In Tennessee, if you are in possession over a certain amount of marijuana, you will likely be charged with felony. You can also be charged if you possess less than the threshold amount, but other circumstances are present that indicate the manufacture of or intention to sell marijuana--such as plants, scales, and baggies.
A breathalyzer does not actually measure blood alcohol content or concentration, which would require a blood sample. There are big differences between the small handheld breathalyzers used in the field, and the larger devices at police stations. Small breathalyzers may justify an arrest, but may not be admissible evidence in court. So how does it work?