LI-7500 Calibration

There are two aspects of calibration associated with the LI-7500; factory calibration and user calibration.  The factory calibration consists of determining the values of the calibration coefficients.  User calibration (performed weekly or monthly) consists of setting analyzer zero and span.  The accuracy of the LI-7500 depends on both calibrations.  The calibration coefficients determined at the factory should be valid for several years.  The zero and span settings make the analyzer's response agree with the previously determined factory response at a minimum of two points.  Calibration gases of 1% accuracy can often be obtained without too much difficulty; for higher accuracy the user should obtain WMO standards which are within the range of concentrations to be encountered during experimental measurements.  It is recommended that the user keep these WMO gases as primary standards for checking less expensive working gas calibration tanks.

Factory Calibration

CO₂

LI-COR uses a series of thirteen working calibration gases ranging in concentration from zero to 3,000 ppm.  The thirteen working standards are verified against a set of primary WMO standards known to an absolute accuracy of 0.06% or better.  Factory calibration coefficients are obtained from a 5th order polynomial fit to the entire range of concentration from zero to 3,000 ppm.  The nominal accuracy of the polynomial fit across the entire range of concentrations is about 1%, but the absolute measurement accuracy obtained by the user depends primarily on the accuracy of the calibration gas uses to set the analyzer span.

H₂O

The calibration procedure for water vapor is similar to that for CO₂, except that water vapor mole fractions are generated using the LI-COR LI-610 Portable Dew Point Generator.  Fifteen data points between 0 °C and 40 °C dewpoint are used during the calibration procedure, and a 3rd order polynomial is fit to the data.  The LI-610 is certified by the National Institute of Standards and Technology (NIST) to have an absolute accuracy of 0.2 °C.  The calibration coefficients determined at the factory should be valid for several years.  The zero and span settings make the analyzer's response agree with its previously determined factory response at least at two points.

User Calibration

The LI-7500 is calibrated by inserting the calibration tube into the optical path.  The tube volume is then flushed with a flow of zero gas to set the zero, and a known concentration gas to set the span.  In general, the analyzer zero is stable over a several month period; the span stability affects H₂O the most during large variations in temperature, so the frequency of user calibration is largely dependent on the operating environment.  Atmospheric or diurnal pressure variations are generally not a concern.  The calibration is performed with the Windows® interface software.