The air conditioning system life and the most efficient use of air conditioning depends upon the chemical stability of the refrigeration system itself, to a large extent. The most widely used gas used in modern systems is R-134a. It is very important for all materials in the refrigerant system be chemically compatible. The only suitable compound for use with 134a systems is PAG, Polyakylene Glcol, a synthetic lubricant.

Generally speaking, coaches need about 1 ounce of PAG for each 7 feet of hose after the first 15 feet of hose. Roughly, a 40-foot coach will use about 92 feet of refrigerant (depending upon conversion, OTR A/C and a variety of other factors).

Take 15 feet off the measurement and you have 77 feet of line. Divide the 77 by 7 and you will find that you should have about 11 ounces of PAG from a fully evacuated system.

Taking the matter one step further, the 11 ounces will comprise about 18% of the entire capacity of the refrigerant system. This will create a total volume of about 61 ounces, or 3.8 pounds of R-134a.

R134a is a non-explosive, non-flammable and non-corrosive gas. There is hardly any odor to it and much heavier than ambient air. Yet, proper care and handling of 134a is required. At sea level, with mean atmospheric pressures and temperatures it will evaporate so quickly that it will freeze just about anything with which it comes into contact. The open container boiling point for 134a is -21.7F. This low boiling point makes it an excellent refrigerant. The tremendous heat transfer which occurs when a liquid boils forms the basic principle of an air conditioning system. The amount of heat required to raise or lower the temperature of one pound of water by 1F equals one BTU.

On a fully charged, enclosed system, the high side of the compressor can be as much as 200PSGI.

Yet, this is not a universal measurement, as ambient temperatures significant affect the temperature.

Here is just a sampling:

At 20F PSGI=18.43
30F=26.10
50F=45.48
70F=71.19
90F=104.40

Most experienced air conditioning mechanics do not measure the amount of 134a introduced into an evacuated system. Instead, they input air conditioning into the low side (Always the larger hose) at the compressor. They then slowly run the compressor, with an eye to the sight glass. Once the sight glass gets foamy and then stabilizes, they then consider the air conditioning system to be fully charged. This does away with the considerations of ambient air temperature and pressure regulation.

This is important for coach owners.

A quick measure to determine if an air conditioning system requires charging is to look at the sight glass while the air conditioning system is on (compressor running). If the sight glass appears to be 'foamy,' it is time to charge up the system with the requisite 134a and PAG, often sold in the same container.

Do not buy higher priced 134a refrigerant that has a 'leak stop' component, especially if you have OTR A/C. The Carrier compressor has very fragile reeds and seals on the inside and they can easily become clogged if PAG is introduced above the indicated levels.