In short, semi-conductor sensors can be triggered by substances other than alcohol. Fuel cells sensors are considerably more reliable and accurate but are more expensive.
Breathalysers certified for evidential use fuel cell sensors.
Traditionally, breathalyzers were all designed around a device called a fuel cell. In order to produce a more economic device for personal use, various semi-conductor based sensors have been developed. These use varying levels of software complexity to translate their readings into equivalent values such as BAC%, mg/l and microgrammes.
Semi-conductor based sensors are more susceptible to drift (where the values produced gradually vary as the unit gets older or is used more), saturation/contamination (for example if the user has been smoking or drinking recently) and variations in temperature. However, for general personal use a semi-conductor sensor can produce some perfectly acceptable results, provided some margin of error is allowed by the user.
Semi-conductor based sensors also have a narrower range of sensitivity and are more complex to calibrate. For employers or enforcement agencies requiring a reliable and consistent reading over the full range of use, only professionally-approved devices with fuel cell sensors such as the AlcoDigital EON or Draeger 6820 are going to produce the required levels of accuracy and reliability.
Users should also bear in mind that the accuracy of a particular sensor quoted in the specifications has been measured under strict laboratory conditions immediately following calibration. Due to the variations described above, and particularly the limitations of sampling, it is unlikely that such specific accuracy is likely to be obtained on a repeatable basis by the user in a real world scenario. Sensor saturation with alcohol, or contamination with smoke during a test, can quickly destabilise the sensor software and lead to unreliable results. Anyone using a personal breathalyzer should leave a substantial margin of error and take into account general factors such as what and when they’ve been drinking. You cannot rely solely on a personal alcohol detector to determine your level of intoxication.