Skip to content
Crimes | Types of Criminal Charges » DUI Defense in Minnesota » Breath Test Machine | Minnesota DWI Defense

Breath Test Machine | Minnesota DWI Defense

The government likes to call it a “breath test.” But does their machine pass the test of science? Is it reliable and scientifically valid?

Breath Test Machines for alcohol are unreliable and invalid

The basic idea behind the machine is that alcohol vapor in a person’s breath blocks infrared light.

The machines trap a person’s breath in a glass tube. Then, it puts infrared-wavelength light through it. And sensors on the other side of the tube detect how much of the infrared light made it through.

The machine presumes that alcohol blocks any infrared light that didn’t make it through. Problem? Lots of reasons other than alcohol can cause a false reading.

ETOH Alcohol Dog Molecule
ETOH Alcohol Dog Molecule

Trial by Machine – the Machine Age


We know that many factors can cause the machine to report a greatly inaccurate alcohol level. And one of these is the presence of interferents in the target’s body and breath.

Thousands of common household and industrial (work) chemical products contain Volatile Organic Compounds (“VOCs”). VOCs absorb into the target’s blood, and later dissipate from the person’s body. And VOCs dissipate out of their lungs and breath.

We call them “interferents.” They interfere with the operation of infrared breath alcohol machines. Interferents block the infrared light the same way alcohol molecules do. They cause a false reading for alcohol.

Infrared Breath Test: Volatile Organic Compounds (VOCs)
Infrared Breath Test: Volatile Organic Compounds (VOCs)

Other physical causes of false readings on a breath test

Other problems with the breath test machine include:

  • radio interference,
  • inadequate observation periods by police,
  • mouth alcohol contamination, including that caused by acid reflux,
  • diabetes and rapid weight loss (ketones as an interferent),
  • environmental factors, during administration, as well as
  • other factors relating to the targeted person.

Motive: In addition, the law rewards police for labeling a person “a refuser.” True or not, the label saves them time and work. They can then skip the breath test. And a police officer can also use the “refusal” label to punish a driver. Because penalties are worse for refusal.

So the law gives police a motive to abuse their discretion and call someone a “refusal.”

Not only that, the State actually trains police to blame the driver, when a machine malfunctions. These malfunctions include mouth alcohol, blocked breath tubes, etc. But rather than try to troubleshoot the machine malfunctions, just blame the driver. However, a good DWI defense attorney can spot this.

General themes of DWI litigation are:

  1. the government valuing expediency above justice; and
  2. deliberate efforts by the government to ignore the truth, hide the truth, and deny accused drivers access to the truth.

This contradicts a core purpose of the court: “the search for the truth.” There are many examples of this. Yet thousands of innocent Minnesotans suffer as a result of false “evidence” every year.

How does the government deliberately try to hide the truth? The destruction of exculpatory evidence is one way. Curtailing the right to pretrial discovery of evidence in “implied consent” driver’s license cases, is another.

Context: Breath Test Machine vs Blood & Urine Chemical Tests

Minnesota has three types of chemical tests for alcohol in drivers. And each requires a search for a sample of the subject’s bodily fluid or gasses.

The three types are tests of a breath, urine or blood sample.

A lab must be test urine and blood samples. And that takes time. It also costs more money for the state than a breath machine test. So, for more on the urine test, see: Minnesota urine test.

Common confusion: two types of breath test machines

Most Minnesota DWI arrests involve two different breath tests. And this confuses drivers. Police, lawyers, and judges tend to be blind to this reality. So, what’s the problem?

What is the PBT?

The first breath test machine a driver encounters is a Preliminary Breath Test machine (or Portable Breath Test, “PBT”). The PBT is a screening test machine.

PBT, Portable Breath Test
PBT, Portable Breath Test

A screening test is too inaccurate for use as evidence in a trial. But it’s accurate enough for use as a screening test for a DWI arrest.

As with any breath test machine, a PBT is vulnerable to machine errors as well as human and environmental errors.

After a traffic stop, if the statutory preconditions exist, police may request that the driver blow into a PBT. If the driver then refuses, police can arrest the driver. And if the driver allows the search and blows 0.08 or more, police arrest the driver.

Police use the PBT to support probable cause to arrest. But the prosecutor cannot use it in the criminal trial.

What is the Evidentiary Breath Test machine?

Minnesota now uses a Datamaster DMT Breath Test machine as its evidentiary breath test machine. The state views it as accurate enough for use as evidence of breath alcohol concentration.

But it has many problems that make it inaccurate and unreliable. We already saw some of them above.

Minnesota DMT Breath Test machine
Minnesota DMT Breath Test machine

They “meant to do that!”

Defective Minnesota version: The Minnesota DMT has both infrared and fuel cell technology, as an accuracy check.

But when the two technologies consistently failed to match, what did the state do? Fix it? No. They simply turned off the fuel cell technology in the machine! That was years ago.

So now the DMT uses only infrared light technology, just like the old Intoxilyzer machine did.

Driver confusion: “Is this a trick? I already did a breath test!”

Driver confusion arises when a police officer asks a driver at the police station: “Will you take the breath test?”

Many say: “But I already did, at the side of the road. You want another one?” And sometimes this leads to the police officer deciding to label the driver as a test refusal.

You see, the normal DWI investigation includes a handheld, PBT breath test at the side of the road. And then comes a larger, tabletop, DMT breath machine at a police station. But the two “breath tests” confuse the average driver.


The laws are too complex. But, it makes sense to learn about them ahead of time. Of course, avoiding driving near or above the arbitrary 0.08 alcohol concentration is the best defense. But what is plan B?

For an in-depth look at prevention, our article: Countermeasures at a DWI Stop: the Party Question.

For now, a few highlights:

  • Never do field exercises, Field Sobriety Tests, like “Nine Step Walk & Turn,” or “HGN watch the pen.”
  • Always call a lawyer before consenting to a breath blood or urine sample for chemical testing
  • Always get an independent, Additional Test of your blood or urine soon after.


If it’s too late for prevention, you need a good DWI defense attorney for the cure.

So, we look for causes of an inaccurate reading as well as police mistakes that hurt the driver. And when we find them, we can win the DWI case for our client.

DWI Defense Attorney Thomas Gallagher

Attorney Thomas Gallagher has 35 years of experience winning DWI cases in Minnesota.

And Thomas Gallagher has taught many CLE classes to lawyers and judges on DWI law for years.

Gallagher-empowering-600 Minneapolis Criminal Defense
Contact DWI Attorney Thomas Gallagher about your case

Call DWI Defense Attorney Thomas Gallagher at 612 333-1500.

He helps clients remove alcohol-related incidents from their drivers license records. And this saves them money – much more than attorney’s fees.

For some clients with prior convictions, their priority is avoiding prison or jail. And DWI Attorney Thomas Gallagher helps them do that. Sobriety is a big help, but there is more to it.

DWI Frequently Asked Questions: Minnesota DUI Law

Minnesota DWI posts

Guide to DUI Defense in Minnesota

Call Now Button