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A calibration gas delivery system is a component of any extractive sampling systems designed for meeting regulatory requirements in the United States and Canada. In North America, CEM systems are required to be calibrated against NIST‐certified calibration gases meeting established traceability protocols (Schakenbach et al. 2002; U.S. EPA 2012). Monitoring system calibration is required to be verified daily, and monitoring system linearity must also be audited quarterly using “protocol” gases injected as close to the probe as possible, passing through all components of the extractive system to the analyzers.

The calibration gas system typically extends from the calibration gas cylinders, through a gas pressure regulator, to an automated gas injection control system, through a dedicated line in the umbilical, and on to the injection port (Figures 3‐15 and 3‐21). The gas returns through the sample line to the analyzer(s). The calibration gas line may be either in the umbilical (which may be heated or unheated) or separated from the main umbilical in an unheated line. Components typically in contact with the calibration gas will be the calibration gas cylinder pressure regulator, calibration gas delivery line, the sample gas probe, the sample line, and the internal surfaces of the analyzer sample cell.

The use of cylinder gas pressure regulators is often taken for granted and can be a factor in the delivery of reactive calibration gases. Reactive gases such as HCl and NH₃ can stick to surfaces in the regulator as well as to the calibration gas delivery line, where the gas adsorption is dependent upon the materials of construction, flow rate, temperature, and the prior status of the delivery system. Moisture or other contamination in the regulator can have effects that may be difficult to uncover if proper precautions are not taken. Water in the air can condense in a regulator and regulators used for other purposes may have an unknown history that could lead to losses of the gas. Similar to calibration gas lines, regulators treated with SilcoNert^® reduce surface absorption.

For reactive gases, calibration system problems are often blamed for the quality of the calibration gases. However, problems are more often due to the lack of consistent procedures used to deliver the calibration gas to the monitoring system. It is recommended that for reactive gases the regulator first be flushed with nitrogen for a half hour, installed on the cylinder, and then purged with the calibration gas. The regulator should remain on the cylinder and should be back‐purged when it is removed to exchange cylinders. Recommendations provided by Marshak (2015) are as follows:

 Use new regulators – leave on the cylinder all the time.

 Purge for 30 minutes prior to installing on cylinder.

 Pressure purge with the cylinder gas 10 times.

Gas vendors are not always helpful on these issues, assuming that their purpose is to ensure the quality of calibration gases and not their usage.