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The Blood Alcohol Content (BAC) for an Interlock Device is very low, around .025 (less than 1/3 of legal limit). It is recommended that a person does not eat or drink right before trying to start their vehicle as this may prevent them from starting their car.
Here's a great site that lists many of the faults with taking a BAC that your gentleman friend can use as a guide:
As for a % of false positives, it depends on the specific device. Some states won't use them, other states have devices that is even alcohol specific.
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