The primary purpose of the third digit on the Alco-Sensor III display is to enable more accurate interpretation of test results and more accurate calibration of the instrument. The industry standard for an evidential-quality breath alcohol tester is to read an alcohol level of .100 with an allowable error of +/- 5%. Readings from .095 to .105 are within this allowable margin of error. Only instruments that display results to three decimal places are capable of demonstrating this level of accuracy. An instrument that displays results to only two decimal places (.09, .10, .11, etc.) cannot meet the requirements of an evidential device.
In practice, many alcohol test results are reported to two decimal places: for example, .08, .02, and .04. When using an Alco-Sensor III and reporting results to two decimal places the operator should drop the third digit, rather than rounding up. For example, a result of .086 would be reported as .08, and a result of .009 would be reported as .00.
Does a reading of “.001” indicate alcohol use?
No. Even for programs with a “zero tolerance” policy we recommend using a cut-off level to define a positive test. When using an Alco-Sensor instrument we recommend a cut-off level no lower than .01 for substance abuse treatment programs, and no lower than .02 for workplace.
That means that for substance abuse programs a result of .009 would be treated as a negative test, and a workplace program would treat a result of .019 as a negative test. See our related article on zero tolerance testing for a more in depth discussion.