Humidification of Generated Vapor

As mentioned earlier, detection devices should be tested under diverse environmental conditions. Therefore, the basic generator system was modified to include humidification of the vapor stream. Since moisture will degrade the source substance, the carrier stream passing over the substance must be kept at zero moisture content to prevent sample degradation. Thus, only dilution air streams can be humidified. Fortunately, the amount of carrier flow through the source is much less than the amount used for subsequent dilution, except for very low volatility compounds. That permits generation of a vapor stream with controlled RH. 

To ensure that the air is saturated when it passes through the water reservoir kept at the same temperature as the environment, the flow is maintained at the minimum level to provide adequate volume (e.g., 3 1/min is adequate for most detectors). Saturation efficiency needs to be considered when higher flows are used, however. By keeping the humidifier water at the same temperature as the environment, the relationship will hold. The air passing through the humidifier can only become saturated at the same temperature and thus, when it is mixed with the dry air, the percentage of RH is directly proportional to the ratio of wet and dry flows. This technique minimizes the need to manipulate flow adjustments to control RH if and when a heated water source is employed. It also eliminates condensation problems associated with heated water or steam injection techniques. 

---