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:

1. This does not indicate a uniformity requirement. When uniformity requirements do exist, they are much wider than operational fluctuation requirements.
2. 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.