This was the pre-C2 "defence" (actually negating the presumption), of raising a reasonable doubt contradicting the approved instrument results through "evidence to the contrary", which in practice meant proof of drinking scenario plus toxicologist called by defence. In fact "evidence to the contrary" could include other unreliability issues. See the example in R. v. Phillips, 42 C.C.C. (3d) 150, ONCA, re partition ratio.