It is usually the condensing unit where a leak test begins. The reason is that at this unit, a leak is the easiest to find and repair. The most important tool the technician has is his eyes. Look very carefully for signs of oil. Small amounts of oil attract dirt which make it very visible. Care should be exercised when handling used oil. In certain cases when a compressor has had a burn out, remember that acid is formed and carried by the oil. Heavy concentrations of acid can damage the skin. In addition to your regular hand tools, a few extra specialty items are needed for leak testing.
A bottle of nitrogen will be essential in some leak testing situations. Remember the nitrogen regulator; never use this gas without the proper regulator. Electronic leak detectors are good in isolating an area of the leak. In certain cases where the concentration of refrigerant fumes is very high, the electronic leak detector may be over sensitive to the condition and thus unable to pin-point the leak. A halide leak detector is a must. The concentration of the leak will be indicated by the color of the flame. Difficulties in leak testing with the halide might be encountered on windy days. The wind might carry away the refrigerant fumes faster than the halide leak detector can pick it up. For this reason I recommend you have a six-foot by six-foot piece of construction plastic to use as a tent. On windy days the plastic is used to cover the entire condensing unit. With the addition of refrigerant pressure to the unit, one corner of the plastic is lifted and the hose of the halide leak detector is placed under the plastic. The hose sniffs for the refrigerant. A concentration of refrigerant fumes should occur now that the wind cannot dissipate the leaking refrigerant.
One of the biggest mistakes a technician makes when using the halide leak detector is to use too large a flame in the chimney. It is only necessary for the tip of the flame to touch the underside of the detector disc.
At times, leaks develop and only show at high pressure. This is one of the reasons that nitrogen is used. On the data plate of most air conditioning condensing units, the test pressure is marked. This pressure is applied to the unit in the factory to make sure there aren’t any leaks, defective joints, or materials. When applying nitrogen pressure, don’t exceed these limits. The operating pressure of the unit is lower than those ratings on the data plate. Nitrogen should be added to the system up to the pressure at which the refrigerant of that system would operate. With the addition of the nitrogen pressure, the high pressure leaks can be detected without having the unit operate. Another thing to remember is the pressure of the unit on a really hot day would cause the units pressure to operate at a higher pressure. You might be looking for a leak on a cool day, and the pressure would not be high enough to detect the leak.
A product called Leak Detector is on the market. It is similar to the bubble solution a child makes bubbles with. If you do not have this solution available to you, ordinary liquid soap detergent used in the home will suffice. When the halide or electronic leak detector localize the refrigerant leak, the bubbles of the solution will form at the exact location of a leak.