The State’s primary witness in most DWI cases in North Carolina is not a human person, but rather the breath-testing device used to prove a person’s alcohol concentration.

Attacking the credibility and accuracy of this non-living witness is essential to defending a DWI case.

This article is a brief introduction into just a few attacks that can be mounted on the breath test results. Effective DWI representation requires continual devoted study of the technology and science behind breath testing.

Machines Break

Remind your jury that machines malfunction.

“Have you ever taken a child’s temperature and knew that the reading was not accurate?”

This is a particularly effective argument in cases where the State would have difficulty showing appreciable impairment.

“You heard from the officer that my client did well on field sobriety tests, did not have slurred speech or sway while standing, had no problems communicating with the officer, and was polite and cooperative?”

Why trust an arbitrary number from a machine instead of all the observable evidence that would tend to show a lack of impairment?

Observation Period

Harp on the observation period.

Under the North Carolina Administrative Code, “Observation Period” refers to the 15 minute period immediately prior to a breath test that the chemical analyst makes sure that the person has not eaten, drank, vomited, or smoked. 10A NCAC 41B .0101.

Most officers will explain that they are trained to restart the observation period if someone so much as burps during the observation period.

They are also trained to have a person remove tobacco and gum from their mouth during the entirety of the observation period.

The Observation Period is one of the safeguards in place to protect against a false high reading. Emphasize to your jury the precarious nature of a machine that depends on a person not having burped in the last 15 minutes in order to give an accurate result.

Point out to the jury that no Observation Period is required when the chemical analyst requests a blood draw, because no mouth alcohol contamination is possible in the event of blood.

Also, point out that the breath testing instrument is designed to protect against mouth alcohol and should indicate the same if any mouth alcohol is detected. If we so trust the machine to detect mouth alcohol when is an observation period required?

For many experienced chemical analysts, a mouth alcohol detected message will have been seen at some point during their years of running breath tests.

In that case, how can an officer explain that he did not see anything unusual during the observation period? Short answer is he can’t.

Henry’s Law

To attack breath testing it is absolutely essential to have an understanding of Henry’s Law and the theory that breath testing is based on.

In short, alcohol is a Central Nervous System (CNS) depressant and will not cause impairing effects until it reaches a persons brain. Alcohol reaches the brain by travelling through the blood. This is why a person’s blood alcohol concentration is important to show a person is impaired.

Henry’s Law states that if a gas and liquid are in a closed container, at a constant pressure, and in a state of equilibrium, the concentration of the gas in the air above the liquid is proportion to the concentration of gas which dissolved in the liquid.

As applied to DWI investigations this science begins to break down immediately, because in the case of breath testing the closed container is a human body (specifically the lungs). Also, breath testing assumes that the temperature of exhaled breath will be 34°C. “Equilibrium” means a state of stable conditions in which all significant factors remain more or less constant over a period, and there is little or no inherent tendency for change.

Henry’s Law as applied to breath testing suggest that if you can determine the amount of alcohol in a person’s lungs (i.e. their breath) you can estimate the amount of alcohol in their blood. The ratio that has been developed is that for every 1 molecule of alcohol you find in the breath, you will expect to find 2100 molecules of alcohol in the blood. Unfortunately this fixed ratio of 2100:1 does not accurately reflect variations in human anatomy and can result in breath testing overstating the likely amount of alcohol in a person’s blood.

Remind the jury that a human person is not a closed container, humans do not exist in a state of equilibrium, everyone has different lung capacity (not a constant pressure), people do not stay at a constant temperature, and an average ratio does not apply to every human person.

Study Henry’s Law and the flaws associated with applying that science to breath testing.

.02 Agreement

In North Carolina the State is required to collect at least two separate samples of breath from a DWI suspect which results must not differ by more than 0.02. 10A NCAC 41B .0322 (“If the alcohol concentrations differ by more than 0.02, a third or forth sample shall be collected.”).

This statutory safeguard of requiring two blows within .02 demonstrates North Carolina’s confidence with the accuracy of the breath results.

For a case where the reported alcohol concentration is .08 or .09, this acceptable accuracy range can be utilized to argue that a concentration less than the statutory limit is within that range.

When looking at the case through the lens of reasonable doubt, this statutory range creates a reasonable argument that the person’s alcohol concentration did not exceed .08.

Accuracy Check

The State constantly builds up its machine as being able to conduct appropriate self-diagnostic checks.

One such check is the accuracy check where a pre-made breath alcohol simulator solution in a dry gas canister is pressure forced into the breath-testing machine to verify the machine is testing the solution accurately.

Unlike a human person described in the Henry’s Law section above, a calibrated canister is a closed container capable of being maintained at a constant pressure and temperature in a state of equilibrium.

The irony is that even with the dry gas canister that is capable of relative predictability, the accuracy check can come back at either .07 or .08.

State v. Simmons

The pattern jury instructions for the substantive charge of DWI include language that “[t]he results of a chemical analysis are deemed sufficient evidence to prove a person’s blood alcohol concentration.” 20-138.1(a)(2) [emphasis added].

Most jurors interpreting this language may believe that if they are presented a breath test ticket indicating an alcohol concentration of .08 or greater that they are required to return a verdict of guilty. However, a statutory presumption that the State has proven its case seemingly flies in the face of a defendants presumption of innocence and the burden of proof in a criminal case being with the State.

Explaining the meaning of “deemed sufficient,” the Courts have found “the results did not create a legal presumption that Defendant . . . an alcohol concentration of 0.08 or more.” State v. Simmons, 698 S.E.2d 95 (N.C. App. 2010) (internal quotations omitted) (holding the breath “results did not compel the jury” to convict for DWI).

Request a special jury instruction based on Simmons that explains how the jury should interpret “deemed sufficient” and that reminds the jury that they are not compelled to a conviction based on the results.

Remind your jury that the North Carolina Supreme Court has ruled that “the Court cannot assume the infallibility and credibility of the State’s witnesses or the certitude of their tests.” Tell the jury that if the Court’s can’t assume the infallibility and credibility of the State’s primary witness, the Intox EC/IR II, and the certitude of its results, then neither should the jury.

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