Hydrostatic head (HH) is a method used to measure how waterproof a fabric is. This measurement is given in millimetres, referring the height of a water column standing on fabric that is needed for the water to pass through.
For example, if the hydrostatic head is 1000mm, that means the fabric can withstand a water column of up to 1000mm in height. If the fabric is exposed to more than this amount of water, then it is no longer guaranteed to be waterproof, with the water likely passing through the fabric.
Different fabrics have different hydrostatic heads, with 1000mm being the starting measurement for something to be considered waterproof. The better the hydrostatic head the more waterproof a material is, meaning it can be exposed to even more water before soaking through.
The method of testing was traditionally done using a physical water column. This involves placing a column of water above a section of the fabric to measure how much volume of water it can withstand.
Yet this method is no longer universal, as some fabrics have such high waterproof ratings that measuring them using a physical water column isn’t viable. For example, some fabrics have a waterproof rating in metres, meaning the water column would literally be several metres in height.
As this would be a huge hassle to create in a testing environment, a newer method has been developed – the hydrostatic head test. This involves a machine that duplicates the water pressure that a large water column (e.g. 30m) produces without having to physically create the water column.
By applying increasing levels of pressure onto the fabric, the machine replicates the same water pressure of a water column to calculate what volume of water is required to penetrate the fabric. The resulting pressure required for each hydrostatic head test is then converted into a water column, informing us the size of water column in millimetres needed to penetrate the material.
What Hydrostatic Head Means in Tents
As a hydrostatic head is used to measure how water proof a fabric is, it features heavily in outdoor lifestyle products such as tents and jackets.
As one of the most important features of a tent is how waterproof it is, consumers should pay close attention to the hydrostatic head rating, which indicates the volume of water the tent’s material can withstand before soaking through.
All waterproof tents must come with a HH rating of at least 1000mm, which is the bare minimum required to be considered waterproof. However, this is only waterproof to volume of 1000mm, after which the water will penetrate.
As a result, a HH rating of 1000mmm is only considered waterproof in light rain conditions, as the water pressure from heavier rainfall will be higher than 1000mm. Therefore, a HH rating with 2000mm is required for a tent to be considered waterproof in heavy rain.
That said, the wind can also increase the water pressure the tent faces. So, if the rain is heavy and it’s also windy, then the 2000mm rating may not be enough to keep the fabric waterproof – a HH rating above 2000mm is necessary in this case.
Simply put, the higher the hydrostatic head, the more waterproof the tent is.
A good option for those camping in the UK is to opt for a HH rating between 3000mm and 5000mm. These are generally the most waterproof ratings for UK weather, although more severe conditions may result in water passing through, which is why many camping enthusiasts seek out higher ratings.
Lower HH ratings are still useful of course. They are great for budget tents and for camping in the summer where there is little to no rainfall. When camping during autumn or winter, a higher hydrostatic head is recommended to keep the tent dry.
Is the Hydrostatic Head the Only Thing That Impacts How Waterproof a Tent is?
No, the hydrostatic head isn’t the only thing that influences how waterproof a tent is. For instance, the stitching and seams of the tent need to be high quality to ensure nothing soaks through gaps in the fabric, while the zips may also cause water to penetrate through the tent.
Tents can also feature waterproof coatings that further enhance this, so look for a material with a high HH rating, quality stitching, and a good overall design to ensure the tent remains dry.
What is the difference between oncotic pressure and hydrostatic pressure?
The main difference between these two types of pressure is that hydrostatic pressure pushes pressure away whereas oncotic pressure draws pressure inwards.
What does 10,000 mm waterproof mean?
Waterproof materials are described using numbers in millimetres, so when a tent is described as 10,000mm waterproof, it means the material of the tent can withstand 10,000mm of water before it begins to leak through.
For instance, if you were to take out a small section of the fabric with a 10,000mm waterproof rating, placed a square tube that measures 1” x 1” on top and then filled it with water, the fabric could withstand 10,000mm of liquid before it leaks.
What is a water column on a tent?
The water column on a tent is a measurement of how much volume in a single column of water is required before water leaks through the material. It’s called a water column because a physical column of water is typically used to measure the waterproofness of a tent’s fabrics.
While usually measured in millimetres (mm), certain materials are so efficient at keep water out that their water column is much higher than just millimetres – some have water columns that reach metres. Therefore, a hydrostatic head is used to measure higher volumes of water that cannot realistically be made into a physical column of water, instead replicating the pressure of these higher water column volumes.
What are waterproof tents made of?
Waterproof tents are made from various materials, with polyester being one of the most prominent fabrics used. That said, there many other materials used depending on the size of the tent. For instance, many smaller tents are made from nylon.
The biggest similarity between waterproof tent materials is their coating. All the materials feature a waterproof coating, with the type of coating impacting how waterproof the tent material is. PVC is one of the most widely used coatings for tents, offering high-quality waterproofing.