When evaluating dry gas standards, have you ever noticed the large “C” on the label of your dry gas tank?  Or maybe your tank has a large “U” on the label.  The “C” and “U” on the label stand for Corrected and Uncorrected.  Here’s what that means. 

“Uncorrected” ethanol gas is manufactured using a scientific formula that calculates how much ethanol in ppm (parts per million) is necessary to achieve the desired BrAC level.  The scientific formula works 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.

The effect is predictable; an uncorrected tank will give readings on a fuel cell breath test device that are lower by about 4% than readings obtained with a wet bath simulator.  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 labelled with a large “C;” an uncorrected tank is labeled with a large “U.”

Your supplier of dry gas tanks should be knowledgeable enough to provide you with the appropriate tank for your application; most users may not even be aware that there are two types of gas.  But it’s a good idea to know something about this issue to be certain your program is using the appropriate dry gas tank.  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.