When water is mixed with R12 a hydrochloric acid forms which will eat holes through your condenser, evaporator and compressor. Moisture enters the system when ever you open it up and expose it to the surrounding atmosphere or when you have a system leak. The receiver drier attempts to remove moisture in the system with its desiccant. The drier’s desiccant can only absorb so much moisture. As little as a one or two drops of water in the system and you will have performance issues. Moisture will freeze up inside the expansion valve or other components causing restrictions or functional problems.

When you are converting from R12 to R134a there will be residual R12 in the residual oil left in the system, such as in condensers, original hose lines, evaporators, etc. It is better to run a “pure” state of R134a rather than a mixture of two refrigerants. So evacuation is the best method to pull out the remaining gases trapped in the residual oil.

We suggest that when you are evacuating a 911 or 930 system you do so for at least 3 hours (pump running). Some shops seem to feel 1/2 hour is enough, though we have found the longer, and sometimes “more than once” the better for a few reasons:

  1. This 911, like most two condenser cars, has nearly 40 feet of a/c hose, plus we have three condensers an evaporator and drier. This adds up to a lot of volume for stored moisture and refrigerant trapped in any residual oil in components you have not replaced. Remember, if the outside humidity level is 50% and you open up the system, you will have a 50% humidity level in the system.
  2. Beside air, “moisture” is a killer to any refrigerant system. Moisture (water) can turn to ice and freeze up the expansion valve and plug up small passages. By pulling a simple vacuum you will not remove all the water. You need to “boil out” or evaporate the water. Water boils or starts to evaporate at 212 F at sea level (sea level is approximately 14.7 psi.). Since we can’t fit the 911 in a toaster oven we have use another method to boil out or evaporate the water. If we can lower the pressure in the system (by pulling a vacuum) to let’s say 29.5 in. Hg. (inches of mercury) of vacuum (at sea level), we can lower the temperature at which the water will start to evaporate. The lower the vacuum level the better. The outside air temperature and the elevation you are at all will effect the point of evaporation. There is a vacuum chart available that cover this topic. Also, keep in mind that most a/c gauge sets are analog (dial) rather than digital. Analog dials can be difficult to read accurately because of the increments, needle size and your viewing angle.
  3. Just because you pull a vacuum low enough or long enough to “boil off” the moisture does not necessarily mean you removed all the moisture from the system. Water can change from its liquid state to a gas, and then to a solid state (ice) as the vacuum increases.

So we hooked up our small 1.5 cfm.. vacuum pump with our gauges and let it run for 3 hours. After 3 hours we stopped the pump and shut off the valves and waited 30 minutes to see if there were any drastic vacuum leaks (many practitioners only wait 15 minutes, we prefer a longer observation time)
When you evacuate a system you want to do it when the air temperature is above 70, the warmer the better as moisture in the system will boil off sooner at hotter temperatures than at colder.
We were fortunate in this project as we had brand new barrier hoses, a new compressor, new drier, the new Kuehl condenser and evaporator. We were concerned with the 19 year old original condensers however we did inspect them for signs of oil residue, twisted tubes and corrosion.

A vacuum leak down test gives you a good opportunity to check for major system leaks. You could skip the leak down test only to find a major leak later on, which is an expensive and time consuming way of doing things. So after we ran our vacuum pump for 3 hours we closed the gauge valves, shut off the pump and waited 30 minutes. Luckily the low side gauge stayed put, near -29 (in. Hg) inches of mercury on the scale.

Had the gauge moved upward, for example from -29 to -20 or so, then we know we have a problem. It could a leak where atmosphere is entering the system from a poor seal or faulty component, or it could be related to frozen moisture as the liquid moisture boils off into a vapor.

A more accurate method to determine the quality of the vacuum in the system is to use an electronic-digital gauge that displays the value of the vacuum in “microns”. You could use other units of pressure such as “torrs” or “Pa” but microns is more common. Today Griffiths uses electronic gauges that measure in microns to determine the quality of the evacuation. There are charts available which show you the relative unit of vacuum required to boil off moisture at a given temperature; very handy.

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