Guide To Avoiding Glass Failure
Simplified Frost Point Test
IG seal failure is one of the major problems that this guide intends
to help the film applicator to avoid. As noted previously, there is no technical
information available that establishes a causal link between the application of window
film and the failure of the IG seal. It is widely accepted that when properly researched
and tested the addition of coatings or tinting to ordinary glass does not create a seal
failure /problem for IG units. It can be inferred that the effects of window film would be
similar to the effects of coatings and tintings in glass. This notwithstanding, there is
sometimes a perceived link between the application of window film and the observation of
the signs of seal failure. This link could very well be nothing more than the effects of
added attention on the IG units as a result of the application of the window film.
The first thing that must be realized about IG units is that
all IG unit seals allow water vapor to migrate into the gas space at some rate.
Water
vapor can begin to migrate through the IG seal as soon as the IG unit is manufactured. The
migration of water vapor through the IG seal is not the result of abuse or failure of the
IG seal, but rather, it is simply the nature of the product. Quality IG units are
manufactured with quality materials and have multiple seals that greatly retard the
migration of water vapor through the seal and thus extend the useful life of the IG unit.
The water vapor migration through the IG seal continues until the IG desiccant becomes
saturated. At this point the excess water vapor that migrates through the IG seal no
longer is adsorbed. It is then only a matter of time until the build-up of water vapor in
the IG unit reaches a point where condensation becomes apparent on the inside surfaces of
the IG unit. The overall thermal performance of the IG unit is minimally affected by the
build-up of water inside the gas space of the IG unit unless the gas space was filled with
an inert gas such as argon or krypton. However, the IG unit is generally considered to
have failed from an aesthetic standpoint.
The best thing that a film installer can do with respect to
the IG seal failure problem is to be sure to properly inform the consumer of the inherent
character of IG units, and to avoid application of window film to IG units which are in a
state of imminent failure.
The best indicator of the condition of an IG unit is a measurement
of the frost point of the IG unit. The frost point of an IG unit is determined through the
application of a fairly straightforward test whereby a very cold metallic surface is
placed into contact with the exterior surface of one of the glass plates that is
incorporated in the IG unit. This contact is maintained as heat flows from the glass to
the metal surface. Then, the temperature at which water vapor contained within the IG gas
space condenses and freezes to the inside surface of the glass plate is recorded.
This
temperature is referred to as the frost point of the IG unit. The lower the frost point,
the lower will be the content of moisture or moisture vapor within the IG unit gas space.
A precise determination of the frost point of an IG unit is a very complicated and
detailed procedure which involves the use of highly precise and expensive equipment, and
detailed test procedures. The administration of a controlled frost point test for every IG
situation where a film application is under consideration would be too time consuming and
costly for the average film installer. Therefore, a simplified frost point test was
devised to identify those IG units which are in a state of imminent failure.
To perform the simplified frost point test requires the use of
an IWFA frost point instrument. The IWFA frost point instrument consists of a device which
is fabricated of rigid copper tubing. The end of the tubing that is to be placed in
contact with the IG unit is plugged and the other end is fitted with a vented removable
cap. The entire assembly is fitted with an exterior wrapping of insulation so that the
energy gain through the sides of the pipe is minimal.
To use the IWFA frost point instrument it is necessary to
remove the cap from the copper pipe and fill the pipe with several chips of dry ice and
ethyl alcohol. Then the removable pipe cap should be replaced. During this operation, the
operator should be careful to avoid direct contact with the copper pipe or the dry ice.
Dry ice is solid carbon dioxide which vaporizes instead of melting. It is a convenient
clean refrigerant to obtain subzero temperatures when used in an alcohol bath.
Gloves or
hand protection should be used to avoid any contact with the copper pipe, cold alcohol, or
dry ice. Dry ice reaches a temperature of -109.1° F (-78.5° C).
The sealed end of the IWFA frost point tester should be held in
contact with the exterior surface of the IG unit for a period of 2 minutes for 1/8 (3.0
mm) or thinner glass and 4 minutes for 1/4 in. (6.0 mm) or thicker glass.
At the end of
this time, the frost point tester should be removed and the contact surface of the IG unit
should be quickly wiped with a cloth or tissue dampened with alcohol. Any accumulation of
ice, fog, or water over 1/2 in. in diameter on the gas space surface of the glass will
indicate a leak in the seal of the IG unit. Film should not be applied to this window
unless the owner of the building issues a waiver of responsibility for the use of window
film. If there is no observable buildup of ice, fog, or water on the inside surface of the
glass, it does not mean that the IG unit will never fail. As stated earlier, all IG units
will eventually fail as the result of excess moisture in the gas space.
It does mean,
however, that the unit is not in immediate danger of failure.
Guide
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