System Residues hide leaks. Sludge and residues coat the interior of system components and temporarily seal corrosion pits, fissures, seams, seals, o'rings, and other small leak points. Some of those residues include refrigeration oils, acids, desiccant, pulverized metal, Teflon piston ring material, brazing fluxes, dye particles, etc.
Overcoming Residue Surface Tension: Leaks are harder to find because leak testing is performed with the system turned off. Lost are the benefits of an operating system: a) constant washing of interior surfaces of components; b) higher operating pressures that encourage leaks. With a system at rest, the undisturbed residues mentioned above are able to coat the insides of the evaporator, condenser, compressor, and other components. When Residue Surface Tension is greater than the interior pressures' ability to displace it, there is no leak occurrence, thus no leak detection.
Residue Displacement: If surface tension is the culprit, then how do we overcome it? Answer: By adding 4-ounces of chlorine-based refrigerant R-22 to the system. R-22 disturbs the surface tension. We follow that by pressurizing the system with 175-200 psi of nitrogen. Both are cheap.
At the higher system pressure, the R-22 overcomes the residue surface tension and forces the leak path to reopen. Now, enough R-22 gas is available so that leaks are detected easily. The R-22 is the residue-displacing agent.
R-22 summary. R-22: (a) creates a leak path because of its oil solubility and residue displacement potential; (b) is more readily detectible by electronic leak detectors than HFCs; (c) is non visual-dependent, unlike dyes, thus can be used effectively to locate leaks in enclosed areas; (d) will not cross-contaminate nor cause any harmful consequences to CFC, HFC, nor blend refrigerant systems (i.e., after leak testing an HFC (R-134a) system using R-22, there is typically zero percent cross-contamination if evacuated afterwards).
Nitrogen summary. Nitrogen Gas has the following qualities: (a) inert gas , very dry and non-flammable; (b) does not go into solution with refrigeration oil to create non condensable pressure problems (i.e., after leak testing an HFC (R-134a) system using R-22 and nitrogen, there is typically zero percent non condensables (nitrogen or air), if evacuated afterwards).
Diluted Liquid Soap. Speed is important to technicians when performing a leak test. Large and medium size leaks can be quickly located in exposed areas such as under hood components by applying diluted liquid dishwashing soap directly to suspected leak points.
Multiple Leaks. If a large leak is discovered repair it and perform a follow-up leak test. Leak tests should be performed following each repair until all leak points are found and fixed.
Pinpoint versus Area Testing Pinpoint Accuracy. Technicians need to know exactly where leaks exist. Consequently, leak testing needs to be pinpoint accurate, regardless whether by visual or non-visual means. With dyes, the detection is strictly visual and general area, not pinpoint. Dyes do not provide the rapid and finite definition of the Combination Method [non-visual: electronic leak detector detecting the nitrogen-pressured R-22, and, visual: diluted soap solution bubbling].
Electronic Leak Detectors. Heated diode leak detectors are currently the best non-visual, dependent leak detection instrument for sensing a gaseous leak. They have the necessary characteristics of sensitivity, repeatability and recoverability (after a leak), which are so important.
Releasing Test Mix. The EPA approved the releasing of the “test mix” (R-22 & nitrogen) with the stipulation that: “All existing refrigerant within the system be recovered properly, and a 102mm (about 4”) mercury vacuum drawn on the system.”
By their definition, the R22-nitrogen test mix used for leak testing is not considered a refrigerant and therefore may be released to atmosphere. Their rationale is that one tiny loss which results in the discovery and repair of a leak reduces the greater loss over the life of a system. It's better than multiple recharges of refrigerant and multiple losses to atmosphere.
Note: We've learned that the R22- Nitrogen Gas test mix should be released outside the building via copper (or other) tubing to prevent fouling the air inside the shop. This prevents false alarms by the leak detector. Also, always maintain a well-ventilated work area.