H, 25% H, 50% H, 75% H, 100%
N, H, 3% H, 6% H, 12%
Figure 3 2F spectra of hydrogen in the concentration range of 0-100%
challenge was approximately 10 minutes, with a return to the zero gas baseline value between most of the challenges. The response time (T90) was measured to be 90 seconds and was limited by the propagation of the gas in the sampling cell with a flow rate of 1 L/min. The data acquisition rate was two seconds per measurement. Repeatability as a degree of agreement between replicate measurements of the same quantity was expressed in terms of standard deviation of the measurements. Standard deviation of the readings on each of the challenges was 0.7%. The value of the accuracy evaluated at the levels in the range of 0-100% was less than 1% at all tested hydrogen levels.
Instrumental drift for 15 hours is shown in Figure 5 . During the drift test, nitrogen gas was run through the sample cell at a flow rate of 1 L/min. No significant trends or correlations with the environmental temperature or sample pressure were observed in the data. Over the 15-hour period, a mean value of 0.36%, with a standard deviation of 0.345, was recorded. No drift larger than 1% H₂ was observed. The performance of the instrument was also evaluated over a range of sample pressures from 10 to 25 psia. Spectra recorded for hydrogen over this pressure range are shown in Figure 6 . It should be noted that, because of Dicke narrowing, the amplitude of the 2F signal grows with increasing sample pressure. Significant reduction of the hydrogen linewidth with pressure and a corresponding increase in peak amplitude have been reported by others earlier.
Figure 4 Hydrogen analyser performance validation
Figure 5 Hydrogen analyser zero drift
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