We fond the answer to that question in an article which first appeared in
the May/August 2001 edition of Cool Profit$
Magazine. This article was written by John Noble, and has been reprinted
here with permission from Cool Profits Magazine.
Understanding the Basics
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
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 has the following qualities: (a) inert, 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
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 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.
There is currently no equally effective (in all situations)
substitute for the combination leak test method described above. It has neither
the problems nor shortcomings of dye. I have heard the argument that nitrogen is
dangerous. Well, the stationary industry has been using it for at least 60
years, and my company has been using it since 1973. Some of the major stationary
manufacturers think that is important enough to dedicate a section of their
training manuals exclusively to its use for leak testing their a/c systems. It
is not unsafe, unless one uses it improperly. Mobile a/c technicians are simply
untrained in its use. What a shame, it is so simple and effective and easier,
safer, and cheaper, etc.
Once upon a time, folks were afraid to fly in an airplane that didn’t have a
propeller—it was thought to be too dangerous. I encourage any a/c technician to
get a good quality nitrogen regulator and bottle of nitrogen from a welding
supply store. Set the regulator to 200 psi. It is safe to use with all
conventional a/c systems, and will not damage any system components.
Make sure that the nitrogen bottle is secured to a wall or in a dolly, and
that everyone in the service department is taught not to fool with the regulator
setting. Or, install an Allen head set screw and lock nut with the regulator so
that it can’t be adjusted once it is pre-set. We have safely tested a/c system
components at 200-psi nitrogen pressures on over 40,000 tests. Remember, 200 psi
is only about 50 psi higher than the compressed air lines used by you and every
other technician nation wide. I encourage all technicians who aspire to become
more professional in a/c service to invest in a good quality nitrogen regulator
and bottle. Then take the time to practice the method. You will be doing a
service to yourself and your customers. $$$
About the Author: Before opening an automobile repair business, Cool
Flow, Inc. founder John Noble spent 7 years in the aircraft service industry,
and prior to 1971 worked as an FAA certified airframe & power plant
technician. During the next 4 years he began to make the transition to
specializing in automobile a/c related services. Today, the company is currently
undergoing another transition into development of several a/c related products.