Charging for Perfection

We used a standard “virgin” (virgin is new as opposed to recycled or recovered refrigerant) 30 pound canister of Dupont’s Suva brand R134a for the charging. We placed the 30 pounder on top of our digital weight scale. This is similar to the system you find in most well equipped a/c shops, theirs might be enclosed in a cabinet or “station”. You can purchase R134a in small 12 or 16 ounce cans but this will impose a problem if you are not using the entire can’s contents. Let’s just say it is difficult to measure out 26 ounces of refrigerant from two 12 or 16 ounces cans. You should be cautious of small cans of refrigerant as they sometimes contain additional refrigerant oil. You don’t want too much oil because it will saturate the evaporator and reduce the evaporator’s ability to transfer the heat. Some small cans contain “sealants”. Frankly we have not read any good news on sealants in the industry. In many cases the sealants will collect in small passages and plug them up as well as damage your compressor. Inexperienced DIY’s (do it yourselfer’s) can have problems when putting the second or third can of refrigerant into the system because air can enter the service line.

When you are charging with R134a you are primarily concerned with the high side system pressure compared to the ambient outside air temperature, what is referred to a P&T’s or Pressures and Temperatures. There is a common chart called a P&T chart which will help guide you to finding an acceptable high side pressure related to the outside temperature, and naturally you want to look at the vent temperatures the low side pressure. For the moment we won’t be addressing low side pressures.

Finding the “perfect” quantity of R134a

As we mentioned earlier our friend Mr. Tom Charlesworth has had much success in converting the 911 from R12 to R134a. Though our system would be different than systems Tom has worked with in terms of total refrigerant requirements, we wanted to get a hint on what to look for when things got critical. So with Tom’s suggestions and our charging system in place we started to find the perfect quantity of R134a to use in the Mr. Ice Project. We started off with putting 16 ounces into the system. Our procedure then entailed monitoring the high side pressure gauge and noting the vent temperatures with a digital probe in the center and Kuehl vents.

During this procedure the engine deck is down, gently resting on our ac service hoses. The cockpit thermostat switch, located on the left side of the center floor console, is turned to max cold. The fan speed switch is set to its third setting or maximum speed. We documented our observations and began to plot a “chart” to our perfect amount. The car was on black top in the sun during this operation. The outside temperature was 85 F degrees. The interior car temperature was 96 (it had been sitting for about 3 hours taking in lot’s of heat).

Carefully we added 2 ounces of additional refrigerant (in a gaseous state, meaning can upgright with valve on the top), with the engine running at idle through the low side service port on the compressor. Now we had 18 ounces total in the system. We waited 5 minutes for the system to settle down or homogenize, noting high side gauge pressures and vent temperatures.

Optimally we want the lowest possible high side pressure with the lowest vent temp possible. Normally when charging the 911 with R12 you would observe the drier’s site glass as an indication that the system has been “filled” with R12. With R134a we disregard the site glass completely. Our objective is to obtain the lowest vent temperatures with the least amount of refrigerant and to maintain “acceptable” high side gauge readings. There are some basics here though. You do need a minimum amount of refrigerant so the oil in the system will bathe the compressor (it is the refrigerant that carries the oil through the system; if you don’t have enough refrigerant you won’t have enough oil moving to the compressor….. and then you’ll get compressor lock up). As you recall we have incorporated out “low-high” pressure switch that cuts out the power to the compressor’s clutch in cases of too low or too high pressures.

We repeated the procedure of adding 2 ounces of R134a refrigerant to the system, ran the system to 5 mins., and checked our pressures and temperatures, and recorded our results. We finally stopped at 30 ounces as we noted the pressures were rising above what we wanted based on the P&T chart and we were not seeing any gains or lower temperatures at the vent. So we evacuated out approximately 4 ounces which left us with our optimum quantity of 26 ounces total in the system based on this particluar cars set up.

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