Re: Commpression checks

[Jon Pike <jhpike@domain.elided>]

> First of all, it's a "compression" check. The density of the air has no
> effect on what the reading will be because the piston will just keep
> increasing the psi until it leaks past the rings or valve seats or whatever.
> It will pump it up to the same value regardless of the difference in
> altitude. It may take more revolutions to reach that value but it will hold
> the same regardless.
>
> The real question concerns the gauge, weather or not that reading is
> relative. In other words, is it calibrated to read accurately at sea level.
> I'd guess it probably is so...
Ah.. Paul brings up a good point.

I (and some others) theoretical ideas were based on a single stroke of a 
piston..  while a "real" compression test is a cumulative average 
(courtesy of the little one way valve in the tester) of 4-5 strokes of
the piston.

As for the relative thing.. depends on the if the gauge is absolute or
differential.  I think 99.9% of compression testers there are 
differential.. i.e it reads pressure relative to ambient, i.e. no matter 
what altitude you are at.. that's "0" psi.

So, it should be accurate at sea level, AND at 14,000 ft.

An absolute gauge has a vacumm chamber as it's reference, so it would 
read 14.7 psi (plus or minus weather) for "0", and less as you go up in 
altitude.  Your indicated pressure would be X + 14.7 (or whatever) psi..

Jon
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