An ultraviolet hygrometer has been developed that uses an optical technique to provide fast response, stable calibration, and immunity to airborne contaminants. It is a two-wavelength two-path instrument that relies on the differential absorption by water vapor of two wavelengths in the vacuum ultraviolet. The differential absorption technique employs a reference path and a second wavelength to address the problems of detector drift, changes in lamp output, and window contamination.

The measurement process involves transmitting two narrow-spectrum light beams of different wavelengths, one highly and one slightly absorbed by H2O, one at a time through an air sampling chamber and along a reference path. Two wavelengths are used: 121.6 nm (H), highly absorbed by water vapor, and 123.6 nm (Kr) for reference. Both wavelengths are split to produce two pairs of light beams. One beam is transmitted through the air sampling chamber; the other is transmitted along the reference path to measure the strength of each source, in turn. These four measurements plus two others of detector dark current allow six different values to be analyzed in determining water vapor concentration.

By measuring the differential and received intensities of the two sets of light beams, the level of contaminants in the air sampling chamber for both of the wavelengths can be determined, since the second wavelength is subject primarily to contamination.

Power consumption is <250 W at 115 VAC, and dew point range (for half sensitivity) -23 °C to 11 °C for 5-mm sample path and -11 °C to 27 °C for 2-mm sample path.

The UV hygrometer was specifically developed to provide National Center for Atmospheric Research (NCAR) scientists with the precision not found in commercially available humidity measurement devices. A working model of the hygrometer is currently in use at NCAR. Two more instruments were used by the National Oceanic and Atmospheric Administration in a coupled ocean-atmosphere research project in the western equatorial Pacific.

Among the instrument's features is fast response time of 0.1 second and accuracy of ±0.05 g-kg¯1 random. Output in fundamental units of mixing ratio (gH2O per kg dry air) is easily convertible with software to other units such as relative humidity, dew point, or vapor density.

While the UV hygrometer was designed for airborne use, applications extend to industries seeking similar exacting measurement, such as meteorological research, process control, and clean rooms.

This work was done at the University Corporation for Atmospheric Research Foundation. For more information about licensing, contact the Technology Commercialization Program, UCAR, PO Box 3000, Boulder, CO 80307; (303) 497-8580; fax: (303) 497-8561.

Photonics Tech Briefs Magazine

This article first appeared in the April, 1998 issue of Photonics Tech Briefs Magazine.

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