Russell,

On Thu, 07 Sep 2006 03:44:57 +1200, Russell McMahon wrote:

>...
> > What would be a suitable material to stop the Hydrogen making its 
> > way through the tank itself?
> 
> Anything with a decent density that's not chemically reactive with 
> Hydrogen would do. Thin or semiporous low density materials allow 
> Hydrogen diffusion at a distressing rate at low pressures.

Would plastic-coating the inside help, or are plastics a bit Hydrogen-porous?  What about enamelling (basically glass-coating) ?

> Normal 
> party balloons will lose an appreciable amount of Hydrogen through the 
> rubber in 12 hours. A balloon that floats in the evening will often be 
> on the floor by morning. Helium is not as bad and the balloons sold 
> for Helium use have less permeable rubber. 

Over here Helium ballons seem to be made of metalised poyester these days.  Don't know if they're less permeable, but they are much easier to print 
onto, and more decorative when they are.

> A bit hard to make Helium 
> by electrolysis though and it doesn't burn very well in most gases :-)

Indeed - but then *I* didn't mention Helium!  :-)
 
> Seams and joins are an issue for Hydrogen permeation - it treats 
> almost anything as an open doorway. A well built tank of quite 
> lightweight metal would do well enough over the time spans liable to 
> be relevant. There are almost certainly plastics that would be 
> suitable at a reasonable price. One thing to be watched is reverse 
> diffusion of Oxygen!. Much slower but it happens. This is something 
> the Hydrogen airship people tended to keep a watch on. Understandably.

Quite.  In our case it would be useful to some extent, as long as the ratio didn't get too high and they were reasonably mixed, as it would help 
combustion a bit.  You could have a fan to help the mixing, but you might end up with an Apollo 13 problem...

 
> A floating metal or plastic shell gasometer in water would do a good 
> job. It needs to be twice as tall as required capacity or to have 
> folding skirts or to have a "slot" for the storage float to fit into 
> when empty. Weighting the float could provide pressure if desired, but 
> it's probably better to build it to work at minimum pressure and pump 
> the gas.

Well the float will have some weight, so you will get some pressure whether you like it or not (unless the hydrogen-in-air  buoyancy contributes 
enough lift, which I doubt).  One thing you wouldn't do is have telescoping multiple-ring floats, like quite a few commercial gasometers do (see your 
East London reference - the railings about half way up mark the join of two units).  Because these need seals between the rings, and I'm sure 
Hydrogen would love that as an excape route.
 
> Gasometer description here
> 
>         http://en.wikipedia.org/wiki/Gasometer
> 
> or operating principle only picture here
> 
>         http://upload.wikimedia.org/wikipedia/en/f/f3/Gasometer.png
> 
>         from the above page.

I'm sure some others won't know what a gasometer is, but I've known them all my life.  At one time every town had its own gasworks, making "town 
gas" from coal, with the attendant dreadful smell, and had a collection of gasometers to store the stuff.  When Natural Gas came in the gasworks 
were dismantled, and some of the gasometers too, but about 25% of them remain in situ to form local storage of gas and act as "capacitors" to 
smooth the supply/demand imbalance.  There are two only a couple of miles from here.  If you want some photos of those just ask and I'll take my 
camera next time I go to the supermarket that's next to them!
 
> Nice big one (larger version of one on above page)
> 
>         http://en.wikipedia.org/wiki/Image:Gasometer_in_East_London.jpg
> 
> Gasometers at the oval - large picture
> 
>         http://upload.wikimedia.org/wikipedia/en/4/4a/Gasholders_at_the_Oval.JPG
> 
> I had no idea where "The Oval" was geographically until I saw the 
> famed Millenium Wheel in the background.

It's in Kennington, South London - second-most famous English cricket ground, after Lords, I'd say.  Scene of many an England Batting Collapse, but 
also of the stuff... valiant defeat of Australia, resulting in our regaining The Ashes  :-)  The gasometers are always visible in TV coverage of matches 
played there, as they are only a few feet from the fence.  Interesting to see how the gas/tank levels vary over time.

Cheers,


Howard Winter
St.Albans, England


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist