Hydrostatic head, what does it mean?
Hydrostatic head is used to define how waterproof a fabric is. A column of water will exert a pressure on the fabric, as the height of this column is increased eventually water will penetrate through the fabric. The height at which this happens will be the fabrics hydrostatic head.
Heavy wind or driving rain will exert more pressure on a fabric therefore a higher hydrostatic head is required to stop the water from penetrating. Likewise the ground sheet of a tent will require a higher hydrostatic head as the weight from being walked/sat on will increase the pressure of the water underneath.
What does this tell me about the fabric?
It tells you how waterproof the fabric is. For example a lightweight waterproof coat designed for use in showers will have a low (typically 1200mm) hydrostatic head. Whereas the ground sheet of a tent where the pressure of you walking/ sitting in it could force water through (typically 10,000 mm).
Why not just use the highest possible?
Normally the higher the hydrostatic head the less breathable and heavier the fabric is. If you are a marathon runner there is no point having a super waterproof jacket if you just get soaked by your own sweat. Therefore a compromise needs to be made. Likewise if the fabric is for the ground sheet of a tent designed for family camping, the weight is not so much of an issue.
How can you test a fabrics hydrostatic head?
- Find a suitable piece of pipe. For example drain pipe
- Tape the fabric sample to one end of pipe. Don’t forget to ensure the “outside” of the fabric is facing inside the pipe as this is where the water will be.
- Stand up the pipe with the fabric at the bottom.
- Slowly fill the pipe with water until it starts to seep through the fabric. If it starts to come out round the edges you tape isn't strong enough.
- Measure the height of the column of water in millimetres. This is the hydrostatic head.