A method and an apparatus for detecting a necking error during semiconductor manufacturing. The positioner can slidably move the distal end of the optical probe to maintain the distal end position with respect to an area of a material undergoing combustion. A scope cable portion transmits data between the scope distal portion and the image processor portion. The charger may have charging circuitry and an indicator indicating a recharge state of the rechargeable device. A processing unit of the gas detecting system then performs calculations on the sensing signal, such that an analysis unit may determine the presence of a target gas in the particular space, and further the composition and concentration of the target gas within the particular space, thus providing a detection that is accurate, rapid and convenient.