What is "SWOM" weathering test?

A. The abbreviation WOM is often used in short reference to a Weather-O-Meter (now Weather-Ometer), a trademark of Atlas Material Testing Technology LLC. The original Weather-Ometers were based on enclosed carbon arc (ECA) technology which were a pair of carbon rods burning an electric arc in a Pyrex glass globe to provide an oxygen-deficient atmosphere to enhance UV output.

In the 1940's an improved carbon arc technology which provided a closer spectral match to outdoor terrestrial sunlight in the UV portions of the spectrum was developed. This used three pairs of rare-earth element doped copper-clad carbon rods, usually filtered by Corex-D flat-glass filters. This was referred to as the Open-Flame, or "Sunshine Carbon Arc" Weather-Ometer, sometimes abbreviated as SWOM. This is probably what your reference refers to.

In the 1950's the low-pressure long-arc xenon gas discharge lamp was developed by Heareaus Xenotest in Germany, now part of Atlas MTT. The xenon arc, appropriately filtered, provides the closest spectral match to outdoor terrestrial sunlight or sunlight filtered through window-glass for interior material testing. Because the carbon arc's were spectrally limited, and were not a close match to the daylight spectrum, they are not directly equivalent.

Most test methods today for most materials have moved away from carbon arc testing and use xenon arc technology.

A. You cannot simply run a certain number of hours more in a Sunshine Carbon Arc instrument and claim an equal test. For in-house purposes you may be able to over-expose in the Xenon Arc instrument for a "go/no-go," but ultimately to qualify the material you must test to the standard requested. Once you test in both instruments a number of times you can, for your own qualifications, establish that multiplier for a particular material, testing for a particular property loss, under specific test conditions and use it with some confidence. You shouldn't, however, use that multiplier on other materials testing for other properties.

Sunshine Carbon Arc instruments are expensive to run. They match the solar spectrum fairly well in the shorter wavelengths, but have a rather large mountain of longer UV and near blue to the tune of two to five times as much as natural sunlight.

This makes for an odd test, but when claiming you are meeting the standard with your materials you need to test in the instrument specified to the standard.