The sight glass should be fairly clear, with only a few bubbles passing
by to show the flow. If it is white and hazy, then the system likely
needs a charge. A plastic bag to increase head pressure? Head
pressure is a term that isn't always applied correctly, but what he
acheived is an increase in the pressure at the condensor coil because
the bag aound the unit prevents air exchange which doesn't allow the
condensor to shed heat from the compressor. Why he considered this to
be better accuracy evades me, because a running system is checked on
both sides of the compressor (only an example: the high side after the
compressor having 115psi and the low side having 25psi). It's the
pressure difference that causes cooling. The simplest analogy is an
air hose. An air compressor pressurizes the air in the hose. As it
leaves the hose the pressure drops, and the air is cooler. For a crash
course, take a look at
http://home.howstuffworks.com/ac2.htm