The portable one is used as an indicator.

A positive result will get you arrested and taken to the station, where they have the (non-portable) court admissible calibrated kit.

Why would a certified calibrated breathalizer test be inadmissible in court? How is it any different from catching speeders with a laser gun, or doing a DNA test?

And if giving every cop a calibrated breathalizer is too expensive: give them a reasonably-accurate one for in the field, then take everyone who fails it to the station for a retest on an expensive calibrated one.

This is changing. Most states have “permanent” properly calibrated breathalyzer at every dui checkpoint now. And in an increasing number of regular vehicles