Breathalyzer sensors – Semi-Conductor vs Fuel Cell

How does a breathalyzer work? 

There are two types of sensors found in personal breathalyzers: Fuel cell and Semi-conductor.  Each of these sensors measure alcohol levels in a similar way but Fuel cell sensors are  generally considered more accurate.

To explain why, it is best to break down the breathalyzer test into stages:

Stage one: A breath sample is blown into the breathalyzer.  This sample may contain a percentage of alcohol.

Stage two: The breath sample reaches either a fuel cell or semi-conductor sensor.  The alcohol in the breath that reaches the sensor will generate an electrical charge.

Stage three: The electrical charge generated from the fuel cell or semi-conductor sensor passes onto a second sensor which measures the electrical charge and converts this into an alcohol level/breathalyzer reading.

What is the difference then between a semi-conductor sensor and a fuel cell sensor, and which is better?

When a sensor – either fuel cell or semi-conductor – comes into contact with alcohol, a small chemical reaction takes place.  Simply put, this reaction generates electricity in the sensor producing a tiny electrical charge. The more alcohol in the breath that’s been measured, the larger the electrical charge that this reaction will generate.

The amount of electricity that is generated through this chemical reaction is so minuscule that the device has to be extraordinarily well designed to measure such changes. The more accurate the sensor the bigger the size, making it more capable of generating a larger electrical current. Generally a fuel cell sensor will have both size and accuracy over a semi-conductor.

Large fuel cell sensors can detect alcohol levels more accurately because they have a larger surface area for the alcohol particles to hit, consequently detecting a larger electrical current. A smaller sensor is unable to register all the alcohol ‘particles’ because it has a smaller surface area, consequently not always being able to produce an accurate result – reducing the possible alcohol content it can detect.  The diagram below shows how a larger sensor can detect higher levels of alcohol and more accurately because of the greater surface area.

What can go wrong when performing a breath test?

The main cause of questionable sample results is mouth contamination – ie. substances in your mouth. When you are blowing into a breathalyzer it is not measuring how much alcohol is in your mouth but rather in your lungs.  If you have just had a drink or even just used mouthwash this will trigger a much higher reading than it should.

The way to prevent this is to not eat, drink or smoke anything for up to 20 minutes before performing a breath test.  Not following this can damage a sensor because of the unusually high concentration of alcohol.  The smaller the sensor, the easier it will be to damage – meaning the accuracy will be affected.