How Does A Breathalyser Work?
How can a person's breath show how much that person has had to drink?
Alcohol that a person drinks shows up in the breath because it gets absorbed from the mouth, throat, stomach and intestines into the bloodstream. Alcohol is not digested upon absorption or chemically changed in the bloodstream. As the blood goes through the lungs, a physiologically predictable amount of the alcohol will moves across the lung membranes and into the lungs themselves.
Once in contact with the air in the lungs, it evaporates and is exhaled. The concentration of the alcohol in the air in the lungs is directly related to the concentration of the alcohol in the blood.
The ratio of breath alcohol to blood alcohol is 2,100 to 1 (and called the partition ratio), so the alcohol content of 2,100 milliliters of exhaled air will be the same as for 1 milliliter of blood. The maths are simple from there and leads to blood alcohol readings expressed as a percentage of alcohol in the blood.
The partition ratio can vary between 1700 and 2400 depending upon the individual and local environmental conditions, leading to a breath analysis reporting either a higher or lower calculated blood alchol reading.
Since it is a physiological response, the partition ratio can be raised or lowered by:
Body Temperature: The widely used blood-to-air partition ratio of 2100 is based on a normal body temperature of 98.6°F. A higher body temperature of the individual will overestimate the actual BAC because of the higher volatility (or vapor pressure) of liquids like alcohol at a higher temperature. An elevation in body temperature of 1°C (1.8°F) results in a 7% higher value in the result. Therefore, a person with a body temperature of 100.4°F, and with an actual blood alcohol of 0.0935%, will register a value of 0.10% by the breath test. Be careful if you have a cold or the flu.
Cellular Composition Of Blood: Blood contains suspended cells (e.g. red and white cells) and proteins, and is therefore only a partial liquid. The partition ratio of 2100 is based on an average cell volume of blood of 47%. Cell volume values range from 42 to 52% in males and 37 to 47% in females. Therefore, a person with a lower cell volume will have a falsely elevated blood alcohol based on a breath test, however this variability has been estimated to be relatively small, ranging from - 2 to + 5%.
In most countries the partition ratio is presumed to be 2100:1 unless the defendant presents evidence that his/her personal ratio should be reasonably be assumed to be significantly higher or lower.
What Are Field Sobriety Tests?
Field Sobriety Tests (FST's) are psychophysical tests used to assess a person's physical and/or mental impairment. They focus on the abilities needed for safe driving. Most of the more reliable psychophysical tests are known as "divided attention" tasks. They require a person to concentrate on more then one task at the same time.
To safely drive a car, a person needs to be able to simultaneously control steering, breaking, and acceleration; react to constantly changing driving environment; and perform many other tasks. Alcohol affects one's ability to adequately divide attention, thus causing drivers to concentrate on more difficult tasks while ignoring simpler ones (i.e. ignore a traffic signal while concentrating on one's speed). Even if impaired, most people can successfully concentrate on a single task fairly well, but when impaired, most drivers cannot successfully divide their attention between multiple tasks at once.
Divided attention tasks are designed to evaluate mental and physical capabilities a person needs to safely drive a car. They include information processing; short-term memory; judgment and decision making; balance; steady, sure reactions; clear vision; small muscle control; and coordination of limbs. A good FST will combine any two or more of these capabilities simultaneously. A test must also be reasonably simple for the average non-intoxicated person to perform.
The most common FST's used by the police include the three standardized tests consisting of the Horizontal Gaze Nystagmus tests, Walk & Turn test, and the One Leg Stand test. These three tests have been validated as reliable indicators of intoxication, although they are not 100% accurate. Other commonly used, but non-standardized, tests include counting backwards, saying the alphabet (or a portion of it), finger count, and the stationary balance (Rhomberg) tests.
In reference to the three standardized FST's, the government has admitted, and it is printed in the police officers DWI manual, that "IT IS NECESSARY TO EMPHASIZE THIS VALIDATION ONLY APPLIES WHEN: THE TESTS ARE ADMINISTERED IN THE PRESCRIBED STANDARED MANNER; THE STANDARDIZED CLUES ARE USED TO ASSESS THE SUSPECTS PERFORMANCE; THE STANDARDIZED CRITERIA ARE EMPLOYED TO INTERPRET THAT PERFORMANCE. IF ANY ONE OF THE STANDARDIZED FIELD SOBRITY TEST ELEMENTS IS CHANGED, THE VALIDITY IS COMPRAMISED."
Horizontal Gaze Nystagmus (HGN)
This test refers to the involuntary jerking of the eye as it gazes to the side. When this occurs, the person is unaware of the jerking, and cannot control it. This involuntary jerking becomes noticeable as persons' blood alcohol increases. This is the most reliable of the FST's. However, nystagmus is a natural, normal phenomenon. Alcohol and certain drugs do not cause this phenomenon.
When the HGN test is administered, the officer hold a stimulus (usually a pen) 12-15 inches in front of you eyes and asks you to follow the stimulus with your eyes, without moving your head. The officer will always start with the left eye and they are looking for 3 specific clues:
1. Lack of Smooth Pursuit:
As the eye moves from side to side, does it move smoothly or does it noticeably jerk (bounce)? The jerking is similar to how windshield wipers jerk across a dry windshield. There is a standardized pace the officer is to have the eyes move side to side. From the center of the face, they are to move the eye all the way out to the side in approximately 2 seconds, then 2 seconds back to center, approximately 2 seconds to the other side, and 2 seconds back to center. They are to repeat the procedure. If they have the eyes move too fast, not only are they performing the test improperly, the government has admitted tha
2. Distinct Nystagmus at Maximum Deviation: When the eye moves as far to the side as possible and is kept in that position for several seconds, is there distinct jerking (bouncing) of the eye. The eye is to be moved all the way to the side, and kept there a minimum of 4 seconds. Interestingly enough, some people exhibit slight jerking of the eye at maximum deviation even when unimpaired! Also, if the eye is moved to the side too quickly, this may cause the nystagmus.
3. Onset of Nystagmus Prior To 45 Degrees: As the eye moves towards the side, does it start to jerk (bounce) before it reaches a 45-degree angle? When moving the eye out to 45-degree's, the movement of the eye should take approximately 4 seconds to reach 45-degrees. It is important to take the full 4 seconds when checking for onset. If the stimulus is moved too fast, the officer could take your eye beyond 45 degrees, or if the eye is moved to 45-degrees too quickly, th
The maximum number of clues in each eye is 3, for a total of 6 clues. The original research has shown that if 4 or more clues are present, the person is intoxicated. This test has been shown to be accurate 77% of the time; therefore, it is inaccurate 23% of the time!
