Current breathtest devices are completely automated which means that these units have been made "Idiot Proofed" for police officers.   Being able to operate a breathtest device does not make the police Operator, a scientist,... by any stretch of one's imagination.  There is nothing in these breath devices do to adjust its outcome to what occurs, individualistically, in you, & there by this can cause significant error agaisnt you in a breath test.    

By your client's repeated intakes of alcoholic beverages &/or drugs, over time, YOU can become VERY TOLERANT to their drug effects on YOU.... which mean that he or she  has become RESILIENT to their drug effects.   Many individuals, by my testing on them since 1970, are not  actually impaired nor intoxicated, even when their % BAC was well above the Political Legal Limit of 0.08%, & the current adopted POLITICAL Agenda.   You have heard about: "being politically drunk, but not actually drunk"?  It happens, often, by my own observations, since 1970, due to alcoholic beverages being consumed at many social, fraternal,  & sporting events in America,... numerous times, throughout the year...many DWI defendants are very tolerant  This, also, has  been helpful in defending a bar or tavern in a Dram Shop case because so many drinkers are very tolerant & these individuals won't display outward drinking symptoms to an experienced bartender or a doorman, [or] to a host or hostess at a house party.   When  I started to help to defend DWI defendants, the Legal Limit was 0.15% BAC,... I was, v.  easily, able to show that many defendants were not intoxicated nor impaired, well over 0.20% BAC. The human body has not changed since then.

One major manufacturer of American beer produces Many Millions of 12 oz containers of their 'LITE' beer, each day of the year.   They, also, produce Many Millions of 12 oz containers of their 'Regular' caloried beer,each day .... & Many Millions of containers, each day, of their 'ICE' beer.  They don't even close down their production lines for Christmas or for the 4th of July!  And that is just one American beer manufacturer.    Americans drinking a lot of beer, each day, would be quite tolerant individuals, & not impaired at the state's overly simplistic Legal Limit.     
You can have street drug[s]  in your urine, without even using them. For MORE on my street, Rx, or OTC drug defenses,... boot up my website: www.streetdrugconsultant.com