The Alcosense breathalysers are incredibly sensitive pieces of equipment, capable of picking up the minutest amounts of alcohol on someone's breath. As they are sensitive, the detection systems can drift out of their operating range just through general usage, blowing too hard, knocks and so on. This would result in a false figure, and may read that you're safe when in fact you're over the limit.Alcosense recommend that you recalibrate your breathalyser every 12 months, by sending it off to Alcosense to be tested and reset. This service costs £19.99 for the Alcosense Lite and Elite models, and £24.99 for the Pro, Ultra and Excel models.