Abstract: An embodiment of the present invention is a method for creating and maintaining a partial vapor pressure in a sample chamber of a sorption analyzer. A first dry gas flow is purged from a first mass flow controller through a saturator to produce a near-saturated gas flow. The near-saturated gas flow is mixed with a second dry gas flow from a second mass flow controller to produce a mixed gas flow with a predetermined partial vapor pressure. The mixed gas flow is directed into the sample chamber. If the lowest possible vapor pressure is required in the sample chamber, the first dry gas flow is directed around the saturator to the sample chamber and vapor migrating from the saturator is vented to the atmosphere.