The DUI Breath Sampling Process. How The Machine “Looks” at the What A Person is Blowing  Into the Breath Machine

Under the protocol for the administration of a breath test in South Carolina, after a twenty minute “observation period” to allow any residual mouth alcohol to evaporate through normal respiration, a 120 second period is allotted  for a person to provide a breath sample.

South Carolina law provides that a person must provide an  “acceptable” breath sample.

Acceptable to whom or what?

This is where things get really interesting.

A refusal to blow clearly occurs where the person being tested states “I refuse” and never places his mouth on the mouthpiece attached to the breath tube when offered the chance to do so. This identification of a “refusal” isn’t complicated. Also, if a person simply sits silently in the chair by the breath test machine, and the 120 second window to provide a sample expires, that is a clear refusal.

This 120 second period is denoted by an audible beeping. When a person IS blowing into the mouthpiece, this beeping tone changes to a steady tone. This establishes that the person is blowing at a minimum flow rate of 2.8 liters per minute, which is the first required parameters set by the machine manufacturer for a “valid sample.”  This means that a person IS blowing into the mouthpiece attached to the breath tube leading into the machine. This parameter assumes that every person can blow at a minimum rate of 2.8 liters per minute. This is determined by the mass pressure sensor or thermistor in the device.

Is It Possible for A Person to Blow Into the Machine (Produce Steady, Audible Tones) and Despite That Fact, The Officer Conclude That He Refused to Blow?

Yes. That is where things get complicated. It may very well be the case that, for reasons difficult to easily discern, the “machine” did not “like” the breath being induced into the machine. How can that happen? Is acceptance by the breath alcohol machine of a person’s breath sample dependent upon aspects the person cannot control?

Well. assuming that the person meets the minimum flow rate, a second required parameter for a “valid sample” is that a person must blow a minimum volume of breath of 1.5 liters during any breath segment.

Most of the time, a person being tested does not know a Datamaster DMT model from a mixmaster in the kitchen. He or she has no idea at all what these two parameters, flow and volume, are all about-and they are not told by the officer administering the test. Even if they were told, it is unlikely that they would understand the concepts-certainly not in a way they could follow. Unfortunately, some lawyers taking DUI cases haven’t the foggiest idea of any of these issues.

Officers administering the breath tests and not trained in any of these issues as well. They are simply trained to push the buttons on the machine to make it function. Accordingly, what they advise a person being tested, about how to blow into the mouthpiece can vary.

In fact, some officers do and have advised a person to “blow hard” when this is precisely what the machine manufacturer advises in its manual against. That manual, unless it is rewritten since this issue is coming to light, states that a person should NEVER be told to blow hard.

Some persons tested are told to blow “until I tell you to stop,” which is a concern I will address in another blog post. Suffice it to say here that this advice is in direct contravention of the third required parameter for a “valid sample.” That third parameter is the opposite of the first required parameter. The person’s flow rate must drop below 2.8 liters per minute.

If these three required parameters are met, then the machine looks to the fourth required parameter which turns to the machine’s quantification of the alcohol in the breath sample. The machine is “looking for” the rising alcohol values in the breath sample to “plateau” or level out indicating that particular part of the breath sample is deep-lung, or alveolar air.

Deep Lung or Alveolar Air


The fourth required parameter. When the 120 second window begins for a person to provide a breath sample, the machine starts “looking at” the breath. Every quarter of a second during this period, the machine is capturing a “data point” for the breath flow rate and for the alcohol measurement. Of course, is the person is not blowing or has not started blowing, the data points for breath flow and alcohol value would both be zero.

But as a person begins to blow, and meets the first required parameter of providing a minimum flow rate of 2.8 liters per minute (detectable by the audible tone changing from beeping to steady), data point values for flow rate and alcohol are collected and stored. a calculation is being made associated with the second required parameter-the total volume amount amassing during a single breath or expiration of breath.

As mentioned above, that must reach a minimum of 1.5 liters total expended breath during a breath, while the flow rate continues at the rate of 2.8 liters per minute.

These raw data points are collected and stored by the machine. Yet for reasons still to be determined, the manufacturer of the device has elected not to collect and save any flow rate data below 2.8 liters per minute. If the rate is below 2.8 liters per minute, the data point value is reflected as zero.  (Look for a future blow post on that topic.)

Consider these data points, and looking at required parameter three, the flow rate dropping below 2.8 liters per minute, move on to the fourth required parameter. When during a single expiration of breath, these three of four parameters are met, the device then “looks” to the last four data points of alcohol measurement. Remember, these occur in quarter-second intervals.

This is to determine if the rising, measured value for alcohol in the breath has reached a “plateau” reflecting that the breath expelled has finally reached deep-lung or alveolar air, the 2100:1 rule can apply, and the last alcohol data point can be considered a valid and accurate measurement of the amount of alcohol in a person’s blood.

Those last four data points reviewed must meet this requirement: the average of the last two points, compared to the average of the next to last two points must be .001 or less. If not, the machine continues to analyze the breath, not just as long as someone is blowing, but even when not. If not blowing, breath that is left in the sample chamber continues to be analyzed for alcohol content.

What Else is Going On As A Person is Blowing into the Machine?

This is beginning to go into more depth than I wanted to present in one blog post, but suffice it to say, that through the constant assessment of the breath sample, a number of other calculations are being made.

The manufacturer of the device insists that no breath alcohol result should be used as evidence in court unless it is a “valid sample.”

Well, of course, this suggests that an “invalid sample” might exist and occur. I will address this aspect in another blog post.

But other circumstances can occur as well. There are several. RFI or radio frequency interference with the testing process might occur. Alcohol in the ambient air in the testing room might be detected. The simulator might fail as to expected ethanol value or temperature. A pump might fail. Voltages might be off.

More about this issue in detail to follow.

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