In North Carolina, some people refer to the Alco Sensor FST as the “breathalyzer” or “PBT” (portable breath test). It is is a handheld breath alcohol screening device used by law
Both the Alco Sensor FST and the EC/IR II (products of the Intoximeters corporation) detect the presence of ethyl alcohol (ethanol) in breath using an electrochemical fuel cell. Under N.C.G.S. 20-16.3, the Alco Sensor FST is approved for roadside screening. A positive or negative indication for alcohol may be considered when determining whether there is probable cause to arrest.
The numerical result of the AlcoSensor FST is generally not admissible to prove a violation of N.C.G.S. 20-138.1. A numeric portable breath test value may appear in limited administrative proceedings, but it is generally not used to establish the elements of the prima facie elements of the criminal offense of driving while impaired in North Carolina.
In a N.C.G.S. 20-138.3 “after consuming” prosecution involving an under-21 driver, subsection 20-138.3(b2) permits use of alcohol screening results to determine whether alcohol was present, provided the test is performed with an approved device and in compliance with procedure.
Courts and agencies have treated that subsection as a specific authorization for zero-tolerance cases.
Whether the exact numeric reading from a PBT should be published at trial in cases like “underage DUI” remains contested, as the statutory element is presence, not a particular concentration or BAC.
Understanding how the AlcoSensor FST works, the science behind its measurements, and the legal limits on its use, are important in critically analyzing an impaired driving investigation.
This article examines the breath testing technology, the assumptions built into its design, and the reasons its numerical results should not be admitted to prove impairment in a North Carolina DWI trial.
How does a “Breathalyzer” Work?
At the core of the AlcoSensor FST is a small fuel cell coated with a catalyst known as platinum black. When a breath sample reaches the fuel cell, ethanol molecules are oxidized at the catalyst surface, releasing electrons and generating an electrical current. The device converts that current into a displayed numeric value.
The AlcoSensor FST can also display a simple “POS” or “NEG” (positive/negative) result instead of a number. This feature allows the operator to determine whether alcohol is present without seeing a concentration figure, reducing the risk of undue influence on probable cause decisions.
Reported BAC and the AlcoSensor FST
The displayed value on a PBT device is not a direct blood measurement. There is a technical difference between a BAC – Blood Alcohol Concentration and BrAc – BREATH Alcohol Concentration.
That’s an important distinction.
BrAC is an extrapolated value, estimating the true BAC.
It is the product of a chemical reaction in a sensor, processed through firmware (not subject to critical analysis), and presented as a number that is intended to correlate with alcohol in the body (within the bloodstream).
The correlation depends on several premises that deserve at least some level of scrutiny.
AlcoSensor FST, key premise #1:
The manufacturer (Intoximeters, the corporation that also sells the Intoximeter EC/IR) designs the device to wait for sustained exhalation and then to capture a breath portion later in the blow.
The theory is that the longer the subject blows, the sampled breath better represents air that has exchanged in the tiny sacs of the lungs known as alveoli.
Defense lawyers and other legal professionals may refer to that as a “deep lung sample” or “alveolar sample.”
One would be remiss in failing to note that how a person breathes during a DWI stop and resulting impaired driving investigation, can change the sample that reaches the sensor.
Breath holding (hypoventilation) can raise a reading by allowing more ethanol to diffuse into the airways. Unfortunately, law enforcement is often taught to instruct the subject, “take in a deep breath, hold it, and then blow,” thereby almost certainly elevating the reported BAC.
The duration of the breath hold, the hypoventilation, is directly related to and affects the resultant BrAc – Breath Alcohol Concentration.
The converse is also true. Hyperventilation can lower a reading breath reading.
Both deep and shallow exhalation (even panting at an accelerated rate) may never deliver what the device expects.
The AlcoSensor FST is also notoriously susceptible to false high results due to residual mouth alcohol.
Burping, regurgitation, or recent use of an alcohol-containing product can flood the mouth and upper airway with alcohol, which are different from air that has equilibrated in the lungs.
The device tries to infer that the sample is suitable by monitoring flow, pressure, and time.
It does not confirm the physiologic origin of that air, to wit: a deep lung, alveolar sample or gases emanating from the gastric tract including the stomach esophagus.
AlcoSensor FST, Premise #2: Selectivity
Fuel cells are thought to be relatively selective for ethanol (ethyl alcohol or “EtOH”), which is one reason police use them in the field.
That said, fuel cells are electrochemical sensors, not miniature laboratories.
They can respond to other short-chain alcohols (defined by the number of CH2 – methylene groups), and they do not separate compounds the way a laboratory device would.
This is not a flaw so much as a design tradeoff.
A roadside PBT device prioritizes portability, speed, and ease of use.
Those priorities can be valuable during an investigation, but they also explain why the law does not treat a roadside readout as courtroom proof of an alcohol concentration.
The Alco Sensor FST purportedly contains safeguards that serve field reliability rather than legal admissibility.
The Alco Sensor FST does not perform an automated “air blank” the way the Intoximeter EC/IR II does in evidentiary testing.
There is no programmed purge and zero-point confirmation with a printed record before and after each test, including pre-calibration between testing.
At most, a police officer can confirm the display is clear before beginning. There is no manual methodology to observe the reading over time.
Earlier handheld models such as the AlcoSensor and AlcoSensor III allowed an operator to depress (and hold) the test button and see whether the fuel cell was still generating a reading from a prior breath sample.
That capability is not present on the AlcoSensor FST.
Like other fuel-cell devices, the FST can exhibit “holdover” effects, where residual breath sample and resulting electrical charge from prior samples very likely influences the next reading.
Repeated use without adequate recovery time (which is common at DWI checkpoints) can cause the sensor to respond sluggishly or display an inflated “carry-over” value unrelated to the current sample.
- The FST lacks a manual “residual check” feature older models had.
- Fuel cells can and do exhibit carry-over effects.
- Repeated sampling can cause measurable drift or ghost readings.
Operating temperature limits reduce out-of-range behavior.
Low-power warnings can help prevent erratic performance.
Those protections support screening.
They do not transform a roadside screen into a forensic analysis suitable for a jury.
Fuel Cell Degradation and Carry-Over in the AlcoSensor FST
Fuel cells are electro-chemical sensors.
Inside the AlcoSensor FST, a catalyst-coated surface oxidizes ethanol molecules and produces electrons, ultimately creating an electrical current that can be measured and displayed as a BrAC reading.
This is not a one-time, perfectly repeatable process.
Over time, the catalytic layer can lose efficiency, and the electrical characteristics of the cell can change.
This “tiring” effect is well documented in fuel-cell devices.
The more samples they process, the longer the recovery time required for the sensor to return to a true baseline.
Older portable devices, such as the AlcoSensor and AlcoSensor III, gave officers a very simple, reliable way to check for residual “hold over.”
By holding down the test button, an operator could see whether the fuel cell was still stripping electrons from prior exposure and generating a reading even without a new breath sample.
The AlcoSensor FST does not have that function.
Once the operator starts the next test, the device assumes the fuel cell is at baseline.
In reality, if the sensor still contains residual alcohol from a previous test, or if the fuel cell has not fully recovered, the electrical output from the cell can be higher than it should be.
This can create a holdover or carry-over effect, where part of the reading in the next test comes from leftover charge, not from the subject’s breath.
Frequent, back-to-back testing, especially in cold weather or in high-volume enforcement situations, can magnify this problem.
Carry-over is not the only concern.
As fuel cells age, they can respond more slowly, produce less consistent output, and become more sensitive to environmental contaminants.
In the context of roadside screening, that means the displayed value is only as reliable as the device’s maintenance history (which is rare, if not nonexistent in North Carolina), the spacing between prior uses, and the physical condition of the sensor itself.
From a medico-legal standpoint, these factors matter for two reasons.
First, they go directly to the scientific reliability of the reading, which is the foundation for any later use of the test result.
Second, they highlight why North Carolina law should, at best, treat the AlcoSensor FST as a probable-cause tool, not an evidentiary chemical analysis.
A device that can produce inflated values due to residual charge or degraded sensor performance is worrisome, at best.
Problems with the AlcoSensor FST
The AlcoSensor FST is built for screening.
That purpose explains its strengths and its limits.
You get a portable device that can quickly help an officer decide whether alcohol is present.
You do not get the full chain of safeguards that North Carolina requires for a chemical analysis that comes into evidence.
The North Carolina Checkpoint Law, N.C.G.S. 20-16.3, allows use of an approved screening device and allows a court or agency to consider whether the result is positive or negative for alcohol when evaluating reasonable grounds (probable cause) to arrest.
The statute does not allow the State to use the numeric alcohol concentration from that device to prove a violation of G.S. 20-138.1.
That is not a loophole. It is a policy choice.
For more information about the North Carolina Administrative Code relative to breath testing protocols and approved devices, download for free our North Carolina DWI Quick Reference Guide.
The legislature drew a clear line between screening in the field and proof in the courtroom.
The technology supports that policy line.
In evidentiary testing, two sequential breath samples must agree within a narrow window (.02 of one another).
If they do not agree, the test is not valid.
The EC/IR II uses a fuel-cell sensor to quantify alcohol concentration. Its infrared component does not produce a second BAC result.
Instead, the IR system monitors sample quality, purportedly verifying deep-lung breath and detecting mouth alcohol and triggers the fuel cell when appropriate.
North Carolina procedure ensures accuracy through sequential sample agreement (within 0.02) and built-in diagnostics, not by duplicate BAC measurements on the same sample
The AlcoSensor FST also does not create the kind of detailed record that evidentiary testing produces, such as a printout with control checks, diagnostics, and maintenance information tied to the exact test event.
Those differences matter because they bear on repeatability, reviewability, and the ability to reconstruct the test later in court.
Sampling conditions are another concern. The device expects a steady, continuous exhalation.
Real roadside encounters are not always steady.
Cold weather, wind, anxiety, or a subject’s pulmonary limitations can disrupt the blow.
If the officer does not follow a proper observation period after possible mouth alcohol reading, the reported BAC can reflect alcohol lingering in the mouth or esophagus rather than alcohol from the lungs.
Even with an observation period, reflux or belching can reintroduce alcohol into the mouth moments before the test.
None of this means the device is useless. It means you should treat a displayed value as a screening signal that needs confirmation under the procedures that North Carolina recognizes for evidence.
Environmental and operational factors can also influence performance.
Finally, selectivity must be discussed honestly.
Fuel cells are built to favor ethanol. They are not immune to other volatile organic compounds or “VOCs.”
Certain medical conditions can alter breath chemistry.
Ketosis can raise acetone levels. Gastroesophageal reflux disease can introduce recently ingested alcohol into the mouth.
Alcohol-based mouth products can leave residue that a short waiting period does not fully clear.
None of these explanations fit every case. They are simply realistic variables that a court should consider when deciding how much weight to give a roadside screen.
What the IR component of the EC/IR II actually does
- The Intox EC/IR II uses both a fuel-cell (electrochemical) sensor and an infrared (IR) sensor, but each serves a distinct function.
- The fuel cell is the analytical workhorse. It quantifies alcohol concentration by measuring current produced when ethanol is oxidized.
- The infrared sensor does not generate an independent BAC value (at least one that is recorded, printed, and/or observable by the LCA – licensed chemical analyst). Instead, it is thought to continuously monitor the quality of the breath sample. It is said to detect mouth alcohol and verify that a deep-lung (alveolar) sample is being delivered. Again, purportedly, the IR aspect of the EC/IR II also helps identify the correct moment to trigger the fuel-cell sampling.
- Neither operation duplicates the BAC result in a separate analytical stream. The EC/IR II’s IR pathway does not provide a BAC, it is intended to ensure the proper type of sample is being analyzed and help guard against contamination like mouth alcohol.
So when someone says the device runs two BAC measurements for accuracy, that is not correct for the EC/IR II.
It does take two sequential tests in short order, using the same electrochemical fuel cell. The reported BAC as measured by the fuel cell is not compared, confirmed, or contrasted with an infrared BAC reading.
Older IR-only models like the Intoxilyzer 5000 did rely on IR for quantifying alcohol.
For North Carolina’s EC/IR II, the infrared is a sampling-quality monitor, not a secondary quantifier.
It should be.
Field use of the PBT in North Carolina
On the roadside, the AlcoSensor FST is a decision tool.
An officer uses the device to help decide whether there is probable cause to arrest and whether to proceed to an evidentiary test.
The device belongs in the investigative phase, not the proof phase.
Once probable cause is established, North Carolina relies on an evidentiary breath test at a facility or a blood draw performed under state procedures.
Those methods bring with them an extended observation period, internal checks, and the requirement that two sequential breath results agree within a relatively close margin (.02).
The State’s device at the station produces a printed record designed for later review.
The roadside device does not.
The practical point is simple.
When you read a police report that lists a number from an Alco Sensor FST, treat that number as an investigative clue, not evidence.
The Intoximeter FST Portable Breath Test: Practical takeaways for North Carolina cases
The Alco Sensor FST helps an officer screen for the presence of alcohol.
It can be useful when used properly and within its design purpose. It is not a laboratory device.
The PBT is is not the evidentiary test that North Carolina relies on at trial.
The numeric readout from the Alco Sensor FST is not admissible to prove a violation of G.S. 20-138.1.
DWI Defense Lawyers: Powers Law Firm
Powers Law Firm defends clients accused of impaired driving in Mecklenburg, Union, Iredell, Gaston, Lincoln, and Rowan Counties, and in select cases involving felony-level DWI-related charges such as Felony Death by Vehicle or Felony Serious Injury by Vehicle across North Carolina.
Our work often involves examining the science behind portable and evidentiary breath testing, as well as other technical evidence.
Every case begins with a detailed analysis of the circumstances and applicable law, with scientific review incorporated whenever it can strengthen the defense. To speak with Bill Powers in confidence, TEXT or call 704-342-4357
About Bill Powers
Bill Powers is recognized across North Carolina for his work in defending impaired driving cases. His teaching experience includes regular instruction at the Charlotte-Mecklenburg Police Training Academy, as well as serving as chair of the 2025 North Carolina Advocates for Justice Criminal Masters of Advocacy. He has been a member of the Governor’s Statewide Impaired Driving Task Force since 2014.
In 2025, Bill Powers was named among the “Top 100” lawyers in North Carolina and the “Top 25” in Charlotte by Super Lawyers® North Carolina Magazine. He is also listed in Best Lawyers in America® for 2025, with Powers Law Firm PA recognized in Best Law Firms in America® in the same year. In 2020, the North Carolina State Bar honored Bill Powers with the John B. McMillan Distinguished Service Award for his contributions to the legal profession and community.
For more on the selection and inclusion criteria for these honors, see the Super Lawyers® North Carolina Magazine Selection Process and the Purely Peer Review® Methodology and Process for Best Lawyers in America® and Best Law Firms in America®.