Typical interferences from other gases are summarized in Table 2, all of which were measured with nitrogen as the makeup gas. The interference data indicate the fractional response of an analyzer to a gas other than the one being analyzed. The strongest interference is NO with O₂, where a 962 ppm concentration of NO is interpreted by the O₂ sensor as 500 ppm O₂. This interference arises because O₂ and NO_x are paramagnetic, with the strength of the NO and NO₂ paramagnetic susceptibility being roughly 43 and 28 % that of oxygen. Oxygen concentrations are accurately determined so long as they exceed those of NO by approximately an order of magnitude. This behavior is typical of all paramagnetic-style oxygen analyzers and is a primary reason such analyzers cannot be used to measure low oxygen concentrations precisely in combustion flows. The remaining interference ratios are two to five orders of magnitude lower and present little concern under typical combustion conditions. Entries that indicate 0.0 indicate no interference was noted. Blank entries indicate no measurements were made.

A second gas analyzer is used simultaneously with the first to prevent temporal drifts in measured NO_x concentrations from biasing the results and interpretation. This analyzer is portable and commonly used on field trips, and is therefore configured with more flexibility but has less stability than the dedicated system. When not in the field, it is used in the laboratory so we can make simultaneous measurements during experiments. The portable analyzer provides continuous monitoring of CO, CO₂, NO_x, O₂, and SO₂ (no THC, as in the first analyzer) with ranges and techniques as summarized in Table 3. Non-dispersive infrared detectors (NDIR) measure CO, CO₂, and SO₂ concentrations. A galvanic cell oxygen analyzer provides oxygen concentrations. Chemiluminescence is used for NO. The analyzer (Horiba model PG-250) includes particle filtration systems and minimal sample conditioning (moisture control). The analyzer repeatability is ± 0.5% of full scale except when CO and NO are below 1000 and 100 ppm, respectively, where repeatability for these gases is ±1% of full scale. Zero drift is no more than ±1% of full scale per day except for the SO₂ analyzer, where zero drift is ±2% of full scale per day. Span drift performs similarly. Linearity is within 2% of full scale. Minimum detectable limits are no more than 0.5% of full scale. Analyzers are typically calibrated before each experiment.

Gas samples are extracted similarly to the approach indicated above for the first analyzer.

Interferences from other gases are summarized in Table 4, all of which were measured with nitrogen as the makeup gas. The interference data indicate the response of an analyzer to a gas other than the one being analyzed. This instrument is relatively new and we have not measured the interferences in the laboratory any more precisely than the manufacturers specifications, which is what is represented in the table.