H2 tank leakage
 

Another question I have on hydrogen is regarding the energy storage problems:
wont hydrogen, stored in a conventional metal (SCUBA) or steel tank, leak out of the tank if stored for say a day (24 hours), and how much of it will leak out of it (in %) depending on the pressure the hydrogen is stored inside it ?
Pressures I'm thinking of are: 7 bar, 250 bar, 350 bar and 700 bar.

This would matter if you would ever want to use hydrogen in say a car, and would not drive sufficient distance to completely drain your tank (and so instead would keep whatever hydrogen remains after driving, stored in the tank in your garage untill the next day).

It's a very small amout.
I have a spare helium tank that leaks a very tiny amount.
I has lost about 1/3 of its pressure in 3 years.

Plus you have hydrogen enbrittlement with hydrogen.
So eventually your hydrogen tank will fail catastrophicly with out warning.

I'm guessing you have a car that was made to run on hydrogen ?
So it probably has carbon fibre tanks and uses high pressure (700 bar) ?
Won't leakage be a more serious problem when you use say 250 bar pressure in your tank (as proposed for conversions using CNG kits -these only allow up to 250 bar-)

IIRC those CNG cylinders are quite overbuilt and could stand up to 900 bar, but I'd still not try such a narrow safety margin using them with hydrogen.