The CEU consists of a sampling probe attached to a control unit by a 5 foot retractable cord. The control unit is a miniature computer that completely automates up to three breath tests on the subject. Test results are stored in memory and may be recalled at any time prior to testing a second subject. The results are displayed on a digital readout, which insures that the correct value for the subject's blood alcohol level was correctly recorded. Alcohol from the subject's breath is oxidized on the surface of the fuel cell in the sampling head and generates an electrical signal proportional to the amount of alcohol present. The signal is amplified and processed by the Control Unit, which is a microcomputer. The operation of the various controls on the front panel of the Control Unit are explained, including the Step-Display switch, which generates all data on a particular case according to a special code number. A diagram of the control unit illustrates the unit's major functions: to store test data, receive and route input signals from the Sampling Probe, utilize about 3500 preprogrammed instructions in programmed storage, and display test results upon request. In conclusion, the Mark V's modular instruction, small size, and tamper-proof features (including insensitivity to interfering substances) make it an efficient and practical breath alcohol instrument capable of supplying good evidence close to the time of arrest. Seven photographs and one diagram are used to explain the operation of the Mark V.