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Breath Testing: What an “INVALID SAMPLE” reading means on a BAC DataMaster

Dayton DUI Attorney Charles Rowland > DUI Law  > DUI Process  > Breath Testing: What an “INVALID SAMPLE” reading means on a BAC DataMaster

Breath Testing: What an “INVALID SAMPLE” reading means on a BAC DataMaster


“I was standing by the machine.  It beeped and the officer told me to blow.  I blew and blew.  The officer got really mad and told me that I was messing with the machine.  He said I had to do it again.  The next time I blew again…really hard.  He told me that I was being difficult and told me that he was going to say I was refusing to take the test.  This was horrible.”

-Client Testimonial-


After hearing a story like the one above, the client hands you two pink BAC DataMaster tickets each showing a “invalid sample” reading.   You can assure your client that he or she was not doing anything wrong.  An “invalid sample” reading is NOT an indication that your client did not provide an adequate breath sample.

If your client was fake-blowing or sucking in on the machine, it would produce a different result.  The machine will produce an “INCOMPLETE TEST” or a “SUBJECT REFUSED? <y/n> response, depending on the particular software that the BAC DataMaster is using.  In fact, if the machine does produce an “invalid sample” when someone sucks in, it represents a failure of the one-way valve and is indicative of a faulty machine.  You should make a point of checking multiple calibrations to determine if the machine has been or should be taken out of service for repair.  The requirements of a breath sample for the BAC DataMaster are quite easy to obtain for a vast majority of the population.

BAC DataMaster Sample Acceptance Parameters:

1.       Minimum flow rate (~3.7 liters per minute) must be maintained;

2.       Increase from one two-consecutive point average to the next be less than or equal to 0.001 g/210 L;

3.       Minimum total volume of 1.5 liters must be delivered (that is less air than a two liter bottle of pop); and

4. Flow rate must drop below threshold (~3.7 liters per minute).

An “invalid sample” result does not mean that your client blew saliva or another foreign body into the sample chamber.  The plastic mouthpiece has a special chambered design which would prevent any saliva from getting into the chamber.   As counsel, you should also guard against jumping to the conclusion that the “invalid sample” result was due to G.E.R.D. (Gastroesophageal reflux disease).   This is but one of many pieces that you will need to construct a winning G.E.R.D. defense.

Whenever you have a client tell you that the officer accused him or her of manipulating the blow when the machine renders an “invalid sample” test this means that you have an ill-informed or poorly trained operator.  Keep this in your back pocket and move on the officer at the motion to suppress.  Lock in the fact that he did not know the cause of the “invalid sample” actually was.  This is particularly effective when you are able to present as your theory of the case that the officers “rushed to judgment.”  “Ladies and gentlemen of the jury, my client did not stand a chance.  See how they treated him unfairly on the field tests and then how wrong the officer was to accuse him of faking a blow.  Never once was my client treated as innocent until proven guilty.  You can make that right by being fair now.”


An “INVALID SAMPLE” response is an indication of mouth alcohol from the refluxing of alcohol containing stomach contents on the BAC DataMaster or alcohol trapped in the oral cavity.  If you can imagine that a normal breath sample is represented by a bell-shaped curve.  The presence of mouth alcohol sufficiently alters that bell shaped curve so as to alert the machine that it should kick the test out.  This is done by a part of the machine known as the “slope detector.”  A negative slope is characteristic of the presence of mouth alcohol. As the subject blows into the machine, the amount of alcohol in the mouth decreases, thereby decreasing the alcohol concentration the machine measures.  The machine samples the alcohol concentration in the sample chamber every ¼  seconds. A negative slope occurs when three consecutive negative comparisons of two-point averages are obtained.

Some of the cause of mouth alcohol may include: food, vomit, dentures or other foreign materials in the mouth which may contain alcohol, gastroesophageal reflux disease (GERD), or some drug use. Some examples are Nifedipine, Nitroglycerin, Nonsteroidal anti-inflammatory agents (e.g., indomethacin, ibuprofen, naproxen, etc.).

According to Dr. Alfred Staubus, “The machine is good at detecting mouth alcohol contamination if only mouth alcohol is present and there is little or no lung air alcohol. Slope detector often does not work if both mouth alcohol and lung air alcohol are present.”  It is always a good idea to have an expert get involved in an “invalid sample” case early as they may be of assistance in diagnosing machine issues, or in identifying a valid G.E.R.D. defense scenario.

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Charles Rowland


Charles M. Rowland II has been representing the accused drunk driver for over 20 years. Contact him at (937) 318-1384 if you find yourself facing a DUI (now called OVI) charge.

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