The figure schematically illustrates an improved apparatus for generating a flow of gas with precise, stable humidity. Such flows are needed for calibration of dew-point, frost-point, and other humidity sensors.

Thermal Mass-Flow Controllers are used instead of differential-pressure mass-flow controllers, and are placed upstream of the saturator.

Like a prior humidity generator, this apparatus utilizes a divided-flow technique, in which an initially dry gas to be humidified is divided into two streams, one stream is humidified in a saturator, then the two streams are recombined to obtain a single stream with the required humidity. The flows in the prior humidity generator were regulated by use of differential-pressure flow controllers, which required manual adjustment, were highly unstable, and gave nonrepeatable flow rates. In addition, the proper operation of the prior humidity generator depended on constant attention by a technician to perform hygrometer calibrations.

The instability and imprecision of the prior humidity generator are eliminated in the improved apparatus by replacing the differential-pressure flow controllers with thermal mass-flow controllers that, in turn, can be controlled by a computer. In addition, the mass-flow controllers are placed upstream of the saturator, preventing contamination and increasing stability. These two innovations provide much greater precision in the flow rates used to produce desired levels of humidity. The improvements also reduce the labor time needed to perform a hygrometer calibration from 8 hours to about 2 hours.

This work was done by Miguel Cerezo of Caltech for NASA's Jet Propulsion Laboratory. No further documentation is available.