The chemical, pharmaceutical, food and automotive industries, petroleum production, natural gas processing and so forth, all have brought great benefits to our modern lifestyle. However, in the last decades, with the development of these industries, the human health and life has been negatively influenced, due to the emission of toxic and flammable gases (e.g. methane) in the environment and the frequent exposure to indoor gases whilst working.
For instance, hydrogen sulphide (H_2 S) is a poisoning gas, which is released from sewage, manure, hot springs and petroleum production and refining.
People exposed to a concentration above 10 ppm of this gas, can experience irritation of the eyes, nose, throat, and respiratory system, while higher concentrations (250 ppm) can cause nausea, shock, rapid unconsciousness, coma, and even death (1) (2) (3) (4). Benzene (C_6 H_6), a volatile organic compound (VOC), is one of the air indoor or outdoor pollutants that can cause several diseases such as allergies, asthma, cancer and emphysema (5) (6). From the motor vehicle ehaust and stationary for example industrial boilers Nitrogen oxides (?NO?_x) whose main component is ?NO?_2 are produced.
The regular exposure to concentrations that are higher than those prevailing ambient air can origin irritation of the lungs and lower resistance to respiratory infections such as influenza. Moreover, flammable gases such as hydrogen, methane are very dangerous components, since under specific conditions they can produce unwanted explosion or fires and then pose risk to the safety of the human being.
Year Place Incident Chemical Causalities
1988 Bombay Refinery fire Oil 35
1990 Nagothane Leakage Ethane 32
1994 Asanol Fire Methane 53
1996 Dhanbad Mine Crash Gas 64
1997 Visakhapatnam Refinery fire LPG 60
2003 Vellore Explosion Explosives 25
2009 Jaipur Fire Oil 12
2013 Visakhapatnam Refinery expl Hydrocarbons 27
Table 1: Industrial disasters in India (9)
Therefore, there is a need and a strong industry-driven demand for developing a new generation of reliable, robust, accurate and cost effective gas sensors with enhanced sensitivity and selectivity, reduced moisture cross-sensitivity and fast response time to facilitate the early detection of such pollutants in the environment, before their concentrations reach potentially hazardous levels and also it is very important to introduce an intelligent monitoring system to check air quality and control heating in order to ensure maximum safety for human life. Methane is a chemical compound composed of one atom of carbon and four atoms of hydrogen (?CH?_4). It is a simple alkane and it is a form of compressed natural gas. Methane gas itself is not toxic, but if it can fill an enclosed room it can displace the oxygen and act as an asphyxiant. An asphyxiant means as methane builds in a room; it begins to take the place of oxygen. According to the Canadian Centre for Occupational Health and Safety, oxygen levels may decrease to a point where they represent less than 18 percent of the air in the room. When this happens, an occupant of the room may begin to feel slightly dizzy and experience a headache. At first, your heart rate may quicken, and you may begin to have some loss of coordination. As levels of methane rise and oxygen levels are depleted, you may begin to feel fatigued, and have emotional upsets and trouble breathing. If not removed from the room, you may begin to get nauseous and be unable to move. Oxygen concentrations of 6 percent or lower can cause death. These effects may occur faster if you are exerting yourself in any way while the methane is filling the room. It is also necessary to continuously check the concentration of methane gas in the air to save the life and secure from the explosion. For the explosion point of view, lower explosive limit of the methane gas is 5% of volume in the air and upper level limit of the methane is 15% of volume in the air.
Figure 1: Upper and Lower Explosion Limit of Methane Gas (10)
To overcome this issue, small and flexible device have been introduced and developed to detect the hazardous substances in the atmosphere, which is called as gas sensor. Gas sensors are devices capable of converting the concentration of an analyte gas into an electronic signal and are an important component of devices, commonly referred to as electrical noses. The electronic conductivity varies with the composition of the gas atmosphere surrounding them. The performance of a gas sensor can be measured by its lower limit of detection (LOD) and its sensitivity. The sensor with the lowest LOD is the one that affords the detection of the lowest level of an analyte gas or we can say lowest concentration of the gas and a highly selective sensor makes the detection of minor amounts of the preferable gas in a mixture of several other gases possible (11). In the past decades, researchers have developed gas sensors based on different sensing mechanisms, such as electrochemical sensors, catalytic combustion sensors, infrared sensors and diffusion fuel cell sensors. All these devices have a variety of applications in industrial engineering, chemical engineering, agriculture, medical and architecture among other sectors. In recent years, significant interest has emerged in the synthesis of nanoscale materials (12). One of the most attractive classes of materials for functional nanodevices is metal oxide semiconductors (MOS), which can easily and controllably be transformed in diverse shapes of low dimensionality. Various means have been reported for the synthesis of semiconducting nanowires and nanorods (13) (14). Materials such as tin been used by most researchers (BajpaiRitu, 2012). MOS gas sensors; in particular nanostructured metal oxide-based gas sensors emerge as potential candidates for viable applications in comparison to other rivals, such as chromatography, mass spectrometry or optical spectroscopy techniques, due to their ease of use, simple fabrication process, low cost and robustness.
After getting the data from the sensor and decide to take an action if any abnormal condition happened as per expectation, raise an alarm and shutoff the control valve will be done by designing gas meter module. For designing this module, there is some restriction regarding power consumption, battery life and so on to be noticed. This design has divided into two parts hardware and firmware, firmware side work on coding and gives command to the IMDM to perform the action where as hardware side made whole architecture. To achieve ultra-low power consumption STM32L486 microcontroller 64-bit has been used because it has ultra-low power UART pin which will be helped full to communicate with the gas meter.
This thesis has been organized in 5 chapters.
Chapter 1 gives an overview to the reader about the motivation for realizing this research and its main objectives.
Chapter 2 presents the state of the art of gas sensors and its classifications. It includes the working principles and the characteristics of metal oxide gas sensors. Next, it reviews the sensing mechanism of methane gas.
Chapter 3 describes in its first part, selection of the gas sensor which will be well suited on required application under given specifications. Firstly, comparison between different technologies of the sensors then benchmarking between different suppliers of the metal oxide gas sensor. The second part of this chapter is to design and development of ultra-low power hardware module.
Chapter 4 focuses in the beginning on experimental setup or experimental architecture and the gas mixture to control the methane gas concentration as well humidity level. Furthermore, discussion in this chapter is evaluation of experimental data. Cross check the sensors responses of three different placing sensors, one sensor placed into the gas chamber without module and without enclosure rest of the sensor placed with module but one with in enclosure and second one out the enclosure then evaluates cross sensitivity, response time, inaccuracy and power consumption.
Chapter 5 presents the results obtained from the sensor with the variation of temperature and humidity levels. At the end presents the conclusion of this master thesis and some suggestion for possible future work.