up to heavier rain, especially if it’s windy and rain is being pushed against the walls of your tent. For clothing, while Hydrostatic Head is an important consideration, keep in mind that a higher rating doesn’t necessarily mean a better garment. For certain activities like whitewater paddling where you’re sure to work up a sweat, you’ll want something with breathability like the NRS H2Core Rashguard.
Next time you’re looking for a new rain jacket or tent you’ll have a better understanding of what to look for when determining waterproof v. water resistant now that you know what Hydrostatic Head ratings are all about!
As we have well and truly moved out of the Summer season, and the weather will become a little more testing and temper mental; you might start to look at new or existing options for your outer layer. With so much choice of brands to start with, it is easy to get overwhelmed with the different styles, materials, and that strange number – Hydrostatic Head. Deciding upon a brand/style is often a case of personal choice and/or budget and is never the confusing part of deciding on a new waterproof jacket. Again, there are an ever increasing number of waterproof material/membranes to choose from, but what largely sets them apart from each other is the hydrostatic head.
So what is Hydrostatic Head? Put simply; it is a lab tested means of arriving at a numerical indication of how waterproof a material is. Generally, the higher the hydrostatic head, the more waterproof a material is regarded to be. The test itself is relatively straight forward, in that a column of water is placed on a sample of the particular waterproof jacket or fabric and the height of the column of water in mm determines the hydrostatic head. In terms of what these lab achieved figures equate to in understandable terms; 1500mm and above is regarded as “waterproof”. Looking at that in real terms; 2000mm is the rating of an average tent flysheet, 10,000mm is around the measurement of most good, breathable waterproof jackets. Moving upwards, 20,000mm is about right for a solid four season hardshell. There are some jackets that reach up to 30,000mm but it is argued that this is far too high a figure to retain any breathability and will possibly lessen how comfortable a jacket is.
The Polartec Neoshell used in Rab’s Neo Alpine has a Hydrostatic head of 10,000mm. New for Autumn/Winter 15 – Keep an eye out for a Second Summit Review.
The thing with a HH rating is, that it isn’t all encompassing. The tests don’t take particular things into account that are very relevant in an outdoors situation.
Extreme Weather – As we well know, the weather can change quickly on the hill and generally HH ratings are accepted to relate to only 35mph winds and driving rain. If the wind is stronger than this, and the force of the rain water hitting the jacket is stronger then its theoretical “waterproofness” is lessened.
Useage Outside Test Variables – Tent floor fabrics are given a hydrostatic head rating, as they are needed to be waterproof of course. One thing that a hydrostatic head test does not take into account in the case of tent floors is the extra force exerted on the fabric when it is being knelt on, or heavy rucksacks are thrown onto it. Similarly to stronger winds, this can lessen the theoretical “waterproofness” of the fabric as the variables are not constants, as in a standard hydrostatic head test. Another example of this in waterproof jackets is the ‘stress’ or ‘high wear’ areas such as the elbows, shoulders and waist (where rucksack sacks will sit), wrists (where straps are cinched).
Longeivity/Robustness Of Fabric – Whilst hydrostatic head tests let us know how waterproof fabrics are at the time of testing, there is no rating of how long this level of waterproofness will last. A fabric with a tested hydrostatic head of 20,000mm may degrade quickly with useage and the rating deplete rapidly – in which case, the fabric is not overly ideal for extensive outdoor use. The fabric may be susceptible to abrasion, tearing, being worn through. All of which will of course superceed the fabric’s apparent hydrostatic head rating.
Garment/Gear Design – Not directly talking about the fabric itself, external factors/ the way in which the garment or item of equipment is designed may affect how waterproof it ultimately is. So hydrostatic head is not the be all, and end all. If zips are not designed well for example, or there is no option to cinch hoods or wrists, the hydrostatic head rating can be pretty redundant.
You will have seen waterproof ratings, on jackets for example, of 10,000/10,000 and 20,000/20,000. But what do the rating numbers mean?
Manufacturers usually describe the waterproof and breathability of fabrics using two numbers. The first is in millimetres (mm) and is a measure of how waterproof a fabric is.
In the case of 20,000 mm fabric this refers how waterproof the fabric is when this test is carried out: If you put a square tube with inner dimensions of 1-inch over a piece of fabric, you could fill it with water to a height of 20,000mm (before water would begin to leak through. The higher the number, therefore the more waterproof the fabric. HH is short for Hydrostatic Head.
The second number is a measure of how breathable the fabric is. It is rated in grams (g). This is the amount of water vapour that can pass through a square meter (m2) of the fabric from the inside to the outside in 24 hours.
So a 20,000g rated fabric allows 20,000 grams of vapour out. The higher the figure the more breathable the fabric is.
The balance of waterproof and breathable
If you want full waterproofing, you need a jacket that is made of rubber! However, this would make you sweat and the inside of the jacket would end up wet. At the other end of the scale is a jacket made of fabric that is very breathable but this would not keep you dry in a big downpour.
Most people want jackets that offer both waterproofing and breathability and there is a balance to be struck. It also depends on how hard you will be exercising in the jacket. Runners for example will want a jacket to be more breathable than, say, sailors.
So there is a balance to be found and in the end the chances are you will have jackets with different levels of waterproof/breathability for different activities.
The properties of waterproof clothing
Lets start with the first truth that no one likes to admit: making something waterproof is extremely easy and cheap – just pick up any plastic bag and you have a waterproof item. Or better, take a big bin liner, make holes for your head and arms and you have a perfectly waterproof garment. It might not be pretty or fashionable, but it will keep you dry.
The only problems with a bin liner (besides style) is that it is very flimsy and offers no breathability at all. When we are being active, we sweat a lot in order to cool ourselves. This sweat, when wearing a bin liner, will stay inside and keep us wet, rendering the waterproof part redundant. So the greatest challenge with waterproof garments is making sure it is breathable. The other thing that a bin liner has none of is durability – walk next to a branch with a bin liner and most of the bag will stay on the branch. Any chafing from backpacks, skis or ice axes will completely ruin your magnificent bin liner attire.
The two things we can give a bin liner all the credit for is being utterly waterproof, really, bombproof even; and it is also very light. You can get slightly thicker bin liners, but they will still remain amazingly light compared to most jackets out there.
So the 4 key characteristics we want from waterproof clothing are: waterproof-ness (I know this is not a word, but we are going with it), breathability, durability and lightness. The last factor will be price. As you can imagine, having the combination of the four properties at the highest quality will cost you dearly, very dearly. Those high end super expensive waterproofs? They have all those great characteristics, this is why they are expensive…..
Waterproof construction – the “only 2” theory
Again here, the universe of construction method claims is endless: bonded, laminated, printed, sprayed, waxed, weaved and more- all have been used to make a waterproof garment, but are they really all unique methods construction? Well, not really.
To make a waterproof and durable item, we need a method that will allow the movement of heat and perspiration out while not allowing water to get in – pretty simple. We can see how the bin liner won’t work here, so what will? We need a fabric that has holes to allow the heat and sweat out, but won’t allow water in. The trick is achieved by using the effects of the surface tension of the fabric (making a fabric hydrophobic, or water repellant) while combining it with a porous membrane to make it breathable.
You might have read/heard about the big drop with tiny holes idea (the idea that a drop of water is too big to fit into the pores of a membrane from the outside in, but that the water vapour from your body heat and sweat is made up of smaller molecules and so will be able to escape), but that is sadly not true. If the fabric on your waterproof clothing is losing that surface tension, the fabric will not be waterproof anymore, and may lose the breathability all together.
Back to the construction claims – as I mentioned, there are essentially only two methods of making breathable and waterproof clothing: laminating and coating. Both methods can be very simple but the varieties within each method makes them complicated.
Lamination is the expensive option in the world of waterproof clothing. Laminated clothes tend to be durable and very breathable, which leads to a higher price point. Laminated fabrics are essentially a “sandwich” of three layers:
- External treated durable material – this is usually a hydrophobic (non-absorbent) material that is very durable such as Polyamide, Polyester or Nylon. The external fabric is treated with a DWR finish (explained further below).
- Membrane – the “heart” of the fabric. Made from a very thin synthetic material, the membrane can feature different configurations (depending on the fabric) of microscopic pores that allow vapour to pass though and blocks water drops from coming in.
- Internal lining – can be a loose fabric or laminated to the membrane on the inside. The lining’s purpose is to keep the membrane from getting contaminated from body oils, salt and grime.
The big names in the world of membranes are Gore-Tex and eVent. Most of the big companies will also have their own membrane fabric that has been developed in house such as HyVent (The North Face), MemBrain (Marmot) and many more.
Coating has a couple of variations: a standard PU (Polyurethane) coating that has very limited diffusion ability, and a printed porous membrane that has more breathability to it (also known as 2.5 layer fabrics). Coating is the cheaper option and is offered by pretty much any company that claims to make outdoor gear.
PU coating uses the same Diffusion concepts as an ePTFE (Elastic Polytetrafluoroethylene) membrane, which is explained in more detail in the Diffusion Membranes section below, but because PU needs to be thicker to bond with other fabrics (like Nylon, Polyamide etc), the PU coating ends up being much thicker than the laminated membranes so diffusion is slower (aka less breathability).
Printed PU membranes are the new age of waterproof clothing. PU’s have much greater diffusion abilities compared with ePTFE membranes and can be printed into thinner, more complex patterns that are robust yet breathable. Printed PU membranes have great breathability while being combined with a variety of face fabrics to allow super-light running clothing or robust heavier ski or mountaineering garments.
With the constant promise for garments that are more and more waterproof, it seems that all we get is a numbers game, aiming to show who “has the biggest”. The most common method of measuring waterproof-ness is called Hydrostatic Head (HH). In this test, a tube with both ends open is placed on the waterproof fabric (on the exterior) and filled with water. The tube usually measures 1″ x 1″ and the measurement taken is how much water can be filled (in millimeters) until water starts leaking through the fabric. Most HH tests are now standardized but variations between labs can exist.
Hydrostatic Head measurement brackets:
- Up to 1,500mm – fabric is not waterproof
- 1,500mm – 10,000mm – waterproof fabric as defined by European ISO standards for waterproof fabrics
- 10,000mm – 20,000mm – outdoors industry standard for waterproof fabrics
- 20,000mm+ – completely waterproof fabrics as made by the highest end manufacturers
Hydrostatic Head might seem like a good way to measure waterproofing, but there some issues here:
- HH is a laboratory test, so real life conditions do not apply
- Not all labs comply with the same standard of testing
- The test does not add the internal pressure made by using the garment (such as joints, or when sitting etc)
- The test does not add external pressures such as packs, wind factors etc
- Usually tests are on brand new fabrics and do not calculate the reduction of performance over time
- Final garments have stitches and tapes that have different waterproof ratings and might damage the performance
Using PSI to measure waterproof-ness:
PSI (Pounds per Square Inch) is a different method of looking into waterproofing. PSI measures how much pressure can be applied on the fabric before it fails (water seeps through). As a rule of thumb HH and PSI can be converted at 704mm=1 PSI, yet this is not a satisfying conversion. Compared to HH, PSI testing is based on active movement, so it can include more factors than just the presence of water, such as wind, movement compared with drop impact (your “collision” with the rain), internal pressure from pressed fabrics etc.
Industry standard considers waterproof fabric as 3 PSI and up, yet this will amount to a light rain with no wind while standing around. To get something that will fight a Scottish mist (what we call a torrential storm), at least 30 PSI should be sought after to stay dry.
I prefer using PSI measurements when it is offered as I think it tends to represent reality a little better, even if tested in lab conditions. There are some things that are important to remember when looking at PSI: when we bend our knee and stretch the fabric of our trousers, 2-4 PSIs are being placed on the fabric from the inside. If we place that knee on the wet ground, we need to add the external pressure that is placed on the fabric from our weight being pressed to the ground, that can be quite a lot too. Between those two factors, you can easily see how 10 PSI on waterproof trousers won’t protect your knees (or bum) when in contact with the wet ground.
DWR – the must-know acronym for waterproof-ness
All the waterproofing technology amounts to nothing without the most important part of the equation: Durable Water Repellent (DWR). DWR is a chemical coating that is treated to the face fabric (the exterior layer) in order to make it completely hydrophobic. Without DWR, the exterior fabric can get “wetted out” and suffer from reduced breathability or even cause a reverse effect in that moisture is “sucked in” to the fabric, causing the garment to get completely soaked inside and out.
DWR is a chemical treatment (usually silicon based) that is applied on the fabric in the factory, but it wears off over time. The main reason for DWR wear is abrasion, so areas that suffer in particular are where straps, harnesses, gaiters and other bits of gear create friction on the fabric. In order to keep the fabric from wetting out, it is important to re-apply the DWR on a regular basis, along with giving the garment a good clean.
Water behviour when DWR is functional
Water behviour when DWR is not functional
I recommend washing and treating your waterproofs regularly. I personally wash my waterproofs (if used) after 10 days of total use (can be a 3 day trip, 5 days out and then 2 day hikes) and treat them with an external spray. I use a cotton cloth to spread the spray evenly and remove the excess. After the DWR has dried, a short round in the drier on very low heat helps “cure” the DWR and restore its repellency.
Limitations of Waterproof-ness
When we are talking about waterproof clothing, it is not just the fabric and its ability to repel water that matter; there are other factors that influence waterproof-ness:
- All waterproof garments have big holes that are weak points: collars, hoods, sleeves, waist bands or anywhere you actually fit into the clothes. Those weak points are inevitable and we just have to accept that with enough rain over enough time, water will seep through those weak spots.
- Zippers are very rarely waterproof, but can be water tight. Again, these are weak spots the can be partially solved with storm flaps (fabric that covers the zip), but those will fail too in some point.
- If you don’t keep the DWR in good condition, water might be sucked into the garment or just saturate the fabric, making it not waterproof anymore. Keep an eye on that DWR.
- Purpose and Scope
1.1 This test method measures the resistance of a fabric to the penetration of water under hydrostatic pressure. It is applicable to all types of fabrics, including those treated with a water resistant or water repellent finish.
1.2 Water resistance depends on the repellency of the fibers and yarns, as well as the fabric construction. 1.3 The results obtained by this method may not be the same as the results obtained by the AATCC methods for resistance to rain or water spray.
2.1 One surface of the test specimen is subjected to a hydrostatic pressure, in-creasing at a constant rate, until three points of leakage appear on its other sur-face. The water may be applied from above or below the test specimen.
3.1 hydrostatic pressure, n.—the force distributed over an area exerted by water.
3.2 water resistance, n.—of fabric, the characteristic to resist wetting and penetration by water.
3.3 water repellency, n.—of fabric, in textiles, the characteristic of fiber, yarn, or fabric to resist wetting.
- Safety Precautions
NOTE: These safety precautions are for information purposes only. The pre-cautions are ancillary to the testing procedures and are not intended to be all inclusive. It is the user’s responsibility to use safe and proper techniques in handling materials in this test method. Manufacturers MUST be consulted for specific details, such as material safety data sheets and other manufacturer’s recommendations. All OSHA standards and rules must also be consulted and followed.
4.1 Good laboratory practices should be followed. Wear safety glasses in all laboratory areas.
4.2 Manufacturer’s safety recommendations should be followed when operating laboratory testing equipment.
- Apparatus and Materials (see 11.1)
5.1 Hydrostatic Tester. 5.1.1 For Option 1, Hydrostatic Pres-sure Tester (see 11.2).
5.1.2 For Option 2, Hydrostatic Head Tester (see 11.3).
5.2 Water, distilled or de-ionized.
- Test Specimens
6.1 A minimum of three fabric specimens should be taken diagonally across the width of the fabric to be representative of the material. Cut specimens at least 200 × 200 mm to allow proper clamping.
6.2 Handle the specimens as little as possible and avoid folding or contaminating the area to be tested.
6.3 Condition the test specimens at 21± 2°C (70 ± 5°F) air at 65 ± 2% RH for at least 4 h before testing.
6.4 The surface of the fabric to be ex-posed to water must be specified because different results may be obtained on the face and the back. Identify that surface on a corner of each specimen.
7.1 Verify the water in contact with the test specimen is regulated at 21 ± 2°C (70± 5°F) (see 11.4).
7.2 Dry the clamping surface.
7.3 Clamp the specimen with the surface to be tested facing the water (see 11.6).
7.4.1 Option 1—Hydrostatic Pressure Tester (see 11.2).
184.108.40.206 Turn on the motor, press the lever to raise the overflow device at the rate of 10 mm/s, and close the air vent as soon as water flows from it. 7.4.2 Option 2—Hydrostatic Head Tester (see 11.3).
220.127.116.11 Select the gradient of 60 mbar/min, press the start button (see 11.5).7.5 Disregarding water droplets that appear within approximately 3 mm adjacent to the edge of the specimen clamping ring, record the hydrostatic pressure at the moment water droplets penetrate the fabric in three different places.
8.1 Calculate the average hydrostatic pressure for each sample.
9.1 Results for each specimen and the average for each sample.
9.2 The material and the side tested.
9.3 Water temperature and type.
9.4 Gradient (rate of increasing waterpressure).
9.5 Tester option used.
9.6 Any modification to the method.
- Precision and Bias
10.1 Precision. The test results are tester dependent. Precision statements for each tester are given in 10.2 and 10.3.
10.2 Suter Hydrostatic Pressure Tester (Option 1).
10.2.1 In 1993, a limited interlaboratory study was completed, which included six laboratories, two operators in each, running determinations on three specimens of two fabrics. No prior assessment was made of the relative level of the participating laboratories on performance of the test method.
10.2.2 The two fabrics were at different levels (Fabric 1 approximately 810mm and Fabric 2 approximately 340mm), and residual variances of the two fabrics were found to be different. Accordingly, precision is reported separately for each fabric.
10.2.3 Users of the method are advised of the limited nature of this study and ad-vised to apply these findings with due caution.
10.2.4 Analysis of the data sets for each fabric yielded components of variance and critical differences as displayed in Tables I, II and III. Differences be-tween two averages of (N) determinations, for the appropriate precision parameter, should reach or exceed the table value to be statistically significant at the 95% confidence level.
10.3 Textest FX3000 Hydrostatic Head Tester (Option 2).
10.3.1 In a single-laboratory study, six different laboratory technicians run determinations on three specimens of five materials.
10.3.2 The five materials were at different levels of approximately: A=103, B=33, C=37, D=12, and E=77. Data obtained in this study is recorded in millibars (SI standard). The residual variance of the five materials were found to be different, therefore, precision is reported separately for each.
10.3.3 Analysis of the data sets for each material yielded critical differences as shown in Tables IV, V, VI, VII and VIII. Differences between two averages of (N) determinations, for the appropriate precision parameter, should reach or exceed the table value to be statistically significant at the 95% confidence level.
10.3.4 Between laboratory precision as not been established for this option.
Until such precision information is avail-able, users of this method should use standard statistical techniques in making any comparison of test results for be-tween laboratory averages.
10.4.1 Water resistance of fabrics can only be defined in terms of a test method. There is no independent, referee method for determining the true value. This test method has no known bias.
11.1 For potential equipment information pertaining to this test method, please visit the online AATCC Buyer’s Guide at http: //www.aatcc.org/bg. AATCC provides the possibility of listing equipment and materials sold by its Corporate members, but AATCC does not qualify, or in any way approve, endorse or certify that any of the listed equipment or materials meets the requirements in its test methods.
11.2 Hydrostatic Pressure Tester (Suter).
11.2.1 The apparatus consists essentially of an inverted conical well equipped with a coaxial ring clamp to fasten the cloth specimen un-der the well bottom. The apparatus introduces water from above the specimen over an area 114 mm in diameter and at a rate of 10.0 ± 0.5mm of hydrostatic head per second. A mirror is affixed below the specimen to enable the operator to ascertain penetration of the specimen by drops of water. A valve is provided for venting the air in the well.
11.2.2 Hydrostatic testing apparatus of the type described is no longer available for sale.
11.3 Hydrostatic Head Tester.
11.3.1 Uses an electronically controlled pump to apply hydrostatic pressure at 60 mbar/min (selectable) to the bottom side of the fabric. A reservoir with a circular test area of 100 ±5cm2 (»4.5 in. diam) contains distilled or deionized water which is applied to the fabric surface. The fabric specimen is secured with a coaxial clamp which is equipped with viewing lamps to aid the operator in seeing the penetration of water droplets. A digital readout dis-plays the pressure. An RS232 data port is provided to transfer the test results for storage and statistical analysis.
11.4 Some laboratories use water at ambient temperature. If testing is performed other than 21 ± 2°C, so state.
11.5 1 mbar = 1.02 cm H2O.
11.6 Lateral water leakage can be minimized by sealing the fabric with paraffin at the clamping area.
The information provided has given you a general idea about hydrostatic head test. The easier way to test it is to use Hydrostatic Head Tester. Further information about the machines can access to TESTEX through the web at http://www.testextextile.com/product/hydrostatic-head-tester-tf163c/.