Hello everyone,

I used to be around here a lot, until it seemed like I spent more time here arguing about dogs, cyclists, and whether or not Covid was real, and sometimes climbing, than arguing, sorry, talking with my partner 

Any way, I have returned as I have a technical question I can't find a good answer to on Google, and I expect someone around here could help...

I'm looking for a conversion from PSI water pressure to hydrostatic head in mm.

I want to get my aforementioned partner a decent winter coat for general everyday use. We have looked at a North Face women's Zaneck coat, with a Dryvent outer fabric. I emailed them to ask about the hydrostatic head figures and they replied with a PSI figure, saying that's what they use. I'm used to hh figures and have an idea what they mean.

The figure they gave is 75 psi. 

Can anyone help?

Thank you

1 bar is 10m of hydrostatic head.

75psi is 5.2 bar (approx)

75psi is therefore 52m (approx) of hydrostatic head. (52000mm)

^That is near enough, but here's a link anyway: https://www.convertunits.com/from/pounds/square+inch/to/mm+of+water

Thank you,

Does that sound right? I'm not questioning your workings, more the answer from North Face. Sounds like a very high figure.

Thank you.

Same secondary question to you. Is 50,000 ish mm a likely figure, or could North Face have given me the wrong psi?

I'm very much not an expert on such things, but no, that figure doesn't seem to make sense.  Maybe they use 'PSI' differently somehow, so that it doesn't just neatly convert to mm of water as a measurement of pressure.  Or maybe they've just given you a silly number.

Here's another link.  According to this the realistic range for a jacket would be a hydrostatic head of 10 - 30 m.

https://www.gearassistant.com/what-is-hydrostatic-head/

Imagine a column of water, one inch square, weighing 75lbs.

How tall is that in metric units?

75lb = 75 * 454g = 34050g

Divide that by the area of the column (in cm), to give height in cm:

Height = 34050/(2.54^2) = 5278cm

Or 53m

Sounds a little optimistic.

50,000mm hydrostatic head is feasible (some sea fishing gear claims that) but it certainly won't be breathable.

That's what I was thinking. I'm used to buying a hardshell jacket, and have an idea about the figures I would be expecting, which tallies with what you say, but this is street wear.

Thank you all for your replies.

I think I'll get back to North Face. The jacket I'm looking at is a street wear winter coat so I think their figures are wrong, otherwise mountaineers and the RNLI would all be wearing them. 

I get 75psi to 52,730.1787 mm/H2O on the conversion app that I use. 

Looks like you may get the same reply. Found this - 

The DryVent technology has been tested in The North Face’s Quality Assurance lab. It is measured in PSI (pound per square inch) which is the pressure resulting from a one-pound force applied to an area of one square inch.  The ratings that have resulted from this test are:

Breathability: 700-750g/m2/24hrs avg. (MVTR) upright cup*

Waterproof rating: 75 PSI avg., 75 avg. PSI after 20 launderings

*Upright Cup numbers will typically be much lower than results from other testing methods.

I realise this is to do with breathability testing, but perhaps something similar is going on here.  Maybe there's a standard test that returns a result in PSI that has a different methodology to another that returns a result in mm H2O, so that they produce results that aren't directly comparable. 

Dunno.  Pressure is pressure you would think, but perhaps one test has a much more forgiving definition of when water has penetrated the fabric than the other or something like that.

Obsessing over these numbers seems like overthinking things a bit for a general purpose "street wear winter coat" tbh.  Especially one that's mostly being bought because it's fashionable.  (C'mon, you'd sack off the TNF option and toddle down to Decathlon otherwise wouldn't you?)

Well beyond my knowledge, but something is not right for a street wear coat. I’ve have/had fully waterproof jackets for mountaineering at 20000 mm HH. Waders are in region of 30000 mm.

Maybe the OP can ask TNF to explain and for a conversion to mm HH, unless someone knowledgeable on here can offer an explanation.

My workings are sound, if approximate!

10,000mm is classed as waterproof, most fabrics are rated between 10 and 30k. 52,000mm seem suspiciously high, though DryVent is only, I believe, a re-brand of HyVent. DryVent is a microporous PU coating, same technology as Pertex Shield.

What do these 'hydrostatic head' pressure figures actually mean when it comes to fabrics? The force that the wind is battering your jacket with rain, for example, or, just nonsense numbers that appeal to us geeky men? 60,000 leagues under the sea type watches springs to mind as another example 😂

It means you can sit or kneel in your waterproofs, and not get a wet bum or knees.

So ... the less heavy one is, go for the cheapest option. In my experience, you still get wet, but they keep the wind off, that's what does for you in the end. 

Depends; the less heavy may have more bony arses & knees...

Although they still make one or two good items useful for mountaineering, by and large.. they make ludicrously overpriced urban wear themed on outdoors/mountaineering. That's why you don't often see mountaineers or RNLI wearing their stuff.

However my mother in law, who despises people who go hill walking and scrambling etc ("they are so selfish! Not only are they risking their own lives by going out on hills, but they risk the lives of those who have to rescue them") wears a The North Face waterproof jacket.

As hinted earlier, an HH of 10,000mm, say, represents the ability to hold the equivalent pressure of a column of water 10m high.

Wading happily with my ill-informed knowledge, PSI seems a slightly odd way of referring to the water-resistant capabilities of a material since in practice they'd surely not provide the same resistance to air pressure?

PSI can also measure water pressure.

You have to remember this a US specification, and they eschew SI units in favour of an idiosyncratic set of units...