It pays to have good equipment and as you know good equipment can be expensive, anywhere from $1,500.00 to $5,000.00 to “properly” service your cars air conditioning system. Unless you are going to be using this expensive equipment on a weekly basis it is not worth buying to try once or twice on your car or the neighbor’s. Reading a few books will give you a baseline to understanding refrigerant theory and practice, and taking the “Refrigerant Test” may get you a license to buy refrigerant. But, nothing is better than hands on experience. There is nothing more frustrating than paying for equipment you can’t use effectively. So when it comes to evacuation charge and testing it is best to job this work out to an experienced ac technician who not only knows the trade but also knows your car. So we highly recommend that when it comes to evacuation, charge and testing, it is wiser to pay the tech $150.00 to do the job right.
Most air conditioning service pressure and vacuum gauges are “analog” needle gauges. Although electronic-digital gauges are available, most shops have needle gauges. Manufacturers of the ac service sets typically use one or two gauge makes. The accuracy of these gauges can be limited. For instance, when you are reading the vacuum gauge the increments or marks are typically increments of two, meaning between 20-30 there are only 3 ticks or slash marks, each representing a value of 2 between the 20 and 30 numerals. The needle on the gauge is usually as wide as one of the slash marks, so it can be difficult to tell if you are at 27″ or 28″ inches (inches of mercury) vacuum when the needle is on the slash mark just before the 30″ numeral. In order to vacuum down to the proper value or number you also need to know the ambient temperature surrounding the system as water boils off a certain temperatures and at certain vacuum readings. When a technician is working on systems that don’t give him problems it is easy to develop a trust for the equipment, however the first time he encounters a problem relative to vacuum moisture issues…… well its not fun.
We strongly recommend that small cans, 12-16oz typically, of refrigerant should NOT be used when charging any system. Though some hackers will argue this thought and brag about their successes, we find that with small cans you always risk the chance of allowing air to enter the system when you swap out the empty can for a new full can. And, though you may be charging by pressures and temperatures, it is very difficult to tell how much refrigerant you added to the system: was it a full can plus 1/2 can or 1/3 can? In the end you want to know exactly how many ounces or grams you have put in the system and the most accurate way to do that is with either a weighted charge scale or charged cylinder and a typical 30 lb container or virgin refrigerant.
Though initial charging, or getting a minimum quantity of refrigerant into the system, can be done with the pressure in the refrigerant can, getting enough refrigerant in the system afterwards requires either a heated charge cylinder or heated blanket. When you initially charge the first few ounces of refrigerant into a vacuumed system, the refrigerant in the canister is at a greater pressure than the vehicles system. After a few ounces of refrigerant the two sides equalize so you need to raise the pressure in the refrigerant canister by heating it.
Knowing how cold your system actually is is best checked using either an analog thermometer that has been checked against a standard or good reference, or a good quick responding digital thermometer or probe and DVM combination. If you don’t have a calibrated-certified-standard to check the accuracy of your analog (dial) type thermometer you can try comparing it to several other thermometers to see if it is in the average range. For instance when you are shopping in a store and you see a display of thermometers, check to see what the average temperature displayed is on the units. Though this method does not guarantee the accuracy of any particular unit it will give you a good idea of which one not to buy if it reads much differently than the others. The advantage of a quick responding digital thermometer is that they are typically more accurate than analog units and the advantage or a quick response is helpful when performing diagnostics.
After you have determined the optimal amount of refrigerant for your particular system, and you are satisfied with the system’s performance, you need to check for refrigerant leaks. There are two methods typically used however one alone is not best but rather a combination of the two is better. The first is the “dye” method where a fluorescent dye is added to the system and a UV or black light is used to check the typical points where leaks might occur (such as compressor seals or hose joints). Unfortunately the dye will only work its way through a leak that is large enough and under enough pressure, and sometimes that can take days or weeks to appear. The second method is the leak “sniffer” which uses an electronic device that can sniff out halogen gases. These units typically will emit a beeping noise or flash LEDS when they encounter a leak. By passing the detector around the typical leak points in a system you can verify whether you have a “detectable” leak. The only limitation with sniffers is their sensitivity or how many parts per million of refrigerant they are capable of detecting. Very slow leaks or minuscule leaks, such as those found in old fashion non barrier ac hose lines won’t show up with either liquid dye or sniffers.