1ml. is a gram, is a cc., when it comes to water (sg =1.0). so....
100mm x 100mm x 100mm is equal to 1 litre.......
which leads on to;
1 metre x 1 metre x 1 metre is equal to 1000 litres which is equal to 1 ton of water. Example 8700 litres is 8.7 metric tons...... Don't you just love metrics!..(sometimes anyway)
Or theres......
1 gallon of water equals 10 lb, and there are 6 1/4 gallons to the cubic foot...... etc.
Why cant you use a small solar panel?. More panel = more voltage = more hydrogen = bigger bang! I guess you would want a switch well away from the tank.
Just set it up at the other end of the paddock, from the house. (or shed at least) Be careful!!
I am still enjoying your thread (as it seems, are many others) keep it going.
You should be able to check out solar panels on the net. This country is full of them.