The bottom of the test chamber is covered with demineralized water and heated up to 40°C. The water evaporates resulting in 100% condensing (or high) humidity inside the chamber. The thermal losses through the chamber walls produce temperature gradient and consequently the dew point is reached in the vicinity of specimens and the vapor starts condensing on the surface of the test samples.
In addition to the high humidity (also known as Constant Humidity "CH") the DIN EN ISO 6270-2 defines two more variants:
- Alternating Temperature (AT): 8 h high humidity, 16 h cooling at room temperature
- Alternating Humidity and Temperature (AHT): 8 h high humidity, 16 h aeration with room air
The Water Condensation (High Humidity) test can be optionally extended with the Kesternich tests. This option introduces the prescribed volume of the SO2 gas into the test chamber during the High Humidity phase. The Kesternich test gains on popularity due to the fact that the atmosphere in many industrialized countries is increasingly poluted with this gas.