Pure water is used mainly to simulate the rain effects on outdoors materials and should therefore not contain contaminants absent from rain water. The water is run through the spray nozzles and sprayed on samples, as well as run through the vibrasonic humidification nozzle.
Consistent and valid test results cannot be assured when contaminants enter the weathering test chamber.
Several water contaminants can impact weathering tests:
The effects of a coating or film caused by any of the contaminants listed above are as follows:
A coating on the black panel in the chamber can decrease its heat absorption, resulting in inaccurate temperature readings for the samples; therefore, the test will be run at the false temperature.
Coatings on the chamber light sensor will cause it to incorrectly read the intensity of lamp. This will cause it to automatically boost power to the lamp to compensate for its lower output (sensed inaccurately). The result is that the test may be run at too high a light intensity, potentially changing the results, and definitely shortening the life of the lamp.
Contaminants may also cause plugging of the spray and humidification nozzles, which will influence the reliability of the test and result in the need to clean or replace the nozzles.
The sample itself is sensitive to the water quality. Using water with high loads of contaminants may result in deposits, spots, rusting at the surface of the sample.
Water Quality Parameters
Organics Organics should be removed to avoid a quick spoilage of the weathering test chambers, requiring more frequent maintenance. Organics may also form a thin film on the surface of the sample. A target level of 50 ppb (µg/L) is recommended.
Silica Silica is one of the most common contaminant that may lead to issues in weathering test chambers. Silica is often the cause of the white powder seen in contamination problems.
Silica forms a film (glaze) almost impossible to remove and which causes the following problems:
It makes samples unsuitable for testing color and gloss
It reduces the reflection inside the chamber
It precipitates on the lamp, which decreases output
It provokes deposit on the radiometer, which decreases sensitivity
In combination with some trace metals, silica can create iridescent glaze.
Metals Calcium, magnesium and iron are common contaminants that affect the temperatures, humidity, irradiance and running costs of weatherometers. In combination with carbonate present in the water, ions such as calcium, barium, and strontium can significantly increase the hardness of the water and generate scaling in the weatherometer and on the samples.
Particles, Bacteria Particles and colloids are obvious contaminants for these tests: the presence of particles in the water would generate aggregates and hard deposits on the samples.
How to Reach the Adequate Water Purity
A combination of technologies is usually required to remove the different contaminants causing the issues listed above.
Colloidal silica is efficiently removed by reverse osmosis cartridges. The reactive silica, form that creates deposits, is moderately removed by reverse osmosis. The addition of technologies such as electrodeionization and ion exchange resins allows reaching levels of silica at the low µg/L (ppb) concentration. The combination of those three purification technologies is recommended to maintain a low level of silica over time.
Those technologies also reduce the level of organics to a TOC < 50 ppb. The particles are removed by the reverse osmosis cartridge as well as by the depth filter typically installed upstream the reverse osmosis cartridge. Ions are removed to a low level by all three technologies mentioned previously: reverse osmosis removes the bulk of these ions, and the ion exchange processes (electrodeionization and / or ion exchange resins) complete the removal of the ion to trace level.