The LI-7500DS is a high performance open-path CO2/H2O analyzer designed for use in eddy covariance flux measurement systems. It features simultaneous, high-speed measurements of CO2 and H2O in the free atmosphere. With the SmartFlux 3 System and a sonic anemometer, the instrument constitutes a complete eddy covariance system. This combination of instruments is used to measure the exchange of gas and energy between the terrestrial earth and the atmosphere using the eddy covariance method.