An interlock device for measuring the sobriety of a potential vehicle operator is coupled to the starting mechanism of the vehicle. The interlock device includes a microprocessor controlled handset and base unit. The base unit encloses relays to preclude the starting mechanism from engaging if a predetermined acceptable threshold level of blood alcohol content has been exceeded. The relays receive a signal, through the base unit microprocessor, from the handset microprocessor which has calculated the blood alcohol content of a breath sample introduced into the handset by the vehicle operator. A breath sampling housing is enclosed within the handset in axial alignment with an intake port of the handset. A water filter and valve are positioned upstream from an electrochemical fuel cell. The fuel cell is coupled to the microprocessor. A pressure transducer measures the pressure of the breath sample. The handset microprocessor calculates a pressure offset through an algorithmic equation and applies the offset to the variable reading across the fuel cell to provide a normalized blood alcohol content measurement.