Part 3: Test methods for measurement of humidity

ISO 8573-3 introduces several test methods to determine the content of humidity in compressed air, describing the typical range of application and limitations of each method.

Applying one of the test methods of ISO 8573-3 is mandatory for the allocation of a purity class according to ISO 8573-1.


ISO 8573-3 doesn’t demand a specific method, but the proper selection of a suitable method for the intended measurement task as well as related measuring device according to the manufacturer’s declaration.

Beside a lot of useful information about humidity measurement, ISO 8573-3 provides a professional method for the calculation of the saturated water vapor pressure in compressed air in its Annex B.

ISO 8573-3 defines the reference condition for humidity statements in compressed air as

«7 bar(e) and 20°C, unless otherwise agreed».

In practice, humidity statements are typically referred to the actual working conditions (pressure, temperature) during the measurement period – without any further reference statement and thus actually non-compliant to ISO 8573-3.