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Bill C-46 Requires Fine Tuning of the Instrument to Set Ambient Threshold


Section 320(18) requires a ceiling of 10 mg/100mls for the ambient fail system. That threshold needs to be set and checked on each individual machine.

Assuming that the software version for a particular province fixes the threshold for ambient fail at 10 mg/100mls, the actual threshold is a function of where zero is (see previous blog entry) and most importantly the actual calibration of the instrument at 10 mg/100mls. The calibration curve for each instrument and each re-calibration of an instrument is unique. (See blog entries re cross-examinations on calibration and calibration curve.) And how do you calibrate at such a low level near the bottom of the measuring interval? That is a difficult task.

If you obtain disclosure of annual or periodic maintenance records, you may find evidence of drift at the lower end of the calibration curve. For example, 50 solution may read on the instrument as 43, more than 10% off what it should be. If you extrapolate that bias to the equivalent of 10 solution the bias may render the reliability of the instrument at 10 mg% quite suspect.

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