Weathering and corrosion test standards usually specify chamber set point conditions with a plus/minus tolerance. For example, ISO 4892-2 contains the following test parameter:
Black Standard Temperature: 65 ± 3°C
What this and other tolerances actually mean is that the tester’s irradiance, temperature, or humidity control system must be capable of maintaining stable conditions at a specified set point, which ASTM refers to as Operational Fluctuation. Typically, each test parameter is measured and controlled by a single sensor at a fixed location in the test chamber. The above example from ISO 4892-2, once chamber conditions have stabilized, the temperature of this single control set point should not rise to more than 68°C or fall below 62°C at any time.
Unfortunately, these tolerances are frequently misinterpreted. There are two important things to keep in mind that will help you from misinterpreting the intent of one of these setpoints:
- This does not indicate a uniformity requirement. When uniformity requirements do exist, they are much wider than operational fluctuation requirements.
- This does not allow the user to program the machine at any temperature to any value within the tolerance range. The tester should be programmed at the value stated.
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