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Action levels are concentrations of a gas or vapor in the air that require a certain response from personnel operating in the atmosphere. For example, the U.S. Occupational Safety and Health Administration (OSHA) sets an action level of 10% of the lower explosive limit (LEL) when operating in a potentially flammable atmosphere. When 10% is reached, which is considered a potentially explosive atmosphere, action must be initiated to protect personnel operating in that atmosphere. This may include ventilation and PPE. If 25% of the LEL is reached, withdraw personnel and take precautions for dealing with an explosive atmosphere.
Upon arrival on the scene of a potential hazmat incident, one of the first actions to take for a known or unknown material is air monitoring. Personnel using monitoring instruments must be trained in their use. Instruments must be calibrated according to manufacturers’ instructions before use and in some cases following use before being stored. If unidentified materials are encountered, first monitor the air for corrosive gases with pH paper to ensure instrument operation will not be affected by the gases. Next, monitor the air for oxygen concentration, radiation and flammable and toxic atmospheres. Instruments are available that can monitor for oxygen and LEL as well as other gases at the same time. Combustible-gas monitors require sufficient oxygen to operate properly to determine LEL. All monitoring instruments have operating condition limitations and personnel operating them should be aware of those limitations.
Combustible gas indicators (CGI) measure the concentration of a combustible gas or vapor, using catalytic bead technology, commonly a Wheatstone Bridge. Detection is accomplished by a platinum filament that is heated by burning the combustible gas or vapor. The increase in heat is measured and a readout displayed that is a percentage of the material’s LEL. Effective use of this and all monitoring instruments requires that the operator understand the operating principles and procedures. Maintenance involves recharging or replacing batteries and calibrating immediately before use. The monitor can be used as long as the battery lasts or for the recommended interval between calibrations, whichever is less. Limitations include lack of valid readings under oxygen-deficient conditions and filament damage by certain compounds such as silicones, halides, tetraethyl lead and oxygen-enriched atmospheres.
Electrochemical indicators monitor toxic-gas and oxygen-deficient atmospheres. These are single-sensor monitors for each of the materials being measured. These sensors use an electrolyte solution, which is a chemical that, when dissolved in water, conducts electricity. The solution is stored inside the sensor behind a permeable barrier. Two electrodes in the solution sense electricity.
Electrochemical sensors are designed for a specific chemical such as chlorine, ammonia, carbon monoxide, oxygen, nitrogen and sulfur dioxide. When the chemical diffuses across the barrier, it dissolves in the solution and contacts the sensing electrode, which produces ions and electrons. These travel to the counting electrode, which produces a percentage reading (in the case of oxygen), a direct digital readout or an alarm based on a pre-selected value that is reached or exceeded. Limitations include: similar chemicals may cause false positives; gases detected may poison the sensor’s electrolyte solution; particles or condensation from hot gases or water vapor can clog the sensor membrane; and going from low to high humidity may inhibit the movement of molecules across the membrane.
Metal oxide sensor (MOS) technology involves the use of a metal oxide and semiconductors to sense a particular gas. Contaminants change the conductivity of the semiconductor. They can sense multiple substances at the same time. Limitations include non-selective sensors and only provide accurate quantitative data for the calibration gas. These monitors are product specific, giving a ppm readout and provide a percent of the LEL. They are used for broad-spectrum general sensing.
Photoionization detectors (PIDs) are used to monitor organic and some inorganic gases and vapors. The sensor technology involves a sample drawn into a chamber. It is then exposed to ultraviolet (UV) light and the light ionizes the contaminant. Ions allow an electrical current to flow and the greater the number of ions, the stronger the current. The monitor interprets the ion flow as the parts per million (ppm) of the chemical. Limitations of the detector include: it does not monitor for methane; response may change when gases are mixed; dust; needs proper lamp intensity and clouds with high humidity and high concentrations; and dirty lamps may affect response.