How do breathalysers work?
When you empty a lung-full of air into a breathalyser, how does it detect how much alcohol is in your blood?
It might come as no surprise that it’s all about science.
As gin and tonic-infused blood circulates through the body, some of the alcohol evaporates into the lungs. Breathalyser units are sensitive to this mix of air and alcohol vapour.
Of the three main types of breathalysers available, the one you’ll encounter at the roadside is a fuel cell unit.
Alcohol entering the breathalyser fuel cell oxidises to form acetic acid, which creates a small electrical current. The more alcohol the stronger the current, which is translated by the machine to display as a blood alcohol concentration (BAC) reading.
So, how does the amount of alcohol in a breath of air determine how much booze is in the blood? It’s been calculated that 2100ml of breath contains the same amount of alcohol as 1ml of blood, and breathalysers are calibrated accordingly.
Deep lung air is required for an accurate test, so mouth alcohol might provide a false reading if you’ve just finished a drink. But mouth alcohol evaporates quickly, and by the time you reach the police station for a secondary test, it should no longer be a factor.
Back at the police station, the secondary test involves an extremely accurate infrared spectroscopy unit. Alcohol absorbs infrared light at a specific wavelength, and the more alcohol vapour exhaled, the greater amount of infrared energy is absorbed. By shooting an infrared beam into a breath sample, the machine calculates the BAC.
The third type of breathalyser is a semiconductor. When alcohol contacts the unit’s sensor, it reduces the electrical resistance proportional to the amount of alcohol, which is converted into a BAC reading. The lower the resistance the higher the reading.
Semiconductor units are cheaper than fuel cell breathalysers but they’re considered less accurate.
What factors can affect a BAC reading?
- People with a higher percentage of body fat can record a higher BAC because fat doesn’t absorb alcohol, so it stays in the bloodstream longer.
- Smaller people have less blood to dilute the alcohol than larger people, so they may record higher readings.
- Women will usually record a comparatively higher BAC than men due to differences in physiology.
- The healthier the liver, the more efficient it is at processing alcohol.
- As we age our metabolism slows, so it can take the liver longer to process alcohol.
- BAC is likely to rise more quickly if you’re drinking on an empty stomach because food absorbs some of the alcohol.
If you plan to buy a breathalyser, make sure it can be calibrated and complies with Australian Standard AS3547, and read the instructions thoroughly before using it. RAA shops have a range of breathalysers available.
Commercially available breathalysers should only be used as a guide to BAC as they may not be as accurate as police-operated units.
If you plan to have a few drinks, ensure you don’t end up behind the wheel.