Although an approved instrument has been properly maintained and operated, and is functioning in accord with manufacturer's specifications, there may exist unusual ambient or physiological presentations not contemplated by the design of the instruments. In those unique circumstances, the results of the analysis may appear to be reliable yet are false or exaggerated positives. See R. v. St-Onge Lamoureux at paragraph 78 and R. v. Kasim 2011 ABCA 336.
As a result, innocent persons may be convicted of "80 and above" in Canada. Section 320.31(1)(a) of the Criminal Code, which results in "conclusive proof", should therefore be challenged constitutionally, since it does not provide for any defence of reasonable doubt as to reliability of the measurement result. Such reasonable doubt is a real possibility where there are unusual ambient or physiological presentations not contemplated by the design of the instruments.
See also our Humour and False Positives Video links
You can learn more about these ambient and physiological presentations by taking the online courses described in the "Members" section of this site.