> R-134a (tetrafluoroethane) is a single chemical compound.
>   I am unclear, however, why this should matter with respect to the
> sight glass.

Larry:

Thanks for pointing this out.

Yes, R134a is a single chemical. I was wrong on that.  I have
read that looking for bubbles in the sight glass won't work
for the mixtures, which are most of the "drop-in" replacements.

Here is a comment about the sight glass and R134a:
> The receiver-drier or Accumulator usually has a sight glass
> in the top. This sight glass is often used to charge the system.
> Under normal operating conditions, vapor bubbles should not be
> visible in the sight glass. The use of the sight glass to charge
> the system is not recommended in R-134a systems as cloudiness
> and oil that has separated from the refrigerant can be mistaken
> for bubbles. This type of mistake can lead to a dangerous
> overcharged condition.
This is from:
http://tinyurl.com/ky723

Also, I found a comment on an EPA page, Choosing & Using Alternate
Refrigerants..." that says that barrier hoses are needed with
any refrigerant containing HCFC-22.  So that implies that R143a
conversions do not need barrier hoses(?)  I suspect there is a
cat drying in the microwave here--urban myths at large.

                                Richard A Jones
                                Boulder, Colorado