LI-7500 Description

The LI-7500 Open Path CO₂/H₂O gas analyzer consists of two components: the analyzer sensor head and the control box which houses the electronics.

The sensor head has a 12.5 cm open path, with single-pass optics and a large 1 cm diameter optical beam.  Reference filters centered at 3.95 µm and 2.40 µm provide for attenuation corrections at non-absorbing wavelengths.  Absorption at wavelengths centered at 4.26 µm and 2.59 µm provide for measurement of CO₂ and water vapor, respectively.  These features minimize sensitivity to drift and dust, which can accumulate during normal operation.  The LI-7500 operates over a temperature range of -25 to +50 °C.  An additional calibration verification of the LI-7500 can allow for operation to -40 °C.

The cutaway representation of the LI-7500 sensor head shows the main components of the analyzer.  The Infrared Source emits radiation, which is directed through the Chopper Filter Wheel, Focusing Lens, and then through the measurement path to a cooled Lead Selenide Detector.  A brushless Chopper Motor rotates the chopper wheel at 9000 rpm (rotations per minute).  The source, detector, and chopper motor have a lifetime in excess of 20,000 hours.

The windows at both ends of the optical path are made of sapphire, which is extremely hard and scratch resistant, allowing for worry-free cleanup of dirt and dust accumulation.

The control box that houses the analyzer electronics is weatherproof, as are the cables and connectors used for data output, power, and auxiliary input.

Making Measurements

The LI-7500 is normally mounted vertically or at a slight tilt (allows water to run off), since this causes the least obstruction to wind over a range of azimuthal directions.  The sensor head can be mounted in any orientation desired without affecting performance.  The signals from the LI-7500 are modulated at 150 Hz, so that variations in ambient light have virtually no effect on operation.

In the eddy covariance method, the flux, Fc of gas c (e.g. CO2, H2O) is given by

where c' is the density fluctuation of gas c (mol/m3), w' is the vertical wind velocity fluctuations (m/s).

Near the ground, vegetation and other roughness elements induce eddies.  To correctly sample these eddies, a high speed, high precision analyzer is required.  The photo below shows a typical eddy covariance station.  The flux of CO2 or H2O from a landscape is obtained from the vertical wind speed (measured with a sonic anemometer) and the concentration of the eddy (measured with the LI-7500).

To minimize flow disturbances, the LI-7500 sensor head has a smooth, aerodynamic profile.  In addition, the open path measurement eliminates the need for a pump, greatly reducing the overall power requirement of the flux system.  This is especially important in remote locations where electrical power supply may be limited.  The LI-7500 requires less than 10 Watts of power to operate (initial power-up can draw up to 30 Watts).  The open path analyzer also eliminates time delays, pressure drops, and sorption/desorption of water vapor on tubing employed with closed path analyzers.

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