Toxic Gas Monitoring
Optimal TGM Selection & Maintenance. Locations of TGM fall into three main categories: area, tool, and gas cabinet. Area monitors measure ambient concentration of target gas. They are placed between workers and a potential source. Evacuation is appropriate at PEL. Tool and gas cabinet monitors are placed in exhausted enclosures. There is a presumption that workers are not exposed to exhaust. Alarm levels are set high enough to avoid unnecessary alarms. Three times PEL or one half IDLH is appropriate. Evacuation is unnecessary. TGM relies primarily on electrochemical sensors and reagent-impregnated paper tape systems. Electrochemical sensors tend to be less expensive. They are smaller, easier to install, and easier to test. They respond quickly if properly maintained and periodically challenged. Electrochemical sensors have the disadvantage of being less specific than paper tape and more susceptible to false alarms. Interference by IPA is common. Sensors mounted directly in exhaust ducts can dry out quickly. Extractive systems are appropriate for monitoring ducts. Paper tape is the more reliable technology and is more specific to target gas. It is also more expensive and more difficult to install. Teflon tubing draws a sample from a detection point to the analyzer. A delay in response is associated with the length of tubing. Tubing can be damaged and needs tested periodically. Dust inside tubing can interfere with detection of highly reactive gases such as HF, HCl, and HBr. Infrared sensors work exceptionally well for gases with specific spectra.