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Calibrating the LI-7000 for
Eddy Covariance Measurements

Many customers use a closed-path Analyzer (such as the LI-7000 CO2/H2O Analyzer) for making Eddy Covariance measurements. When employing this method of measurement, the user will typically use a pump downstream of the analyzer. The pump will pull air (typically between 8 and 15 liters per minute) from the sampling site where the sonic anemometer is located. The sample is drawn through a length of tubing from the sample site to the analyzer's inlet port. The tubing will vary in length depending on the height of the sample site (which could be up to 30 meters or more).

Because the pump is drawing the air down through this long tubing, it creates a decreased pressure in the analyzer. The LI-7000 records the pressure in the sample cell using a pressure transducer. The LI-7000 uses these pressure values in the equations and computations of CO2 and H2O concentrations. The pressure correction algorithm that LI-COR uses is intended to correct for typical diurnal variations of atmospheric pressure of a few millibars, in order to prevent day to day drift, but for very large deviations of pressure (i.e., 50 kPa) the pressure correction algorithm will not be accurate. Atmospheric pressure is typically at 95 to 100 kPa, but inside the analyzer the combination of the pump and tubing could create a pressure drop down to 50 kPa. Since the corrections are not as accurate as they could be, the solution is to calibrate the LI-7000 at a pressure similar to experimental pressure.

The way that many people using closed-path measurements at high flow rates handle this problem, is to not run calibration gas tank at the high operating flow rates, but instead reduce the flow during calibration. This can be done by placing a restriction valve between the CO2 Cylinder and the inlet of the IRGA. Place a ‘T’ junction between the CO2 cylinder and the restrictor that is vented to atmosphere. Turn on the sampling pump (or a smaller pump to conserve gases) and adjust the restrictor valve until the pressure sensor in the LI-7000 reads similar to when the system is running at the normal operating flow rate during measurements. Having adjusted the plumbing to produce the operating pressure and a low flow rate, turn on the CO2. Use an excess flow of calibration gas so that air flows out of the vent to prevent ambient air from being pulled back in through the vent. The gas flowing through the analyzer should only be coming from the cylinder. Check the flow direction out of the vent tube and confirm this before setting the span.