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.

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