Today’s automotive A/C systems need to be charged with accuracy

Today’s automotive A/C systems need to be charged with accuracy

Today’s automotive A/C systems need to be charged with accuracy

The underestimated risk

Several years ago everything was fine in the world of automotive air conditioners. A standard A/C system was charged with about 1.5 kilograms of refrigerant – and it actually didn’t matter if there were 1.3 or 1.7 kilograms of coolant romping about between the compressor and the filter drier, because there was enough leeway in either direction. Those air conditioning systems used to work almost without fail for a long time. And they did so until automotive manufacturers started experimenting with weights and materials where they saw an enormous potential for savings. Today, we are witnessing refrigerant charging amounts of less than 300 grams. 200 grams of refrigerant more or less in these systems can have serious consequences.

Modern vehicles need more refrigerant

That means, modern vehicles call for much more accuracy when it comes to the quantity of refrigerant to be charged into an A/C system. A deviation of +/- 15 grams is commonly considered tolerable. With the latest generation of A/C service units it would even be possible, without any problems, to charge the system accurately to within a gram – if it weren’t for the service hoses.

When a car air conditioner is charged with refrigerant, the first thing that is filled is the service hoses. So far, so good. Theoretically, the refrigerant remaining in the hoses after the service couplers have been removed would be recovered, so the correct amount of refrigerant would arrive in the vehicle. In real practice, however, said service hoses are not filled to the brim with fluid refrigerant. Instead, an unpredictable proportion of gaseous refrigerant remains in the tubing.

While a litre of fluid R 134a weighs about 1.2 kilograms, its weight in gaseous state is only about 30 grams, depending on the pressure and the ambient temperature. However, the weight ratio of the gas and fluid not only depends on the pressure or temperature. The compulsory test run after every A/C service, for example, will also influence this ratio, because the compressor recovers the refrigerant from the low-pressure side and forces it into the system via the high-pressure side. Things are getting truly exciting when the car is serviced on a hoist – about 2.5 metres above floor level, or when the service unit is placed, for whatever reason, at a higher level than the vehicle. The obvious result in the former case is that the service hoses are completely filled with fluid refrigerant, while they will filled with gaseous refrigerant in the latter scenario.

Most workshops do not want to spend time for A/C Service

As if this wouldn’t be enough, there is another “fact of life” we are facing these days: Most workshops cannot afford spending a whole hour or more on an A/C service. Profitable operations call for shorter cycles at the individual workstations. And the easiest way to get shorter work cycles is to reduce the time spent on refrigerant evacuation.

At first glance this method seems to work. In most cases the manometer would show that a proper vacuum has been reached in a few minutes time. The really important phase, however, follows only afterwards: residual refrigerant contained in the refrigerant oil has to be evaporated and then recovered by the vacuum pump of the service unit. This can well add up to an amount of 50 to 100 grams. Low-emission service units by WAECO prevent this effect, because they collect and recycle the refrigerant contained in the hoses.

If the recommended evacuation time is not adhered to, the first 50 grams of refrigerant are already “pre-charged”. All the factors earlier described end up in the following worst case scenario: a vacuum phase too short results in a surplus of about 80 grams, while working at the upper limit of the specified level of charging amount accuracy means another 15 grams of excessive refrigerant charged. This adds up to a total of almost 100 grams of refrigerant overcharging or undercharging, an absolutely critical amount in younger A/C systems.

Air conditioner overcharged?

What happens when a car air conditioner is overcharged? – The temperature at the centre air nozzle in the passenger compartment is too high; the increased mechanical load results in compressor noises; the pressure at the low side and the high side are too high; and, in case of excessive overcharging, the refrigerant pipes can even burst. An overcharged A/C system, by contrast, will cause a rise of the compressor temperature due to the insufficient heat transmission and oil transport; the temperature at the centre air nozzle in the passenger compartment will also be too high, whereas the pressure on the low and high side will be too low. A modest deviation from the target values will “only” result in a reduced cooling capacity of the A/C system. Larger deviations, however, will cause serious damage to the components, which can entail costly repairs – and potential recourse claims from the vehicle owners.

Even if the workshop operator did all the recommended working steps, there’s still a spanner in the works: the refrigerant scales can easily be “readjusted” when the mobile A/C service unit is moved about on uneven workshop floors and the internal refrigerant tank is hammering with its full weight of 40 kilograms on the weighing cell of the service unit. What will then appear as a badly adjusted measuring instrument, will in effect be an irreversible mechanical deformation of the sensitive weighing cell. The first three or four times this happens, the scales can be recalibrated with some effort. Sooner or later, however, the displayed values will deviate to an extent that means that the weighing cell has to be replaced.

WAECO ASC’s special weighing system

To prevent this scenario, WAECO ASC A/C service units are equipped with a special, sandwich-type weighing system. A total of eight vibration absorbing springs reliably dampen any shock load that might damage the sensitive weighing cell. Experiments on uneven flooring and vibrating plates have shown that it is more likely for the all-rubber back wheels of the service station to give in than the WAECO scales to readjust. There’s no need to calibrate the weighing system of these service units. This helps save costs and guarantees highly accurate measuring results – finally resulting in increased efficiency, more reliability, better environmental protection and more customer satisfaction.

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