Using
an electronic leak detector, you wouldn’t be able to pinpoint those
types of leaks, and they are more common than you may think.
First
of all, understand the normal operating pressures of the auto A/C system
range on the high side from 2 to 2-1/2 times the ambient air temperature
in °F. So working on a typical 80°F (27°C) day, you would
expect that high pressures would range between 160 psi (which is 2 times
ambient) and 200 psi (which is 2-1/2 times the ambient temperature).
Now,
those are just the normal operating pressures. With increased temperatures
and high relative humidity, the normal operating high side pressures could
range as high as 300 psi or higher and still be considered normal. Would
you still trust a leak test that is completed at 70 psi? Not likely.
The
most reliable method for leak testing the automotive A/C system is using
nitrogen. Besides leak detection, nitrogen also has other uses when working
on A/C systems, but what’s best about nitrogen is that you can control
the pressures at which you conduct your leak test.
Nitrogen
is readily available from your local welding suppliers as a bottled gas.
Nitrogen MUST be used with care and caution, because it is packaged under
extreme pressures as a bottled gas. You will require a
nitrogen regulator in order
to regulate and control the pressures of nitrogen that you put into the A/C
system. For best results, it is suggested that the nitrogen be connected
to the A/C system through a set of manifold pressure gauges.
Proper
nitrogen leak testing should be conducted at 200 psi. All automotive A/C
components have been proven to withstand those pressures, including evaporators.
With 200 psi of nitrogen, external leak test fluids can be used to help locate
the leak with surprising ‘pin point’ accuracy.
Leak
detection fluid manufactured for this type of leak test are thick enough
that they will not run off fittings and joints, yet thin enough that they
will show the slightest leak as very fine bubbles in the liquid.
The best leak detection
fluid we have found includes a UV dye marker. This fluid works backwards
to the way you think about UV dyes. Typically, you think of UV dyes as a
fluid to show the leak. In other words, when the UV dye leaks out of the
system, you use a black light or UV light to expose that dye. With the external
leak detection fluid, it works almost opposite. The fluid is UV reactive.
When a leak is encountered, the bubble displaces the UV dye and causes the
bubble to appear as ‘black’ in the UV fluid. The excellent contrast
makes external leak detection fluids far more accurate. The UV leak detection
fluid does not have to be used in conjunction with a UV light, but the results
are best with the light. An alternative to specially formulated fluids would
be to use soapy water, just like checking for a leak in a tire.
Nitrogen
is also an excellent way to test for evaporator leaks. Although not the quickest
leak detection method, it sure beats having to remove and reinstall an evaporator
just for test purposes. For a complete A/C system test, the system should
be pressurized to 200 psi with nitrogen. Allow the system to sit for at least
one hour per pound of refrigerant charge. In other words, a 2-1/2 pound A/C
system should be left standing with 200 psi of nitrogen for 2 -1/2 hours.
If there is no degradation in the pressures, the A/C system is free of leaks.
That’s a simple method that can be used to verify an evaporator leak
if you know the rest of the A/C system is leak free.
Nitrogen
can also be used for purging excess moisture from A/C systems and for flushing.
It’s ideal for flushing because of the increased pressures (typically
higher than compressed air) and you are not inducing any moisture into the
A/C system. Nitrogen is known to be a very dry and inert gas. What’s
best is that nitrogen will not cause any other problems with components nor
will it react with any oils or other contaminants that may be found in the
A/C system. Nitrogen is truly the best method for leak detection and other
functions when working on auto A/C system. |
This
was probably one of the best books on recharging air conditioning that I
have seen...
