[MR2] reading a/c pressure gauges

[tbamc]

I bought a used set of gauges for dealing with my a/c system, and 
thought using them was fairly straightforward.  I then noticed that in 
addition to a PSI scale, there is an R12 and an R22 scale.  I 'thought' 
the large PSI scale was the one to use, but then seeing the vastly 
different R12 scale I started to question that assumption.  Anyone know 
the difference, or have a link to a site explaining it?  Google wasn't 
kind to me.

Thanks.

Thon