A/C Machine Accuracy

A/C Machine Accuracy: The Consequence of an Incorrect Charge

To maximize the potential of the new refrigerants, the precision of system operation became more important than before.

Old habits are hard to break, and when you grow up and learn things in a certain way, it can be difficult to retrain yourself down the road. Such is often the case and was with me when it came to automotive A/C systems and R-12 refrigerant. R-12 simply worked well. I always believed it had a better heat transfer rate than subsequent refrigerants, but did it really? Scientifically I’ve heard arguments that go both ways, and rather than fall in that black hole, I’ll just leave it at that.

What I do know is that R-12 cooled big cars and the refrigerant systems had massive capacities. The refrigerant worked well and being down a few ounces really didn’t matter. When you charged these systems, you didn’t worry about exact amounts. You kept an eye on the sight glass, and when the bubbles were gone, the system was charged. It was a process that was far from anything resembling precision, but it didn’t matter. You’d have ice cubes coming out the dash vents in no time.

Don’t get the wrong idea. This isn’t a complaint about ditching R-12. Environmentally we needed to switch to a different refrigerant, and I completely agree with it, but it took a while to adjust my mindset on A/C service. Here’s what made the big difference: R-134a and R-1234yf have different thermal heat transfer rates, i.e., they are chemically different than R-12 and, as a result, the systems were designed to operate at slightly different pressures.

Old 2-cylinder compressor
How times have changed: This old 2-cylinder compressor weighs more than some lawnmower engines.

In order to maximize the potential of the new refrigerants, the precision of system operation became far more important than before. You might say that R-12 systems were much more forgiving, but with the new refrigerants, it became critical to balance the heat transfer rates of the evaporator and condenser. Condenser efficiency is a key component to proper operation, and if the evaporator is more efficient and draws more heat out than the condenser can release, system operation will suffer.

In addition to all this, A/C system capacity is far less than it used to be. Instead of capacities that were multiple pounds, most systems operate with only 12-16 ounces of refrigerant, for many reasons. Compressors are much smaller and more efficient in order to save weight and increase fuel economy. Smaller hoses and lines allow for more precise pressure control, and lower capacities mean less effect on the environment in the event of refrigerant loss.

old R-12 system
2 lbs., 14 oz. of R-12 was required for this old R-12 system, but there was plenty of wiggle room.

Add all this together and the accuracy of your A/C machine becomes an important topic. The most obvious consequence of an incorrect charge is the effect on the cooling ability of the system. Even the slightest over or under charge will throw off the designed pressures of the system, resulting in an immediate effect on duct temperature. A low charge also means low oil circulation, which can cause damage to the A/C compressor.

Refgrigerent caution sticker
Modern system capacities are much lower, and accuracy is critical for proper operation.

Accuracy during refrigerant recovery is an often-overlooked aspect. If an A/C machine reports that it has recovered all refrigerant, but has not actually done so, this presents a number of problems. One, if you open the system, the remaining refrigerant is released into the atmosphere. Two, it’s money lost, and three, if you are performing a recharge service, you could very likely overcharge the system.

No matter how you stack it, an incorrect A/C charge will cost you money, either now or in the long run. How can it be avoided?

The latest standards for A/C equipment are SAE J2788 for R-134a and SAE J2843 for R-1234yf. Machines meeting these standards are required to be able to recover 95% of system refrigerant in 30 minutes or less without applying any heat to the system, and their required accuracy for refrigerant weight is plus or minus .5 ounces.

None of us likes to buy new equipment, but if we’re going to continue to work on modern A/C systems, we’ll have no choice but to make sure our equipment meets these standards. A final point to remember is that all this equipment requires regular calibration and, in many cases, it’s recommended by the manufacturer once a month. This should be part of your normal shop maintenance routine. TS

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