On an approved instrument that uses infrared (IR) spectroscopy as a measurement system, there is no true zero. The instrument reads a number between about 0 and 600 (e.g. 300) coming off its circuitry (on the Intoxilyzer 5000C this was called the DVM) and subtracts the DVM value of maybe "190" (where it thinks zero is) to produce a final result of 110. The 190 value is actually constantly changing depending on the stability of the electronics and the cleanliness of the sample chamber. An instrument that is badly in need of maintenance may have its DVM fluctuating wildly, thus sometimes triggering a diagnostics fail (processor error, processer positive or negative drift). On a properly operating instrument the zero is reset whenever there is an ambient air blank test. In other words, on a properly operating machine (or an improperly operating machine), if room air is compromised then zero is compromised, and the air blanks (checking for excessive ambient alcohol), the cal. checks, and the subject tests are all compromised. Since zero is always changing, even on a properly operating machine, with minimal ambient alcohol, there will be variability in the indicated results.