Re: [Phys-l] fire pistons

[Brian Whatcott <betwys1@sbcglobal.net>]

John Denker wrote:
> This is a practical application of compressing a gas
> quickly enough so that it is approximately thermally
> isolated from the surroundings.  Useful for making
> fire without matches. /snip/
>   
Hmmmm........since reading John M's enlightening note, I am now
skeptical of the text books and contributors who suggest that
bump compression is only about thermally isolating the gas from
its environment by minimizing hot dwell.  
 I prefer to conceive that bump compression
greatly lifts entropy and temperature over that provided by
a highly insulated reversable compression stroke.
> I've never actually made one, but I think I would prefer 
> Schedule 80 CPVC since it has thicker walls and higher 
> pressure ratings than the more common Schedule 40 stuff.
>   
Having been set into skeptical mode, I note that burst pressures
are surprisingly high for plastic tubes of small diameter:
at room temperature,  schedule 40 CPVC 1/2 inch pipe may
not burst until 1800 psi is reached - schedule 80 in the same bore
at 2700 psi.   True, its strength drops greatly as temperature rises,
so a decompression to access the glowing char in the piston head is wise....

Brian W