LI-COR technical support often receives questions about choosing a set of CO2 calibration gases for our analyzers. While we realize that every customer’s measurement situation may be slightly different, we do have a few suggestions for selecting a set of calibration gases.
We recommend having a zero gas, or the equipment to perform a chemical zero at the very least. A zero gas is best if the gas is really CO2-free. It’s not necessary to purchase pure nitrogen or argon if your supplier of air can certify that the air is really CO2-free and that certification can be verified. If the concentration is not zero, there will be a zero offset, which will affect the rest of the calibration as well.
For the best calibration, the instrument should be spanned at the same time that it is zeroed. The span gas tank should be somewhere near the top of the expected measurement range. For example, typical values might be under 2500 ppm, and the top expected value might be 10000 ppm. Therefore, the user wouldn’t need a span gas higher than 10000 ppm. With a wide range of CO2 values such as these, the user might consider getting a second span gas tank and performing a two-point span if the instrument supports it. This allows the instrument’s response to be more accurate because there are two reference points with which it can refine the response algorithm. If a second gas is used, it should be somewhere in the middle of the expected measurement range. In this case, 5000 ppm would be a good midpoint.
The final word of warning is that users should test their calibration gases using a calibrated analyzer to be sure that the stated concentration is the same as the actual concentration. While it may not be necessary to purchase WMO quality gases, users should purchase high quality (1%) gases in order to minimize the chances that the certified concentration and the actual concentration are very different.
Although the same dry CO2-free gas used for zeroing CO2 can also be used to zero H2O, a high-precision dew point generator (such as the LI-610) must be used to span the instrument for H2O. If users do not have access to a dew point generator, LI-COR recommends that they do not adjust the H2O Span.