On the vapor trail
Hydride hazardsSeveral of the hydrogen compounds known as hydrides are particularly harmful. An example is arsine, a colorless gas used to â€œdopeâ€ semiconductors â€“ that is, to alter their electrical properties. Its faint garlic odor can be smelled only at dangerously high concentrations. Besides workers in the semiconductor industry, personnel at risk include those performing metal-handling activities, such as smelting and pickling, where acids are used on arsenic-contaminated materials.
Arsine poisoning has an insidious, delayed effect on the body that can lead to complete urinary stoppage and, potentially, death from asphyxiation. NIOSH lists arsine as a potential carcinogen, although research in this area has proved inconclusive.
Another semiconductor dopant is silicon tetrahydride, or silane. A colorless irritant of the eyes, nose and throat, silane is extremely flammable and can spontaneously combust.
Equally liable to ignite on contact with air is phosphine, which, again, is a gas used as a doping agent. However, it is more commonly deployed as a rodenticide or a fumigant to kill pests in grain silos, with additional applications in textile treatment and epoxy resin production. A severe pulmonary irritant, whose respiratory symptoms include breathing difficulty and a cough producing fluorescent green sputum, phosphine depresses the central nervous system, causing headaches, impairment of vision and gait, back pain, coldness and fainting. Its distinctive smell, variously compared to rotting fish or garlic, can be detected at concentrations as low as 0.15 ppm, half of OSHAâ€™s permissible exposure limit (PEL) of 0.3 ppm. However, as with all poisons, smell alone is an inadequate defense.
Sterilizing substancesPersonnel involved in preparing dressings and other medical supplies may be at risk from the toxic sterilizing agent ethylene oxide (commonly known as EtO). This gas, which has an ether-like smell at high concentrations, is also found in a number of chemical manufacturing processes. As well as irritating the eyes, nose and throat, EtO causes breathing difficulties, blurred vision, headaches, skin blistering, nausea and vomiting. Moreover, it can be carcinogenic and cause damage to the nervous and reproductive systems.
Chlorine and ozone are both well known as disinfectants and bleaching agents, with numerous applications in water treatment, swimming pool disinfection, paper bleaching and other industries. However, detecting these gases can be problematic: OSHA stipulates a PEL for chlorine of 0.5 ppm over 8 hours, while for ozone the PEL is only 0.1 ppm. Extreme sensitivity and stability are required to maintain accuracy at these low levels. Although the distinctive odor of ozone can be detected at sub-PEL concentrations â€“ for example, the minute amounts released by an office photocopier â€“ awareness of the odor decreases as the concentration rises, making ozone especially dangerous.
Hydrogen fluoride is an extremely harmful compound used as a catalyst in the manufacture of high octane fuels; in the production of fluorinated hydrocarbons for use as refrigerants, solvents and propellants; and in etching and polishing glass and metal. Colorless and having a pungent, irritating odor, it boils at only 67ÂºF and so is likely to be encountered both as a liquid and as a gas. It is also found in aqueous solution as hydrofluoric acid.
Permanent installationsFixed detection systems offer the widest choice of sensors for obscure gases. A sensor â€œheadâ€ comprising the sensor itself and the electronics needed to process its output signal is permanently installed near the source of the hazard. In some cases, additional hardware is needed to ensure meaningful readings. For example, it may be desirable to monitor the air inside a duct to ensure that gas levels remain within safe limits. In such a situation, a special sampling unit will redirect the air flow over a gas sensor mounted outside the duct for easy maintenance access.
In other cases, it may be necessary to measure heavy gases like chlorine, arsine or silane, which tend to gather at floor level. Here, the answer is a specially designed pipe assembly that can be configured to pass low over the floor and is perforated along its length with sample holes. Such apparatus is also ideal for wet well monitoring, where a fluctuating liquid level would otherwise make it impossible to monitor gas with a detector mounted just above the surface.
Whether or not additional hardware is used, detector heads must be connected to control equipment. If necessary this can be installed some distance away in a safer location. Should the sensor detect an alarm condition, the control unit will interpret the signal and take whatever action the user has preprogrammed â€“ from triggering a strobe or sounder to shutting down an operation and automatically dialing the emergency services. Control units range from simple, wall-mounted devices offering one-channel operation and basic controls to large-scale, modular installations able to monitor dozens of remote detectors.
It may be appropriate to connect remote repeater panels, so that personnel offsite can monitor changing conditions. Some systems also incorporate â€œnon-gasâ€ sensors, such as smoke and heat detectors, pressure transducers and door switches, to provide the fullest picture of safety across the site.
SIDEBAR: Portable protectionThe simplest gas detector is a portable unit designed to monitor just one gas. Users adjust the sample rate to suit the hazard being detected. Two alarm levels can be set to warn not only when the 8-hour PEL (or equivalent long-term exposure limit) is exceeded, but also when a short excursion limit, usually an average or ceiling level in a 15-minute period, has been reached.
Multi-channel instruments provide more versatility, typically able to monitor four hazards simultaneously. Here, the choice of sensors generally covers most, if not all, obscure gases. A slightly larger instrument means a bigger display, which should allow gas levels for all four channels to be read side-by-side, along with useful information relating to calibration and configuration.