Published by Australian Academy of Science
A fair cop! Accurate breath analysis and speed detection Box 2 | Drager Alcotest 7110

The Drager Alcotest 7110, though much larger than a hand-held breath-testing device, is portable and is housed in a small, aluminium suitcase. As the suspect blows through the mouthpiece, asterisks appear on the display to indicate that the person is blowing; when 16 asterisks appear, the operator will tell the person to stop blowing. The amount of air required for a valid reading is 1.5 litres of deep-lung air.

What happens when this air enters the machine? The Drager Alcotest 7110 uses two different techniques to measure the concentration of alcohol: infrared absorption and electrochemical reaction. Let’s look at the infrared technique first.

Infrared analysis

The breath enters a ‘sample chamber’. At one end is an infrared transmitter: it sends a beam of infrared radiation through the sample chamber to the other end, where it is collected and concentrated by a mirror.

All molecules absorb infrared radiation at particular wavelengths, depending on the nature of their interatomic bonds. The ethanol molecule – the active molecule in all alcoholic drinks – absorbs infrared at two wavelengths: 3.4 micrometres and 9.5 micrometres. The frequency used by the Drager Alcotest is 9.5 micrometres, because molecules other than ethanol (eg, acetone) also absorb infrared at 3.4, so if acetone molecules were present in the breath they would distort the results at that wavelength.

The infrared beam will encounter and be absorbed by ethanol molecules as it travels through the sample chamber. The higher the concentration of ethanol, the more radiation will be absorbed. By measuring the decrease in infrared radiation received at the far end of the sample chamber, the machine can calculate the blood alcohol concentration.

Electrochemical testing

The instrument uses a small sample of the air from the infrared sample chamber in a second, electrochemical analysis. This involves a fuel cell, which is similar to a conventional battery – it generates electricity through a chemical reaction between two substances such as alcohol and oxygen. It comprises an anode (the alcohol), a cathode (the oxygen) and an electrolyte that facilitates the flow of electrons between the two. In the Drager Alcotest 7110, the alcohol in the sample air is chemically oxidised at the anode. Simultaneously, oxygen from the air is chemically reduced at the cathode. This produces electrons which flow between the two electrodes – the higher the concentration of alcohol, the higher will be the flow of electrons, thus producing a greater electrical current.

Comparison of results

The instrument compares the infrared results with the electrochemical results; if they are identical, the machine displays the calculated blood alcohol concentration.

Related sites

• How breathalyzers work (How Stuff Works, USA)

• Breath alcohol instrumentation: A proposal in commercial taxonomy by Professor RJ Breakspere and Dr PM Williams (Shaffer Library of Drug Policy, USA)

• Development of a system for real-time breath alcohol analysis by CM Bell and HJ Flack (Road Accident Research Unit, University of Adelaide, Australia)

• Examining variables associated with sampling for breath alcohol analysis by CM Bell and HJ Flack (Schaffer Library of Drug Policy, USA)

Other boxes

Box 1. Measurement standards