Breath Testing Instruments and the Records Behind a Result
A breath reading is not a direct measurement of blood. It is an inference from a gas sample, produced by an instrument whose accuracy rests on a chain of control solutions, calibration checks and maintenance entries that exist as documents.
The rule in short
A breath testing instrument measures alcohol in a sample of deep lung air and converts it to a blood-equivalent figure using an assumed partition ratio. The result depends on the instrument conforming to published model specifications, on an internal air blank and control check accompanying each subject test, on a documented calibration history, and on an observation period intended to exclude alcohol originating in the mouth rather than the lungs. Each of those conditions leaves a record.
A breath instrument does not measure blood. It measures alcohol in a sample of expired air and reports a number expressed in blood-equivalent terms. The conversion rests on an assumed relationship between the concentration in deep lung air and the concentration in blood, applied uniformly to every subject. That assumption is the foundation of the result, and everything the instrument does afterward — the blanks, the controls, the printout — exists to show that the measurement of the air sample itself was made under known conditions.
What the instrument actually measures
Two detection methods dominate. Infrared instruments pass a beam through a sample chamber and measure absorption at wavelengths characteristic of the ethanol molecule. Fuel cell instruments oxidize alcohol at an electrode and measure the resulting current. Some devices use both, which allows one measurement to check the other. Neither method identifies ethanol with certainty on its own; each responds to a class of compounds, which is why the specifications for approved devices address discrimination against interfering substances rather than assuming it.
The sample matters as much as the detector. An instrument is designed to analyze end-expiratory air from deep in the lungs, and it uses flow and volume criteria to decide when it has received such a sample. A short or interrupted breath may be rejected, or may be accepted at the edge of the acceptable range. Because the reported figure is an inference about blood from a measurement of gas, individual physiology sits inside the result in a way the printed number does not display.
Screening devices carried in the field and evidential instruments installed at a station are built for different jobs, and the distinction is often blurred in description. A handheld screening unit is designed to indicate whether alcohol is present and to inform a decision about whether to proceed. An evidential instrument is designed to produce a number intended to stand as a measurement, and it is subject to the printing, control and calibration requirements that a screening device is not. Treating a roadside indication as though it were an evidential result confuses two categories that the governing specifications keep apart.
Approval, specifications and the control checks
Instruments used for evidential purposes are tested against published model specifications, and only conforming devices may be used for confirmation testing under the federal workplace testing rules. Those rules are specific about what a conforming device must do: print the result in triplicate, assign a unique test number that is readable before the test begins, print the manufacturer's name, the serial number and the time, distinguish alcohol from acetone at the 0.02 level, run air blanks, and perform an external calibration check. Each requirement leaves an artifact on the printout.
Those artifacts are the most useful part of the record. An air blank run before and after the subject sample shows the chamber was clear, and a blank reading that is not zero is a visible problem. A control sample of known concentration, run alongside the subject test, shows the instrument reading a value it should already know. The control has its own paperwork: a solution lot number, a certificate stating the assigned value, and an expiration. A control result within tolerance is only as meaningful as the certificate behind the solution.
| Document | What it records | What it can show |
|---|---|---|
| Calibration and certification record | The adjustment or verification of the instrument against known standards | Whether the instrument was within tolerance, and when it last was |
| Control solution certificate and lot record | The assigned value, lot number and useful life of the reference solution | Whether the check accompanying a test used a solution of known value |
| Maintenance and repair history | Service visits, parts replaced, faults reported and cleared | Whether a recurring fault preceded or followed the test date |
| Printed subject test record | Blanks, control result, subject samples, time stamps, test number | Whether the required sequence was completed and in what order |
| Operator certification file | Training completion and authorization status of the person running the test | Whether the operator was authorized under the governing program |
A common conflation treats a valid certificate on the wall as proof that a particular reading was accurate. It is not. Certification records a condition observed on a past occasion under controlled conditions; the subject test is a separate event, and it either carried its own contemporaneous blanks and control or it did not. The same conflation appears in reverse when an instrument is described as out of service because a routine check was due. A missed service interval is a documentation defect and may or may not indicate a measurement problem. The two questions are distinct and the records answer them separately.
The observation period and mouth alcohol
Nearly every state program requires an interval during which the subject is observed before the evidential sample is taken. Its purpose is narrow and physiological. Alcohol remaining in the mouth or upper airway — from regurgitation, from belching, from a substance placed in the mouth, or from material trapped near dental work — is far more concentrated than deep lung air, and an instrument analyzing it will report a value that does not reflect blood at all. The waiting interval is intended to let any such residue dissipate.
The requirement is administrative rather than constitutional, and the details differ. Some programs specify continuous observation by the operator; others accept presence in the same room. The length of the interval and what interrupts it are set by state regulation, so a practice that satisfies one jurisdiction may not satisfy another. Instruments also include features aimed at the same problem, notably slope detection, which examines the shape of the breath curve for the falling profile characteristic of mouth alcohol rather than the plateau characteristic of lung air.
Interference, tolerance and how the record is examined
Approved instruments include protections against radio frequency interference and against detection of substances other than ethanol, and a detected interferent should abort the test rather than produce a number. Where those protections operate as designed, the printout says so. Where an instrument is producing readings without completing its checks, the printed sequence shows the gap. This is why the underlying test record is worth more than a certificate summarizing it, in the same way that an extraction report is worth less than the extraction behind it.
The showing a proponent must make is that the method is reliable and that it was reliably applied to this sample. Reviewing the second half requires the instrument-specific documents rather than general assurances, which is one reason a defense examination is often directed at the maintenance file rather than at the science. Obtaining that examination proceeds through an application for expert services, and the reliability of the underlying program is the subject of accreditation and its documented scope. Breath programs sit largely outside the accredited laboratory system, which is itself a distinction worth stating plainly rather than assuming away.
Points to carry away
- An evidential breath instrument measures alcohol in expired air and reports a blood-equivalent value using an assumed partition ratio rather than measuring blood directly.
- Federal confirmation testing rules require conforming devices to print a result, assign a unique test number readable before the test, run air blanks, discriminate alcohol from acetone at the 0.02 level, and perform external calibration checks.
- A screening device used at the roadside and an evidential instrument used at a station serve different purposes and are held to different specifications.
- The observation or deprivation period exists to exclude residual mouth alcohol from regurgitation, belching or a foreign substance, and its length is set by state administrative rule.
- Calibration certificates, control solution lot records, maintenance and repair entries, the printed subject test with its blanks, and the operator's certification are separate documents that can be produced and compared.
Questions readers ask
Why is a blood draw treated differently from a breath sample?
A blood draw measures the analyte of interest directly, while a breath instrument measures a proxy and converts it. That makes blood analysis less dependent on physiological assumptions, but it introduces a different set of records: the collection kit and its preservative, the storage temperature and interval, the chromatography run and its internal standards, and the analyst's worksheet. Neither method is self-proving. Each rests on documentation that shows the measurement was made under controlled conditions, and each fails in characteristic ways that the paperwork tends to reveal.
Does a certified operator make a reading reliable?
Operator certification establishes that a person completed a training program and was authorized to run the instrument. It says nothing about whether the instrument was in tolerance, whether the control solution was within its useful life, or whether the required observation interval was actually kept. Those are separate showings resting on separate records. Certification is worth confirming because an expired or lapsed authorization is a real defect under many state programs, but a valid certificate does not substitute for the instrument's own calibration and maintenance history.
What does a refusal to provide a sample change about the record?
It removes the instrument from the picture and shifts attention to the administrative process. Most states attach license consequences to a refusal through an implied consent scheme, and those schemes are creatures of state statute and administrative regulation rather than of the federal rules. Where no breath result exists, the case rests on observations, field testing and any blood analysis obtained by other means. The documents that matter then are the officer's report and whatever chemical analysis was performed, not a calibration file.
Sources
- 49 CFR 40.231, Devices used for alcohol confirmation tests (Cornell LII)Requires conforming devices to print results, assign a unique readable test number, discriminate alcohol from acetone at the 0.02 level, run air blanks and perform external calibration checks.
- Federal Rule of Evidence 702 (Cornell LII)Requires the proponent to show sufficient facts or data, reliable principles and methods, and a reliable application of them to the facts.
- Federal Rules of Evidence (Administrative Office of the U.S. Courts)The adopted rules text governing the admission of instrument-based results in federal proceedings.
- Cornell LII, Wex: Daubert StandardLists testing, error rate, and the existence and maintenance of standards controlling a technique's operation among the reliability factors.
- Organization of Scientific Area Committees for Forensic Science (NIST)Develops and registers discipline-specific standards, including for toxicology practice, through a consensus process.
- Strengthening Forensic Science in the United States: A Path Forward (National Academies)Found that forensic practice needed enforceable standards and systematic change to ensure reliability.
Premier Defense Law is a publication, not a law firm. This article states general rules and cites its sources; it is not advice about any particular case, and the law differs by state and changes over time.
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