How are Secondary and Tertiary standards calibrated for CO₂?

Secondary and Tertiary standards are calibrated by accurately measuring the CO₂ concentration using a Non-Dispersive Infrared (NDIR) gas analyzer, which references CO₂ concentrations from a known to an unknown using the fact that CO₂ is a greenhouse gas and thus absorbs infrared radiation. In this case the knowns are the calibrated Primaries and Secondaries, which are used to calibrate the unknown Secondaries and Tertiaries.

The calibration system is very automated and can calibrate several air tanks at a time due to a network of selection valves and metal tubing connecting multiple air tanks to the CO₂ analyzer. A computer program controls the calibration process, but the new air tanks and the correct standards have to first be connected to the calibration system by hand. The computer program is beneficial for keeping track of calibration results, which could get mixed up or contain errors if done by hand. The computer program also performs complicated mathematical calculations, something that would not be fun to do by hand!

CO₂ Calibration Room: The Secondaries and Primaries that are used to calibrate Tertiaries and Secondaries respectively are placed within a metal rack, which consolidates the tanks within the rather small calibration room and provides a more permanent structure to store the standards that will be used frequently. Notice they are lying flat. This is to minimize the gravitational separation [This is similar to the sugar in your iced tea falling to the bottom of your cup; the sugar is denser just like certain gases are denser.] over time of the different molecular weights of the air molecules. The system is that sensitive! The unknown tanks that need calibrated are placed next to the rack because they will be moved much more often. Over 800 feet of metal tubing connects all the different standards and uncalibrated air tanks to the instruments shown on the right. Notice the straps holding the uncalibrated air tanks; these tanks may look harmless, but if one falls over, the valve could break off and the 2000 psi of pressure (136 times normal atmospheric pressure at sea level) can turn the tank into a rocket. The numbers shown on the colorful computer screen in the right photo are calibration results from multiple runs as well as temperature and pressure readings of the room and inside the NDIR gas analyzer.

• Regulator Preparation

  

  The first step in calibrating an air tank is to place a regulator on it. Placing a regulator onto a tank involves more than screwing it on, we need to take extra precautions to ensure there is no leftover air inside the regulator contaminating the new air being fed to the analyzer once the calibration process begins. This is done by flushing the regulator 5 times with air from inside the tank. On the 5th flushing, the air is left in the regulator overnight to allow the mixture of gases inside the tank to equilibrate on the inner walls of the regulator. Although this might seem overcautious, it prevents non-measurable problems later on.

• Connecting to the Tubing Manifold

  

  Metal tubing (the tubing manifold) is used to connect both the unknown air tanks and the standards to the stream selection valves that feed into the NDIR CO₂ analyzer. Thin tubing is beneficial for our purposes because then there is less surface area within the tubing that needs to equilibrate to the new air from the uncalibrated air tanks. The photo on the left shows these sophisticated valves used to select which known and unknown tanks are sampled, which can each switch between 16 different inlets. One selection valve is for the known standards and the other is for the unknown air tanks being calibrated.

• Running the Calibration

  

  The automated calibration process consists of 8 cycles. Each cycle is completed by measuring three or four known standards from lowest to highest CO₂ concentration through the NDIR CO₂ analyzer shown on the left, intermixed with one to five of the uncalibrated air tanks. Typically it takes a couple cycles to allow the air to equilibrate inside the tubing manifold, so the first couple cycles are thrown out and the last 6 cycles are used to get the true concentration of CO₂.
  
  To be extra certain that the CO₂ concentration is accurate, this 8 cycle process is repeated two more times on separate days in between other calibrations. This measures the "repeatability" of our CO₂ measurements, and they typically agree to within 0.02 ppm. Once the tanks are calibrated with their accurate CO₂ concentration, they are considered "standards."

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