To zero the analyzer of an EC150 or an IRGASON®, any gas that is free of CO2 or H2O, such as nitrogen gas, will work. To span CO2, use mixtures of CO2 in air. It is important that air, not pure nitrogen, be used as the balance gas, so that the pressure-broadening characteristics match that of ambient air. Ideally, use a CO2 span gas concentration that is close to the expected concentration that will be measured at the site.