VCSEL

Instrument Description

Identifying Information

Full Name: Vertical Cavity Surface-Emitting Laser (VCSEL) Hygrometer
Short Name or Acronym: VCSEL

Model if applicable:

Measurements Made and Methods Used

Measurements Provided: Water vapor mixing ratio and dew point.
Manufacturer or Builder of the Instrument: Southwest Sciences, Inc. (SWS)

When Acquired: 7/2008

Operational Status: requestable, ready for deployment
General Description:
- The VCSEL hygrometer employs tunable diode laser absorption spectroscopy to determine the water vapor number density over a dew-point range of -90 to +30 °C. It reports the water vapor number density and approximate dew-point at 25 samples per second. It is mounted on a GV aperture plate, weighs 3.2 kg and draws less than 20W from 120 VAC, 60 Hz (possibly 440 Hz as well, need to check that).
- The hygrometer operates in two absorption modes on two wavelengths: wavelength modulation on a weak absorption line (1853.3 nm, used for high mixing ratio conditions), direct absorption on a strong line (1854.0 nm for moderately low mixing ratios), and wavelength modulation on the strong line (for low mixing ratios). The sample volume is an open-path Herriott cell, giving an absorption length of 375 cm in a volume approximately 20 cm long and 2 cm diameter. The instrument is shown below.

Calibration Methods: Defined by spectroscopic parameters and reference spectrum stored in EEPROM. Checked against dew-point generator and slush baths.
History of Significant Changes to the Instrument:
8/2010 Replaced gold mirrors with more rugged dielectric mirrors.

Description of Software, Data Handling, and Data Flow: Mixing ratio is determined by comparison against a reference spectrum scaled for sample pressure and temperature. Output is mixing ratio and dew point in ASCII text at 25 samples per second over RS232, 38400 baud.
- Software Components (data acquisition, display, processing, and analysis): Internal processing is proprietary SWS algorithms.
- Data Processing Chain (from flight to archival): Data are recorded as generated by the instrument. Data processing follows the standard data path for RAF measurements: instrument -> NIDAS -> nimbus -> netCDF archives. See this page for more information. Sometimes it is possible to produce revised and improved measurements after completion of a project, by reference to post-flight calibration.
QA Procedures and Needs: Instrument is checked for proper operation with dry and humid air before installation. Flight data is checked for agreement with the chilled mirror hygrometers and suitable laser intensity.

Safety Issues: None
Examples of Measurements:
- Data shown below is the one second housekeeping data (beginning with SWS), the 25 s/s mixing ratio and dew point data, and a 5-minute spectrum (SPEC). Lines are terminated with CR/NL.

The 25 s/s data record consists of the mixing ratio (#/cc), laser intensity, dew point (°C), and # of spectral scans averaged, normally 60.

SWS   1199  544 295.9 628.9  9430  9431   408 D 0.0794 2000 30 4095 52 0  7.0 2718
 4.13E+15 1788  -41.16   60
 3.67E+15 2804  -42.28   60
 3.67E+15 2757  -42.27   60
 3.68E+15 2756  -42.25   60
 3.68E+15 2757  -42.26   60
 3.67E+15 2757  -42.27   60
 3.67E+15 2757  -42.27   60
 3.68E+15 2757  -42.26   60
 3.67E+15 2757  -42.27   60
 3.66E+15 2757  -42.30   60
 3.67E+15 2757  -42.29   60
 3.66E+15 2757  -42.29   60
 3.67E+15 2756  -42.28   60
 3.66E+15 2758  -42.30   60
 3.65E+15 2757  -42.33   60
 3.66E+15 2757  -42.30   60
 3.67E+15 2757  -42.28   60
 3.67E+15 2756  -42.29   60
 3.66E+15 2757  -42.31   60
 3.67E+15 2758  -42.29   60
 3.67E+15 2756  -42.28   60
 3.67E+15 2756  -42.27   60
 3.67E+15 2757  -42.27   60
 3.62E+15 2756  -42.40    1
SWS   1200  544 295.9 628.9  9430  9431   408 D 0.0794 2000 30 4095 52 0  7.0 2718
SPEC -1.63E-01 -1.63E-01 -1.63E-01 -1.63E-01 -1.63E-01 -1.63E-01 -1.18E-01 -8.84E-02 -5.56E-02 -1.37E-06  7.99E-02  1.55E-01  2.40E-01  2.96E-01  3.17E-01  3.30E-01  3.38E-01  3.54E-01  3.66E-01  4.03E-01  4.32E-01  4.55E-01  4.96E-01  5.32E-01  5.49E-01  5.83E-01  6.30E-01  6.87E-01  7.37E-01  7.84E-01  8.46E-01  8.96E-01  9.63E-01  1.03E+00  1.11E+00  1.20E+00  1.29E+00  1.39E+00  1.47E+00  1.59E+00  1.70E+00  1.82E+00  1.95E+00  2.09E+00  2.23E+00  2.35E+00  2.49E+00  2.64E+00  2.78E+00  2.92E+00  3.05E+00 3.18E+00  3.29E+00  3.36E+00  3.43E+00  3.48E+00  3.51E+00  3.50E+00  3.48E+00  3.44E+00  3.37E+00  3.29E+00  3.15E+00  3.03E+00  2.89E+00  2.76E+00  2.61E+00  2.46E+00  2.31E+00  2.17E+00  2.02E+00  1.88E+00  1.73E+00  1.59E+00  1.44E+00  1.32E+00  1.20E+00  1.10E+00  9.97E-01  9.06E-01  8.15E-01  7.09E-01  6.18E-01  5.43E-01  4.61E-01  4.01E-01  3.41E-01  2.80E-01  2.03E-01  1.50E-01  1.05E-01  5.29E-02  4.26E-03 -2.84E-02 -5.13E-02 -1.15E-01 -1.60E-01 -1.97E-01
 3.83E+15 1788  -41.88   60
 3.66E+15 2803  -42.31   60
 3.67E+15 2758  -42.27   60
 3.68E+15 2757  -42.25   60
 3.68E+15 2755  -42.26   60
 3.66E+15 2757  -42.31   60
 3.65E+15 2756  -42.32   60
 3.66E+15 2757  -42.31   60
 3.66E+15 2757  -42.31   60
 3.66E+15 2756  -42.30   60
 3.68E+15 2755  -42.26   60
 3.67E+15 2754  -42.29   60
 3.66E+15 2756  -42.30   60
 3.67E+15 2755  -42.27   60
 3.67E+15 2756  -42.29   60
 3.67E+15 2758  -42.28   60
 3.66E+15 2758  -42.31   60
 3.65E+15 2758  -42.34   60
 3.64E+15 2759  -42.35   60
 3.66E+15 2757  -42.30   60
 3.65E+15 2757  -42.32   60
 3.65E+15 2758  -42.33   60
 3.64E+15 2758  -42.35   60
 3.60E+15 2757  -42.46    1
SWS   1201  544 295.9 628.9  9430  9431   408 D 0.0794 2000 30 4095 52 0  7.0 2718

Lead Contacts

Lead Contact: Stuart Beaton

Documentation

Operating Manual VCSEL Operator Manual
Descriptive Publications Vertical cavity laser hygrometer... JGR 2010
Documentation of the Processing Algorithm(s): See the Technical Note on Processing Algorithms at this URL (fix when available)
Calibration History Diao/Zondlo Calibration Report, March 2012
Operating Manual or Instructions - See above operating manual
Final Report VCSEL Final Report

Internal users: See http://wiki.eol.ucar.edu/rafscience/VCSEL (last edited 2012-12-10)