Measuring Principles of the LI‑7700 Trace Gas AnalyzerWatch Now
The optical source in the LI-7700 is a single-mode tunable near-infrared laser, which operates at ambient temperature, allowing for low power consumption. Using Wavelength Modulation Spectroscopy (WMS), a spectroscopic technique widely used in high-precision applications, the laser scans across a single feature in the 2ν3 absorption band of methane near 1.6 microns with high resolution and at a high frequency. The wavelength is modulated at sub-MHz frequency, virtually eliminating 1/f noise of the laser source and allowing detection of fractional absorption (smaller than 10-5). The LI-7700 demodulates the resulting signal at twice the modulation frequency, and then compares it to a reference signal shape to determine CH4 concentration. WMS provides very rapid measurements, allowing the LI-7700 to output data at up to 40 Hz through the Ethernet.
Pressure and temperature induced changes in line shape and population distribution, as well as changes in laser power and mirror reflectivity, are compensated using computational fitting algorithms. This ensures that measurements remain accurate over a wide range of pressure and temperature conditions.This is described in more detail in the following publication:
D. McDermitt, G. Burba, L. Xu, T. Anderson, A. Komissarov, B. Riensche, J. Schedlbauer, G. Starr, D. Zona, W. Oeschel, S. Oberbauer, and S. Hastings, 2010. A new low-power, open-path instrument for measuring methane flux by eddy covariance. Appl Phys B: Lasers and Optics, 102: 391-405.