Case Study CS-TC09 Rev. A

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a)

b)

c)

Figure 5. First harmonics signals of a) methane (5 to 80 ppm), b) ethylene (5 to 85 ppm), and c) ethane (2 to 90 ppm) with

injection laser current in the range of 53 to 100 mA.1

To reduce the impact of cross-interference of the three

gases on the detection accuracy, researchers carried out

broadband multi-wavelength linear regression. This not only

reduces cross-interference errors but also accidental errors

in the design.1 A portion of the DHR signal data is extracted

throughout the whole range of the laser current, and the

selected data with high concentration-signal linearity are

further chosen for multi-wavelength fitting. This can help

obtain the gas concentrations through linear regression with

higher accuracy. Through this process, the error is reduced

to less than ±10% for ethylene and methane and less than

±5% for ethane. This helps the designed PAS system meet

practical application requirements.1

As is seen in

Figure 5, an increased gas concentration of

any sample gas directly correlates to an increased DHR

signal. The detection limits for each sample gas are as

follows: 98.9 ppb for methane, 252 ppb for ethylene, and

33 ppb for ethane with each having an integration time

of 2 seconds. Ethane is detected at a longer wavelength,

making the laser power higher than either of the other

gases and making it have the highest responsivity. On

the other side of the laser power range, ethylene had the

lowest absorption strength and did not have any sharp

absorption peak shapes. Thus, the responsivity of ethylene

is the lowest, leading to the higher detection limit. Although

methane may have a stronger absorption intensity, the

absorption cross-section is much lower than that of ethane.

Figure 6. (a) Multi-wavelength fitting profiles of the mixture 3 in Table 2: (b) The fitting residual plot.1