fine when the ethanol gas standards are used with infrared breath testing instruments— the ethanol gas standard will give the same reading as will a wet bath simulator with only insignificant differences. However, when an uncorrected ethanol gas standard is used with a fuel cell based instrument the ethanol gas will give a slightly lower reading than will a wet bath simulator with the same stated ethanol concentration. Te effect is predictable; an uncorrected
tank will give readings on a fuel cell breath test device that are lower by about 4 percent than readings obtained with a wet bath simulator2
. Manufacturers of ethanol gas
can correct for this phenomenon by adding more ethanol to the gas mixture so that the “corrected” ethanol gas tank will give the same reading on a fuel cell breath tester as will a simulator at the same alcohol concentration. Corrected ethanol gas is labeled with a large “C;” an uncorrected tank is labeled with a large “U.” (See Figure 3) The take-away on corrected vs.
uncorrected ethanol gas standards is this: When using a fuel cell breath test device, best practice is to use a corrected ethanol gas standard. When using an infrared device, use an uncorrected standard.
Adjusting for Effects of Barometric Pressure Te labels on ethanol gas standards state the ethanol concentration at the mean barometric pressure at sea level, or zero elevation. One of the more important things to know about gas standards is that the readings will vary with changes in barometric pressure. Day-to-day fluctuations in barometric pressure caused by weather systems may cause the ethanol vapor of gas standards to change relatively insignificantly by approximately 2–3 percent. However, using an ethanol gas standard
at elevations other than sea level can cause the expected results from ethanol gas standards to change by 20 percent or more. As elevation increases, barometric
www.datia.org datia focus 47 Figure 2
Figure 1
Figure 3
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