Air Flow Inside a Weather-Ometer
Standard ASTM G155 (2021) has been recently revised, and the revision has just been published. There are some significant technical changes and improvements made in this revision. Two major improvements are the inclusion of different category
daylight filters and the inclusion of chamber air temperature (CAT) settings for the most common exposure conditions, which previously were not included in the standard, since its first publication in 2000. Some other standards such as SAE J2527 and ISO 4892-2 already specified settings for the chamber air temperature for many years.
Why is the Chamber Air Temperature so Important?
In a recent study performed by Atlas, 2.5 mm thick HDPE specimens in different colors were exposed in a Ci4000 Weather-OmeterTM without specimen backing. The test parameters were kept constant according to ASTM G155 Cycle 1 except for chamber air temperature, which was varied between 35 °C and 50 °C during the exposure. The specimen surface temperatures were measured using
S³T. Even though the black panel temperature (BPT) was kept constant at 63 °C, the specimen surface temperatures increased with increasing CAT. The differences observed at the different CAT settings for the green specimen (darker, closer to BPT) is about 5 K, while the differences for the lighter colors (closer to CAT) were higher than 10 K.
Correlation between specimen surface temperature and chamber air temperature (CAT).
Even though the black panel (BPT) or the black standard temperatures (BST) are controlled in xenon-arc weathering equipment, significant variability in specimen surface temperatures are observed by not setting and controlling the chamber air temperature. Since specimen temperatures affect reaction rates, a significant influence of CAT on specimen degradation can be expected.
By having a constant chamber air temperature setpoint, the specimen temperature ranges in each instrument used for testing will be the same, between the lower limit (CAT) and the BST or BPT. Even though some minor variability between instruments is still expected, repeatability and reproducibility of testing results will be improved by controlling CAT. This is an obvious and significant improvement for weathering testing in general, but especially for light colored or transparent test specimens.
How was this Addressed in the Past?
Application based standards referring to ASTM G155 Cycle 1 often, but not always, addressed this limitation by defining a chamber air temperature setpoint. For example, ASTM D750 (rubber), ASTM D6754, ASTM D4434 (both sheet roofing), and ASTM D6551 (pressure sensitive tapes) specify 44 °C CAT, ASTM D4956 (retroreflective sheeting) specifies 38 °C CAT, and ASTM D2565 (plastics) specifies 47 °C CAT. Others, such as ASTM D4355 (geotextiles) or ASTM C1900 (safety glass) does not include a setpoint of the chamber air temperature. So, to summarize, since no recommendations for CAT were given for most cycles in in ASTM G155, inconsistent settings for chamber air temperature are used in industry.
Reducing Test Condition Variability
Recommending chamber air temperature setpoints in ASTM G155 will not only improve reproducibility of test results using the recommended settings in the standard, but also, future revisions of the other standards, such as those cited above, might follow with more harmonized chamber air temperature settings.
More Information
You can find more information on surface temperature and its influence in weathering testing
here.
Contactless measurement of sample surface temperature in a Weather-Ometer during a running test is discussed
here.
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