Southwest Sciences Inc HAIPER VCSEL Hygrometer
Southwest Sciences hygrometer installed on NSF's Gulfstream V research aircraft. VCSEL hygrometer, shown mounted on the NSF/NCAR GV aircraft, offers high sensitivity and fast response times for water vapor throughout the troposphere.

Update January 2009: Southwest Sciences VCSEL hygrometer is now flying on the HIPPO mission. Live updates of the instrument status are available at Software at the UCAR web site allows plotting of the atmospheric humidity and other variables in Google Earth.

Southwest Sciences has developed an instrument capable of measuring the entire range of tropospheric and lower stratospheric water vapor. This capability was developed for atomspheric research on the NSF/NCAR Gulfstream-V aircraft (previously called the High-performance Instrumented Airborne Platform for Environmental Research or HIAPER) Gulfstream-V aircraft, also known as the GV. The concentration of water is determined from diode-laser-based infrared spectroscopy in the free air stream, resulting in high sensitivity and fast time response. The hygrometer has been installed and is undergoing flight tests on the aircraft. Specifications for the instrument include:

Frost/Dew Point range: -110°C to +30°C
Sensitivity (S/N=1, 1 Hz): 0.05 ppmv
Frequency: 25 Hz
Accuracy: 5%
Precision: 3%
Power: 10 W of 110 VAC
Mass, instrument: 2 kg
Mass, mounted, total: 5 kg
Interior size: 7 cm x 14 cm x 24 cm (H x W x L)
Operation: unattended

The instrument uses an open-path, external optical cell with penny-sized mirrors separated by 15 cm. The infrared light passes 25 times between the mirrors for an overall pathlengh of 3.7 m. No rack space is required - all electronics lie within the aperture plate liner. Key innovations include of the instrument include:

1. Vertical cavity surface emitting laser (VCSEL): VCSELs have a wider tuning range than normal distributed feedback lasers, and their wide tuning range enables the laser to probe several different strength water vapor absorption lines within a single scan. The entire range of tropospheric and lower stratospheric water vapor can be measured with one laser and one pathlength.

2. Digital signal processor (DSP): DSP electronics acquire data, analyze data, input/output data, and control the laser. The small size and high computational power of DSPs allows all electronics to reside within a 7 cm by 10 cm by 24 cm box immediately below the aperture plate. No rack space is required. All interior components fit within the aperture plate liner.

3. Fiber optics: The use of fiber optics allows the laser to reside within the controlled environment of the aircraft cabin without having to purge the instrument with dry air. Thus, maintenance and operational time and costs are reduced, and ambient water vapor signals are not interfered by residual water vapor within the aircraft cabin.


The instrument is being calibrated and intercompared in the laboratory by a number of techniques. The small size of the optical cell allows for it to be placed inside a small (3 L) vacuum chamber with fiber optic feedthroughs. Ice or water can be added to vacuum chamber, and the entire chamber can be immersed in constant temperature baths. As the partial pressure of water vapor equilibrates to the ice/water vapor pressure, a direct measurement of frost/dewpoints can be obtained. In addition, standard dilution techniques will be used, but the flows will be continual and constant using critical orifices from a regulated pressure on a tank of liquid nitrogen. Recent flight tests include an intercomparison of the HIAPER VCSEL hygrometer with a chilled mirror hygrometer. Additional intercomparisons are planned with commercial instruments made by Li-Cor and DeltaF and with the Harvard Lyman-alpha water vapor instrument. Calibration methods for the instrument installed on HIAPER are being discussed with NCAR personnel.


Sampling frequency:

Although the instrument is reporting data at 25 Hz, the laser scans over the water vapor absorption lines at 1 kHz. By modifying the electronics and acquisition/analysis algorithms, water vapor measurements at frequencies of several hundred Hz are possible. Please contact Southwest Sciences for further information.

Other trace gases:

One advantage of the VCSEL and fiber optic system is that additional gases can be analyzed without having to redesign the instrument mount. For example, isotopes of water vapor such as H18OH and HDO could be analyzed by adding a new fiberized laser, fuse coupling it to the existing fiber optic, and modifying the data analysis/acquisition algorithms. For more information, please contact Southwest Sciences.

For more information, contact Mark Paige.

Contact Information

Southwest Sciences, Inc.
1570 Pacheco St., Suite E-11, Santa Fe, NM 87505
tel. (505) 984-1322/ fax (505) 988-9230



[Home] [Products&Services] [Research] [Technology] [Commercial Partnerships] [Contact]