Beyond the Rule of Thumb
Survival Tip 54
On a cold evening in January, your engine is dispatched to a report of a carbon monoxide (CO) alarm that has activated in a single-family structure. On arrival, you discover that all three residents have evacuated and are being dealt with by EMS personnel who had arrived earlier. One of the medics asks you to “check out” the interior and determine the level of carbon monoxide present. Using the four-gas atmospheric monitor assigned to your unit, you determine the highest level of CO found inside to be 76 parts per million (ppm). Shortly after you complete your assessment, the media informs you that all residents have refused treatment and that they are eager to go back inside. Based on the readings obtained with your meter, should you allow them to return?
General Use of Atmospheric Monitors
The most common type of atmospheric monitor first responders use employs one or more sensors with each sensor intended to detect a specific target gas or perhaps flammable vapors. Many of these devices are commonly referred to as “four-gas meters,” indicating that they have four sensors, typically one each for flammable vapors (combustible gas indicator), oxygen, carbon monoxide, and hydrogen sulfide. The sensors are interchangeable, and some departments instead use those capable of detecting gases such as ammonia or chlorine.
Interpreting Numerical Readings
The basis for interpreting numerical readings from atmospheric monitors involves comparing the reading to a known standard usually published by a regulatory agency such as the federal Occupational Health and Safety Administration (OSHA) or research organizations such as the National Institute for Occupational Safety and Health (NIOSH). Simply put, the reading will either be equal to, below, or above the standard.
To help explain this better, refer to Figure 1, which includes a chart based on a gas with an LEL of 20 percent. The chart is read left to right. As with the readings discussed earlier for methane, for this particular gas, a reading of 5 percent LEL is 1/20th of the concentration in air that would be needed to reach the LEL (1/20th of 20 percent LEL is equal to 1 percent concentration). Likewise, a reading of 25 percent LEL is equal to 1/4th of the LEL concentration of 20 percent, or a 5 percent concentration of the gas in the atmosphere.
Use of Correction Factors
Another concern with using a combustible gas indicator is that readings provided by the monitor are accurate only when attempting to measure the same gas as used during calibration procedures. Since the sensitivity of combustible gas sensors varies with exposure to different types of atmospheres, any attempt to measure the concentration of gases other than that used during calibration will result in a reading that is likely greater or less than the actual concentration. Remedying this situation willrequire the use of a correction factor or relative response curve specific to the gas or vapor measured to obtain more accurate results.
Zero Readings and Interference Gases
When you consider the potential for problems associated with “zero” readings and the fact that most atmospheres contain numerous contaminants and that some of these can “interfere” with the readings displayed, the process of interpreting readings from atmospheric monitors becomes more complex.
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