Problem with Method #1
As the nitrogen is added to the system the pressure
increases above the liquefaction point of the
refrigerant. The refrigerant gas turns to liquid,
separates from the nitrogen and flows to the low
points in the tubing. Since electronic refrigerant gas
detectors are not sensitive to nitrogen, leaks which
are located at the high points (above the level of the
liquified refrigerant) cannot be detected.
Solution for Method #1
Elevating the pressure in a system above that which
would be obtained with refrigerant alone can be
helpful in locating leaks. However, the only reliable
way to identify leaks in a system which is “back-
charged” with nitrogen is to use a soapy bubble
solution or electronic acoustic detector. If you
intend to use only an electronic gas detector you must be careful not to raise the internal
pressure above the condensing point of the gas and temperature with which you are working.
Method #2
This method often used in conjunction with method #1 (described above) but may also be used
with refrigerant alone. The idea here is to attach a gauge set to the system and add refrigerant,
nitrogen (or both) to the system to raise the internal pressure. A note is taken of what that
pressure is. The system is left for a period of time (possibly overnight) and the gauges are then
checked to see if the pressure has dropped.
Problem with Method #2
The fact is, that the pressure indicated on the gauges will generally change somewhat whether
or not the tubing connections are leaking. Most technicians know this, so they end up making a
“best guess” as to whether the amount of the change is severe enough to indicate a problem.
Where is the gas going? Indeed, the gas may be lost through a leak in a joint. However, it is just
a likely to be lost through a leak in a charge hose (all charge hoses have slow leaks) or a
Rotalock service valve (Rotalock valves do not completely seal unless they are fully back-
seated). Neither of these leaks would be a problem once the change hoses are removed and
the Rotalock valves are fully back-seated for normal operation. In some case the pressure will
actually rise. This is due to a change (increase) in the ambient temperature. If the system has
been previously run or the vales are open to the compressor, the “back-charge” gas may be
slowly absorbed by the compressor oil itself. This is analogous to the carbon dioxide gas which
is absorbed into the liquid in a can of soda. As the gas is absorbed, the pressure in the system
falls. How much gas will be absorbed and how long it will take varies depending on the type and
quantity of compressor oil, system pressure and temperature.
Note: A variation on the gas/oil absorption problem can occur when the technician tries to
evacuate the system. As the pressure is reduced (by the vacuum pump) the gas which has been
absorbed by the oil begins to boil off (just at the carbon dioxide comes out of your soda when
you open the top). This greatly extends the time it takes pull the system pressure down to an
acceptable level and gives the technician the impression that he may be pulling air into the
system through a leak.
Page 39
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