Results obtained from this practice can be used to compare the relative durability of materials subjected to the specific test cycle used. No accelerated exposure test can be specified as a total simulation of natural or field exposures. Results obtained from this practice can be considered as representative of natural or field exposures only when the degree of comparative performance has been established for the specific materials being tested.

The relative durability of materials in natural or field exposure can be very different depending on the location of the exposure because of differences in UV radiation, time of wetness, temperature, pollutants, and other factors. Therefore, even if results from a specific accelerated test condition are found to be useful for comparing the relative durability of materials exposed in a particular exterior location, it cannot be assumed that they will be useful for determining relative durability for a different location.

The use of a single acceleration factor relating the rate of degradation in this accelerated exposure to the rate of degradation in a conventional exterior exposure is not recommended because the acceleration factor varies with the type and formulation of the material. Each material and formulation may respond differently to the increased level of irradiance and differences in temperature and humidity. Thus an acceleration factor determined for one material may not be applicable to other materials. Because of variability in test results under both accelerated and conventional exterior exposures results from a sufficient number of tests must be obtained to determine an acceleration factor for a material. Further, the acceleration factor is applicable to only one exterior exposure location because results from conventional exterior exposures can vary due to seasonal or annual differences in important climatic factors.

Variations in results may be expected when operating conditions vary within the limits of this practice. For example, there can be large differences in the amount of degradation in a single material between separate, although supposedly identical, exposures carried out for the same duration or number of exposure cycles. This practice is best used to compare the relative performance of materials tested at the same time in the same fresnel reflector device. Because of possible variability between the same type of exposure device and variability in irradiance, temperature and moisture levels at different times, comparing the amount of degradation in materials exposed for the same duration or radiant energy at separate times is not recommended.

This practice should not be used to establish a "pass/fail" approval of materials after a specific period of exposure unless performance comparisons are made relative to a control material exposed simultaneously, or the variability in the test is defined so that statistically significant pass/fail judgements can be made. It is strongly recommended that at least one control test specimen be exposed with each test. The control test specimen should meet the requirements of Terminology G113, and be chosen so that its failure mode is the same as that of the test specimen. It is preferable to use two control test specimens, one with relatively good durability and one with relatively poor durability.

The use of at least two replicates of each control test specimen and each material being evaluated is recommended. Consult Guide G169 for performing statistical analysis.