Nafion™ functions essentially as a highly selective, semi-permeable membrane to water vapor. If gases inside Nafion™ tubing are wetter than gases surrounding the tubing, drying will occur. If the surrounding gases are wetter, humidification will occur.
In the simplest case, a strand of Nafion™ is suspended in ambient air. If the sample stream inside is much wetter than ambient air (such as breath samples), the sample falls to ambient humidity. If the sample stream inside is much drier than ambient air (such as calibration cylinder gases), the sample rises to ambient humidity.
To dry the sample to lower humidity, the surrounding air must be dried. For simple, portable applications, the Nafion™ tubing is packed in desiccant. The desiccant provides a very dry surrounding purge environment, while the Nafion™ tubing provides selectivity in drying. The desiccant gradually saturates with water and periodically must be regenerated or replaced.
For continuous operation, one or more strands of Nafion™ tubing are suspended within a housing that is purged with a dry gas. For humidification, the housing is filled with water to create a highly humid purge environment.