5. Hydrogen sulfide (H2S) monitoring

#### 5.1. Introduction

The system of CO high sensitivity detection based on TDLAS technology combined with the new type of multi-pass absorption cell basically realizes the high sensitivity detection of CO in the near infrared. The system exhibits good stability and high linearity after long-term measurement experiments. According to the Allan variance analysis, the detection limit of the system is 0.25 ppm with an integration time of 30 s. The system meets the requirements for

Figure 12. The Allan variance of 10 ppm CO.

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Figure 13. The 2f signals with different concentrations of CO.

Hydrogen sulfide (H2S) is an important potential dangerous gas in oil drilling. It is colorless, highly toxic, and acidic; there is a special smell of rotten eggs; the olfactory threshold is 0.00041 ppm. Even low concentrations of H2S can also damage people's sense of smell and have effects on the eye, respiratory system, and central nervous system. It is lethal to detect this kind of gas using a nose [55]. Because there is no smell when the concentration is high (high concentrations of hydrogen sulfide can paralyze olfactory nerves). Hence, sensitive H2S detection is necessary in practical applications. In this part, a 1.578 μm distributed feedback (DFB) laser is used to detect H2S of low concentration [56].

#### 5.2. Wavelength modulation spectroscopy system

The WMS technique is used in the H2S detection system, as shown in Figure 15. The used multipass absorption cell in this experiment is also homemade with a total optical path length of 56 m and a total volume of 0.8 L. A single-mode pigtailed DFB laser with a central wavelength of

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

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 0.014 cm<sup>1</sup> /mA. The laser wavelength is scanned by a triangular wave of 30 Hz. A 20 kHz sine 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 a personal computer (PC) for signal processing via an A/D converter.
