Austest Labs currently performs solar testing in 4 broad spectrum /solar test chambers located in our Adelaide, Sydney and Melbourne labs (separate to our 3 x UVA chambers) and while we use industry best chambers, these are limited to test items 700mm W x 450mm D x 200mm H. What happens when a test item is larger than this? Can we just test a portion of the product or do we need to test the whole item and if the latter, how do we do that?
Before we answer that question, we need to consider whether as a client, you’re interested in only looking at photo-degradation effects or whether the damage from thermal effects also needs to be considered.
Photo-degradation involves the energy contained in photons of light triggering chemical reactions in the material that they fall on – causing the material to alter. Polymer-based materials are especially susceptible to photo-degradation.
Thermal effects are caused by items heating up due to absorbing part of the light energy falling on it. It is important to understand that solar heating is different to heating caused by high air temperature. High air temperature, which is simulated by tests in climatic chambers – causes uniform heating of a test item once thermal stability has been reached. In contrast solar heating biases heating towards the upper parts of items most exposed to sunlight, it therefore causes thermal gradients within an item.
In addition to the usual failure modes that may be caused by high temperature, heating from solar light can trigger additional failure modes related to thermal gradients and the corresponding different levels of expansion it causes within an item. Such thermal gradients can cause mating parts to either bind or become loose, bearings and shafts to distort, unwanted interference between moving parts, electrical contacts to become closed when they should be open and vice versa, the premature or delayed actuation of electrical contacts, gaskets and seals to fail, solder joints or adhesives holding together parts to become stressed and differential air/gas pressure between compartments of an item.
If we are interested only in photo-degradation effects and if the test item has only one or a small number of portions of the product that we are concerned about photo-degradation effects, then if we want to test an item that cannot fit in a solar chamber we can take a sample/s of the product material and test this sample/s in the solar chamber. However, even this method is highly approximate. This is because photo-degradation effects are dependent on test item temperature. This test item surface temperature will likely be quite different when a small sample is tested due to lower thermal mass/capacity of the sample compared to the full size product and due to different relative levels of exposed surface area leading to different levels of heat transfer to the surroundings by radiation and conduction.
If we are testing for potential damage from thermal effects, testing a small swatch size sample simply won’t cut it. As explained above, the surface temperatures and hence internal temperatures and temperature gradients will likely be quite different when the full-size product is tested compared to a sample size. If we need more accurate tests for photo-degradation effects, testing smaller-size samples won’t do either. So what is to be done then?
Austest engineers have solved the problem by modifying existing equipment to develop a setup that tests much larger products than can fit in commercially available solar testing machines. A series of tests performed for customers over the last year has shown this method can be used for products with dimensions up to 850mm x 550mm x 450mm. Using the same method, Austest Labs has developed set-ups for solar testing of products with even larger dimensions – traffic lights for example- should customers require.
Note the method we use is far more controlled than the rough methods that other testing houses are known to use to “perform” solar tests on larger-scale items. In those methods, commercially available lamps are set-up and then adjusted open loop to produce a particular, radiation intensity. This method has three serious flaws. Firstly, the off-the-shelf lamps often used are typically not rated to guarantee the solar spectrum required by most solar testing standards like ISO4892-2. Secondly, since lamp intensity varies greatly with age, open-loop control cannot ensure that the radiation intensity meets requirements throughout a test. Thirdly, the spatial distribution of radiation is often not accounted for in these set-ups. In short, these test methods cannot be said to meet the requirements of solar testing standards.
In contrast the method that Austest engineers have developed for solar testing of larger items ensures closed loop control of radiation intensity to ensure that the test requirements are met throughout a test. Furthermore, through differential, closed loop control of the different solar lamps we ensure that the level of radiation uniformity required by typical test standards is met. And through the use of rated xenon-arc lamps with specific filters we ensure that the radiation spectrum required by test standards is also met. Therefore we are able to produce certification of larger size products to solar testing standards such as ISO4892-2, Test CL2 of DEF STAN 00-35 and Method 505 of MIL-STD-810.
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