The Chemistry of Oxygen Sensors
Direct-Reading Technologies for Protecting Workers in Confined Spaces
Like other real-time detection instruments, oxygen sensors play a key role in protecting worker health and safety. Understanding how these instruments work is integral for achieving optimum performance—and for protecting workers in confined spaces against the potentially fatal effects of oxygen deficiency or enrichment.
An oxygen sensor measures the atmospheric oxygen concentration (or, in some cases, the oxygen partial pressure) to warn of oxygen deficiency or enrichment conditions. Both conditions may pose dangers to health or safety. Either excessively low or high O2 concentrations are problematic. Dry air is composed of about 21 percent O2 by volume, but numerous situations may result in O2 deficiency, which OSHA’s confined spaces standard for general industry defines as O2 content < 19.5 percent by volume. The standard requires the testing of confined space atmospheres to (among other things) verify that the atmospheric O2 content is acceptable. The OSHA standard for confined spaces in shipyards stipulates an acceptable upper concentration of 22 percent O2 by volume. Atmospheres with dangerously low O2 may occur as O2 is consumed without a contaminant being added to the atmosphere. Chemical oxidation (that is, rusting) is possible in enclosed spaces where steel or iron and water are present. The ferrous metals will oxidize in the presence of water and O2 until the available O2 is totally consumed, at which point the system becomes stable and rusting ceases. Combustion is a faster form of oxidation, usually accompanied by flame and smoke. In addition to consuming O2, combustion produces many byproducts, including some that may be toxic, so the drop in O2 percent by volume may also be accompanied by an increase in toxicity. Also, some chemicals can absorb or adsorb O2. For example, newly cast concrete will absorb O2 from the air as it cures. In closed systems that contain living organisms (whether aerobic bacteria or people), O2 will be consumed. Some of the many byproducts of metabolism may be toxic, so this also presents a case where a drop in O2 may be accompanied by an increase in toxicity. Carbon dioxide (CO2) is the chief toxic byproduct of human respiration and may also be produced by aerobic microorganisms. Atmospheric dilution with virtually any gas can also lead to low O2 conditions. Figure 1 depicts fluctuating O2 and CO2 concentrations measured in a new-construction manhole (not yet connected to an active sewer system) following the death of a worker there. This figure shows that atmospheric conditions in the confined space appeared to be safe on some days and were deadly on others; the death could have been prevented with proper testing of the atmosphere within the manhole.