On 23/03/16 08:46, Gareth Magennis wrote:
I would really like a very small, cheap, air tank that I can charge
every now and again at the local garage (i.e. can be filled with a
standard car tyre pump).

A standard type of water-based fire extinguisher can be pressurized
using air through a nozzle the same as a tyre nozzle. That is, you
can fill it yourself and pressurize at the local fuel station.
They have a pressure gauge so you can tell whether your extinguisher
is ready for use.

I have a couple of these, and used one in a gas-flow experiment at
150PSI, no water. It will discharge through a 1mm nozzle for longer
than 2 minutes. I have time/pressure graphs somewhere; that was the
point of the experiment, we measured the speed of sound and calculated
an approximation to absolute zero as a high-school experiment).

If you change the standard water nozzle for one with a smaller
diameter, and don't mind reaching down to the cylinder to discharge
it, you'd need no other changes.

My ones look like this:
http://www.gumtree.com.au/s-ad/sebastopol/miscellaneous-goods/9l-water-fire-extinguisher-refillable-50-each/1086145027

Clifford Heath.