LI-6262 Calibration

Zero and Span Adjustments

Another result of the solid state analyzer design is minimal zero and span drift. To ensure the highest possible accuracy, simple adjustments can be made during operation to compensate for variations in barometric pressure (affects the analyzer span), or analyzer temperature (affects the analyzer zero). The use of a pressure transducer such as the 6262-03 compensates for the effects of span drift due to barometric pressure changes, though for best accuracy the span should be periodically set by flowing a gas of known CO₂ concentration through the analyzer. The zero of the analyzer is checked by flowing a zero reference gas through the analyzer. For unattended operation, the zero and span can be adjusted in software by issuing a set of remote commands to the RS-232 port from an external computer.

CO₂ Calibration

The detector output signal in LI-COR gas analyzers is non-linear with respect to CO₂ concentration. A fifth order polynomial is used to calculate the correct gas concentration. The factory calibration uses 13 different CO₂ concentrations to generate a calibration polynomial that is unique for each instrument. The calibration is checked at three carefully controlled temperatures. The resulting CO₂ calibration range is 0 to 3000 ppm. The accuracy of the factory calibration is ensured by the use of calibration gases that are traceable to the U.S. National Institute of Standards and Technology (NIST). Calibration accuracy is increased by the number of calibration standards, which result in a calibration between the absolute standard and the detector output signal.

H₂O Calibration

The factory H₂O calibration for the LI-6262 is performed in a manner similar to the CO₂ calibrations. A highly accurate (0.1 °C dew point) water vapor generator is used to provide known water vapor concentrations. The H₂O calibration accuracy is 1% of reading from 0 to 7.5 kPa or 40 °C dew point. Note that the optical bench and interconnecting hoses must be maintained above the dew point temperature to prevent condensation.

User Calibration

User calibration of the analyzer consists of adjusting the analyzer zero and span. The analyzer span is proportional to barometric pressure and is generally checked daily by flowing a gas of known concentration through the analyzer. The analyzer zero changes slightly with temperature and should be checked periodically during operation using the CO₂ scrubber provided with the analyzer. Span calibrations for the H₂O channel of the LI-6262 require a dew point hygrometer or the LI-COR LI-610 Portable Dew Point Generator.

Factory Recalibration

The factory calibration remains stable over time; however, recalibration is recommended every two years to ensure accuracy.