ISSN 2249-6343 International Journal of Computer Technology and Electronics Engineering (IJCTEE) Volume 3, Issue 3, June 2013 9 Abstract-Underground mining requires equipment and manpower to operate under the earth surface. Subsurface atmosphere may be contaminated with poisonous gases that displace the necessary oxygen to support life or flammable gases that may cause explosion. Therefore, it is necessary to develop technologies and find ways to accurately measure concentration levels of toxic and flammable gases levels in subsurface atmosphere for safety of underground coal mines. Each sensor has its own advantages and constraints, like some sensors are better for sensing toxic gases and some are better for combustible gas detection. The paper enumerates operating principle, working procedure and application of different types of sensors for monitoring toxic and flammable gases in hazardous areas. Index Terms- Gas sensors, MEMS, Nanotechnology, TLVs I. INTRODUCTION Underground coal mine is characterized by tough working condition and hazardous environment. Many accidents occur in underground coal mine which leads to fatal accidents and huge loss of properties. These accidents have variety of causes, including sudden rise in toxicants such as carbon monoxide (CO), dangerous flammable gases especially methane (CH 4 ) or firedamp and insufficient oxygen for mine workers to breathe. Therefore, for sustainable growth of coal mining industry and safety of miners, it is necessary to develop technologies and find ways to make mines hazard free [1]. To keep atmosphere just right in underground coal mine, the primary requirement is to regularly monitor the levels of gases, like oxygen, methane, carbon dioxide, carbon monoxide etc. This gives miners short and long term trending information in the subsurface atmosphere and allows early warning against explosive and toxic atmospheres at every place where miners normally work or travel. No mineworker should enter any underground work place, in particular those places with poor air circulation (e.g. blind headings), unless the air has been checked therein to ensure a safe breathable atmosphere free from levels of hazardous gases. II. HAZARDOUS AREA Fire, toxic atmospheric contaminant and dust or gas explosion are some critical hazards specifically linked to underground mining. It is necessary to figure out which area needs to be defined as hazardous area so that miners should be alerted in advance (Table I). A. Combustible Gases A hazardous area is defined based on three criteria, namely (i) depending upon type of gas, (ii) ignition temperature of the gas, and (iii) likelihood of gas being present in flammable concentrations. Flammability limit, thus defined, gives the proportion of combustible gases in a mixture, between which limits mixture is flammable.  Lower Explosive Limit (LEL): The minimum concentration of gas or vapour mixed with air (percentage by volume, at room temperature) that will cause the propagation of flames when it comes in contact with a source of ignition. In common terminology, mixtures below the LEL are too lean to ignite.  Upper Explosive Limit (UEL): The maximum concentration of gas or vapour mixed with air (percent by volume, at room temperature) that will cause the propagation of flames when it comes in contact with an ignition source. In common terminology, mixtures above the UEL are too rich to support combustion. B. Toxic Gases As toxic gases can cause harm in low levels over a long period of time (chronic exposure) or in higher concentrations over a short period of time (acute exposure), different countries have established threshold limit values (TLVs) for poisonous gases in order to advance worker protection by providing timely scientific information to occupational and environmental health professionals. TLVs of airborne substances refer to those concentrations within which personnel may be exposed without known adverse effects to their health or safety. Followings are the three types of TLVs: (i) Time Weighted Average (TWA) is the average concentration to which nearly all workers may be exposed over given hours of work shift/week without known adverse effects. However, many substances are sufficiently toxic that short-term exposures at higher concentrations may prove harmful or even fatal. (ii) Short-Term Exposure Limit (STEL) is a time-weighted average concentration occurring over a period of not more than few minutes. It is also recommended that such circumstances should not occur many times. (iii) Ceiling Limit (CL) is the concentration that should not be exceeded at any time. This is relevant for the most toxic substances or those that produce an immediate irritant effect. Application of Gas Monitoring Sensors in Underground Coal Mines and Hazardous Areas A. Kumar*, T.M.G. Kingson, R.P. Verma, A. Kumar, R. Mandal, S. Dutta, S.K. Chaulya and G.M. Prasad CSIR-Central Institute of Mining and Fuel Research, Dhanbad, India *Corresponding Author, Email: eranishkumar@gmail.com