A fair cop! Accurate breath analysis and speed detectionBreath-testing and speed detection are vital for reducing the road toll, but they will only be effective if they can withstand the scrutiny of the legal system.
Key text
Key textIn 1995, a drunk-driver walked free when he appealed successfully against his conviction in a Queensland magistrate’s court. The reason: the breath analyser used to measure the man’s blood alcohol concentration had not been properly calibrated.If courts can’t trust the evidence, it’s reasonable that they won’t convict the accused, because nobody wants to see innocent people punished. But we also know that speeding and drink-driving are offences that endanger people’s lives, so we don’t want the guilty to get off scot-free. Getting the measurements right, then, is critical (Box 1: Measurement standards). Let's take a look at how measurement standards affect breath-tests and speed detectors.
Breath analysis
At random breath-testing roadblocks, drivers are asked to blow through a plastic tube into a small, hand-held breath-testing device that uses an electrochemical fuel cell as a sensor. If the reading on the device is under the legal limit of 0.05, the driver is free to leave. These hand-held devices (in Australia the Lion 502 and 400 series are mainly used) do not print out a hard copy of the reading and are not used as evidence in Australian courts. Any driver registering 0.05 or above is required to take ‘evidential’ breath tests using an instrument that is calibrated before and after the person is tested and provides a hard copy of the results. These can be used as evidence in a court of law. In Australia, the instrument currently used for such testing is the Drager Alcotest 7110 (Box 2: Drager Alcotest 7110).
Breath alcohol vs blood alcohol
Breath analysers are an ‘indirect’ way of measuring the concentration of alcohol in the blood – a direct way would be to take a blood sample from the driver, but this is impractical and potentially unhygienic. The principle of breath analysers is based on Henry’s Law, which states that the concentration of a gas in the air immediately above a liquid is proportional to its concentration in the liquid. In the lungs there is an exchange between the alcohol in the blood and the alcohol in the air within the lungs. In the 'deep lung' region there is an equilibrium situation where the concentration of alcohol in the lung air is proportional to the concentration in the blood. This breath to blood ratio used in breath test analyses states that 2100 millilitres of breath will contain the same weight of alcohol as does 1 millilitre of blood. This ratio has been controversial and is occasionally challenged in court – it varies between individuals and is also influenced by body temperature. Recently, the state and federal authorities in Australia agreed to change the standard of measurement so that the results of the Drager Alcotest 7110, or any other evidential instruments in the future, will be expressed as grams of alcohol in 210 litres of breath. This standard will keep the legal limit at 0.05 in these units, but it will be a ‘breath alcohol concentration’ rather than a blood alcohol concentration. The Drager Alcotest 7110 has its own diagnostics, which means that while the instrument is turned on, it continuously checks that its circuits are working properly. There are currently nearly 1000 units in operation around Australia. Each one is checked on a routine basis by qualified technicians using certified samples of air with known concentrations of ethanol. When the national standard for breath analysers comes into effect, all measurements made by these units and the equipment used to test them will be required to be traceable to the national standard (Box 1).
Speed detection – radar and lidar
Radar traps have been the bane of many a lead-foot’s life since the early 1960s. More recently, laser speed guns – lidar – have added a new dimension to speed-limit enforcement.
Radar
First coined during World War II, the term radar stands for ‘radio detection and ranging’. Using it to measure distance is pretty easy. High-frequency radio waves, a type of electromagnetic radiation, are transmitted towards the object of interest and are reflected back. We know that radio waves travel at the speed of light – about 300 million metres per second. If we know the time taken for the radio wave to reach the object, bounce off, and arrive back at the radar device, we can calculate the distance to the object (using the formula: distance = speed of light × time between emission and reception ÷ 2). Measuring the speed of the object is a little trickier and relies on something called the Doppler effect. This was first proposed by Christian Doppler in 1842. He realised that the pitch of a sound emanating from a moving source varies for a stationary observer depending on the speed of the source and the direction in which it is moving. Imagine you are on a train in a station and you can hear the signals ringing at a rail crossing just down the track. Since both you and the signals are stationary, the signals sound normal. They continue to ring at the same rate as the train starts to move, but now because you are travelling towards them they seem to get faster. In effect, the time between arrival of pulses of sound is being compressed (or shortened) and the apparent frequency is increasing. The result is that the signals sound higher-pitched. This change in frequency is called a 'Doppler shift'. The radio waves emitted by a radar device propagate outwards at a predetermined frequency. When they strike a moving vehicle and are reflected, the frequency is 'shifted'. The radio waves bounce back to the radar device, where the change in frequency is recorded and used as data in a formula that calculates the vehicle’s speed. Radar used from the roadside usually employs something called ‘slant beam’. This means that the radar beam is projected across the road at a pre-set angle; speed is still determined using the Doppler shift. Slant beam radar is used in speed cameras (although some speed cameras use laser guns); slant beams were first introduced in 1991 and are now commonplace around Australia. Attached to the radar or speed gun is a camera that is activated when a vehicle exceeds the speed limit. The resultant photograph records the numberplate of the vehicle, along with the date, time and speed travelled. The radar device need not be stationary itself: it is the relative movement between the radar and the targeted vehicle that is important. Radar devices are often mounted on patrol cars, the speed of which can be determined from the Doppler shift against stationary objects such as houses or trees. The speed of the targeted vehicle can be determined simply by calculating the net speed after deducting the speed of the patrol car.
Lidar
Laser speed guns – known as lidar, or ‘light detection and ranging’ – also use electromagnetic radiation. The term laser stands for ‘light amplification by stimulated emission of radiation’. Lasers are devices that can control the way energised atoms release photons of light: these photons form a very narrow beam of light. The narrowness of this light beam gives rise to the label ‘speed gun’, because it must be aimed at the vehicle by the operator. When the trigger is pulled, the gun sends an invisible infrared laser light pulse. It then records the time it takes for the pulse to strike the target and return to the receiver mounted on the gun. From this time it is possible to calculate the distance to the object (range) in the same manner as for radar. The gun sends out hundreds of pulses per second; if the target is moving in respect to the laser, then the rate at which the distance to the target is changing is used to derive the speed of the target from a number of successive range measurements. The speed of the target is then displayed to the operator. All laser guns in operation in Australia are tested to ensure that the laser light being transmitted complies with the appropriate Australian standard so that it cannot injure a person's eyes if they happen to look directly into the beam. This is done using an optical power meter certified by the National Measurement Laboratory with a certificate issued under the National Measurement Act. The accuracy of all radar and lidar instruments to measure range and speed is tested using equipment called a delay generator, which must also be certified under the National Measurement Act.
The measure of success
In the early years of radar and breath-testing, many people caught speeding or drink-driving contested the scientific basis of the evidence. These days, the courts rarely entertain such challenges. The science, while still evolving, is sufficiently sound to satisfy most objective judges. And the reliability of the instruments can now be verified. That’s probably bad news for drink-drivers and speeders – but good news for road safety. Related Nova topics:
But are these measurements accurate? Ultimately, accuracy depends on the reliability of the measuring device (and the competence of the operator). To make sure the measuring device is reliable, it needs to be calibrated periodically by other measuring devices, which in turn must also be calibrated. Sound complicated? It is, but there is a system in place to make it work.
The national measurement system
In Australia, the national measurement system ensures that measurements are made on a consistent basis throughout the country – not just those made by the police but also in commerce, industry, science, engineering, international trade, health and safety. It guarantees ‘traceability’ – which means that the steps taken to calibrate an instrument through a hierarchy of calibrations of increasingly higher accuracy can be traced back to the appropriate Australian primary standard, which in turn is linked to an international standard agreed to by most nations of the world. The national measurement system comprises six national organisations along with state and territory measurement authorities and is coordinated by the National Standards Commission, a Commonwealth Statutory Authority established in 1950. The basic objective is to ensure that measurements are based on good science and the instruments that make them are reliable. And if the court asks for proof of this, the measurements are traceable to a reliable source. Related site
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
New Scientist 5 September 2006 'Nano-flowers' show promise for alcohol detection (by Tom Simonite) Describes a sensitive nanocrystal for breath alcohol analysis and other applications.
4 April 2006 Drink-driver arm scanner (by Barry Fox) Describes an invention to do roadside blood alcohol tests, without taking any blood.
3 December 2005, pages 28-29 Taking on the drugged and drunk drivers (by Paul Marks) Describes technology to test drivers for the presence of drugs in their bloodstream.
14 February 2004, page 14 Booze tests tell all about our drinking (by Andy Coghlan) A report on tests that reveal alcohol use in past days, weeks and months.
20 March 2002 Drink-drive detector radios police (by Jonathan Beard) Describes a breath detector that is fitted to cars.
11 March 2002 Car computer could detect drunken steering (by Will Knight) Describes a computer designed to detect driver’s eye movement and steering.
5 February 2000, page 17 The wrong trap (by Barry Fox) Discusses a system that uses digital infrared cameras to detect vehicle speeds.
12 December 1998, page 19 Blinded by the light (by Barry Fox) Explains how speedtrap cameras can be fooled by flashguns.
Scientific American March 2001, pages 68-69 Gotcha! (by Mark Fischetti) Explains how radar guns work.
National Measurement Institute (Australia)
Breath alcohol testing basics – Why breath alcohol analysis? (CMI, Inc, USA) Briefly explains why measuring breath alcohol levels is a way of determining
blood alcohol levels.
Evidential breath analysis in New South Wales: an exercise in pragmatism (Schaffer Library of Drug Policy, USA)
Covers the effect of blood alcohol levels on driving skills and explains how the results of breath tests can be used in court.
The history and development of speed camera use (Monash University Accident Research Centre, Australia)
A report that examines the controversies experienced in Australia, USA and UK in the history of speed cameras.
How Stuff Works (USA)
electrolyte. A substance that produces ions (particles with an electric charge) when dissolved in water. The resulting solution (which can also be referred to as an electrolyte) conducts electricity. electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA). electron. A negatively charged particle that is a constituent of an atom. Electrons can move from atom to atom. When they do, they produce an electric current. frequency. A measure of how frequently an electromagnetic wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA). fuel cell. A device that converts energy from chemical reactions directly into electrical energy. The simplest fuel cell 'burns' hydrogen in a flameless chemical reaction to produce electricity. In order to 'burn' the hydrogen a fuel cell needs a source of oxygen and this is usually obtained from air. The only by-product from this type of fuel cell is water. For more information about fuel cells see Fuelling the 21st century (Nova: Science in the news, Australian Academy of Science). infrared. The part of the electromagnetic spectrum between visible light and microwaves. The wavelength of infrared light is between 0. 7 micrometres (0.0007 millimetres) and 1 millimetre. These wavelengths are longer than those of visible light, but shorter than those of microwaves. (The prefix 'infra' means 'below; infrared refers to radiation below the frequency of red light.) legal limit of 0.05. The legal limit of 0.05 refers to the blood alcohol concentration (BAC) and is measured in grams of ethanol per 100 millilitres of blood. For example, men and women with a BAC of 0.05 grams per 100 millilitres have 0.05 grams of alcohol in their body for every 100 millilitres of their blood. The legal limit is lower for certain road user groups (eg, those who hold learner or provisional licences). Depending on the state or territory, this lower limit is either zero or 0.02. national measurement system. Australia's national measurement system is coordinated by The National Measurement Institute. The NMI commenced on 1 July 2004 and is responsible for establishing and maintaining Australia's units and standards of measurement. NMI has been formed from the National Measurement Laboratory (CSIRO) the National Standards Commission and the Australian Government Analytical Laboratories. photon. A photon is the smallest unit of light energy. radar. The use of reflected radio waves to determine the location of an object and its speed if it is moving. It is an acronym derived from radio detecting and ranging. For more information see How radar works (How Stuff Works, USA). radio waves. Low frequency electromagnetic radiation. Radio waves have wavelengths ranging from less than a centimetre to as long as 100 kilometres. The hertz (Hz) is the unit of frequency and means one complete oscillation per second. Many frequencies are much higher than this so other units are used (eg, 1 megahertz (1MHz) = 1,000,000Hz). We divide the radio wave part of the electromagnetic spectrum into bands that are allocated to different uses. These include AM radio (amplitude modulation), FM radio (frequency modulation) and CB radio (citizens' band), television, aircraft communications, satellites, mobile phones and pagers. Within each band, no two transmissions can use the same part of the spectrum – or frequency – at the same time. For this reason, each band within the radio wave spectrum, itself a part of the broader electromagnetic spectrum, must be managed carefully to ensure the best use of this limited resource. The frequency of radio waves used in magnetic resonance imaging range from 1-100 megahertz, depending on the strength of the magnetic field in the scanner. This is close to the range of frequencies used for FM radio (88-108 megahertz). For more information see How the radio spectrum works (How Stuff Works, USA).
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