5.3.2. Linearity and response time

Linearity is an important parameter in measuring instruments. In this TDLAS system, certified H2S gases with mixing ratios of 5, 10, 20, and 45 ppm were tested successively to check the linearity. Figure 17 displays the corresponding background corrected signals at different mixing ratios. Furthermore, the mixing ratios and the peak-to-peak values are linearly fitted in Figure 18. The results illustrate that the system has a good linearity with a fitting coefficient of 0.998.

The repeatability and the response time are also very important for H2S detection. In the extraction system, a 1-L gasbag is used to get different H2S standard gases into the multi-pass absorption cell successively. The response time depends on both the volume of the multi-pass absorption cell and the speed of releasing the airbag. The response time for filling 5 ppm H2S standard gas mixture into the 0.8 L cell with an evacuation flow rate of 3 L/min is plotted in Figure 19. Obviously, the response is accurate and fast. In the system, the data sampling rate is

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy

http://dx.doi.org/10.5772/intechopen.72948

223

Figure 17. 2f signals with background correction for different H2S concentrations.

90 Hz, so the response time is about 4 s.

Figure 16. Background baseline.

The fitting is given by y = 0.00143 + 7.52459x with a fitting coefficient of 0.998.

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy http://dx.doi.org/10.5772/intechopen.72948 223

Figure 16. Background baseline.

1.578 μm is employed in this system. The wavelength of the laser is controlled by a temperature and current controller, which can vary the laser wavelength with a magnitude of about

wave is used to modulate the laser output wavelength. The transmission signal was sent to the preamplifier system whose bias amplifier enhances the weak absorption signals. The parallel circuits amplify the signal and direct it to a lock-in amplifier for demodulation and to a low pass filter for obtaining the triangular wave after passing through the cell. Both signals are directed to

In order to improve the measurement accuracy and the detection limit, it is important to subtract the background spectrum in the spectral measurements. The background and the initial 2f signal are shown in Figure 16. Obviously, the symmetry of the demodulated signal

Linearity is an important parameter in measuring instruments. In this TDLAS system, certified H2S gases with mixing ratios of 5, 10, 20, and 45 ppm were tested successively to check the linearity. Figure 17 displays the corresponding background corrected signals at different mixing ratios. Furthermore, the mixing ratios and the peak-to-peak values are linearly fitted in Figure 18. The results illustrate that the system has a good linearity with a fitting coefficient of 0.998.

The fitting is given by y = 0.00143 + 7.52459x with a fitting coefficient of 0.998.

a personal computer (PC) for signal processing via an A/D converter.

was considerably improved after the background correction.

5.3. H2S concentration measurements

Figure 15. Sketch of the experimental setup for WMS system.

5.3.1. Stability of the background signals

5.3.2. Linearity and response time

/mA. The laser wavelength is scanned by a triangular wave of 30 Hz. A 20 kHz sine

0.014 cm<sup>1</sup>

222 Green Electronics

Figure 17. 2f signals with background correction for different H2S concentrations.

The repeatability and the response time are also very important for H2S detection. In the extraction system, a 1-L gasbag is used to get different H2S standard gases into the multi-pass absorption cell successively. The response time depends on both the volume of the multi-pass absorption cell and the speed of releasing the airbag. The response time for filling 5 ppm H2S standard gas mixture into the 0.8 L cell with an evacuation flow rate of 3 L/min is plotted in Figure 19. Obviously, the response is accurate and fast. In the system, the data sampling rate is 90 Hz, so the response time is about 4 s.

Figure 18. Linearity of the measurement system.

5.3.3. System stability and detection limit

Figure 21. The Allan variance for 5 ppm H2S.

Figure 20. Measurement of 5 ppm H2S standard gas.

the 1/f noise.

The Allan variance is usually used to analyze the temporal stability of the instrument performance. H2S standard gas of 5 ppm was measured for a period time of 11 h and shown in Figure 20 with the fluctuations of less than 1 ppm. Moreover, the Allan variance in Figure 21 indicates a detection limit of 240 ppb with an integration time of 24 s for eliminating the white noise. When the time is increased to 60 s, the detection limit is reduced to 140 ppb for removing

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy

http://dx.doi.org/10.5772/intechopen.72948

225

Figure 19. Response time of the measurement system.

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy http://dx.doi.org/10.5772/intechopen.72948 225

Figure 20. Measurement of 5 ppm H2S standard gas.

#### 5.3.3. System stability and detection limit

Figure 18. Linearity of the measurement system.

224 Green Electronics

Figure 19. Response time of the measurement system.

The Allan variance is usually used to analyze the temporal stability of the instrument performance. H2S standard gas of 5 ppm was measured for a period time of 11 h and shown in Figure 20 with the fluctuations of less than 1 ppm. Moreover, the Allan variance in Figure 21 indicates a detection limit of 240 ppb with an integration time of 24 s for eliminating the white noise. When the time is increased to 60 s, the detection limit is reduced to 140 ppb for removing the 1/f noise.

Figure 21. The Allan variance for 5 ppm H2S.

The experimental results indicate that the system has good linearity, stability, and repeatability, combined with a quick response time and a low detection limit. The H2S detection system based on TDLAS has the feasibility of online monitoring in many applications.
